DOI: 10.1002/cphc.201200097

Directionality of Dihydrogen Bonds: The Role of TransitionMetal AtomsOleg A. Filippov,[a] Natalia V. Belkova,[a] Lina M. Epstein,[a] Agusti Lledos,*[b] andElena S. Shubina*[a]

1. Introduction

Since the 1990s, many examples of hydrogen bonds involvingtransition metal complexes have been reported.[1–3] Amongthem, hydrogen bonding to transition metal hydrides, that is,dihydrogen bonding (DHB), has emerged as a peculiar class ofbonds in which the proton acceptor is a hydrogen atombonded to a transition metal. Like all hydrogen bonds, dihy-drogen bonds rely on electrostatic, charge-transfer, and disper-sion forces.[4–6] Electrostatics and dispersion are less specific in-teractions depending mostly on geometry and charge densityof interacting fragments, whereas charge transfer strongly de-pends on the nature of the base (B). In a classical B···HA hydro-gen bond, charge transfer occurs from an electron lone pair ofB to an antibonding orbital of AH, that is, nB!s�AH. In anMH···HA dihydrogen bond,[7, 8] electrons come from a s bondingelectron pair of an M�H bond localized mainly on the hydrideligand.[9] When the metal is basic enough, formation of M···HAhydrogen bonds is also possible.[1, 2, 7, 10–12] In this case electronsof filled d orbitals of the metal are donated to s�AH.[2]

Hydrogen bonds play a key role as initial stage of theproton-transfer reaction to organic bases.[13] In the same way,hydrogen bonding to metal atoms was found to intermediatemetal protonation of transition metal complexes,[14–16] and di-hydrogen bonding was shown to precede hydride ligand pro-tonation yielding h2-H2 complexes.[7, 17] Basic knowledge accu-mulated over the last two decades suggests that a directproton transfer to a metal lone pair is an unlikely event in thepresence of hydride ligands, which are considered to be the ki-netic site of proton attack even if classical di(poly)hydride werethe thermodynamic protonation product.[18] Still, direct protonattack on the core metal atom to yield classical polyhydridespecies via M···HOR hydrogen-bonded intermediates was sug-

gested for [WH4(dppe)][19] and [ReH5(PPh3)2(PTA)][20] (PTA = 1,3,5-triaza-7-phosphaadamantane) on the basis of spectroscopicdata. Participation of the metal atom in hydrogen bonding ata hydride site was shown to assist direct proton transfer to Wand Os in the case of [Cp*WH3(dppe)][21] (Cp* = h5-C5Me5) and[Cp*OsH(dppe)][22] (dppe =k2-Ph2PCH2CH2PPh2). The study ofproton-transfer reactions via dihydrogen-bonded intermediatesto isostructural hydrides of the group 8 metals [Cp*MH(dppe)]and [(PP3)MH2] (PP3 =k4-P(CH2CH2PPh2)3) showed that the ex-perimentally determined hydride basicity in hydrogen bondingEj

[23] increases evenly down the group, and that the ability of[(PP3)MH2] complexes to undergo proton transfer to form hy-drido/dihydrogen complex changes aperiodically: Fe<Os<Ru.[24] For the [Cp*MH(dppe)] series, passing from ruthenium toosmium changes the reaction mechanism: proton transferyields dihydrogen complex for Ru but cis-dihydride for Os.[22, 25]

In such non-d0 transition metal hydrides the incoming protonin a DHB is faced with two contiguous basic centers : the metaland the hydride. Thus, the effect of the metal atom on thestructure of hydrogen-bonded intermediates and, consequent-

[a] Dr. O. A. Filippov, Dr. N. V. Belkova, Prof. L. M. Epstein, Prof. E. S. ShubinaA. N. Nesmeyanov Institute of Organoelement CompoundsRussian Academy of SciencesVavilov str. 28, 119991 Moscow (Russia)E-mail : shu@ineos.ac.ru

[b] Prof. A. LledosUniversitat Aut�noma de Barcelona (UAB)Departament de Qu�mica, Edifici CnBellaterra, Barcelona (Spain)

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cphc.201200097.

A theoretical study on two series of electron-rich group 8 hy-drides is carried out to evaluate involvement of the transitionmetal in dihydrogen bonding. To this end, the structural andelectronic parameters are computed at the DFT/B3PW91 levelfor hydrogen-bonded adducts of [(PP3)MH2] and [Cp*MH-(dppe)] (M = Fe, Ru, Os; PP3 =k4-P(CH2CH2PPh2)3, dppe =

k2-Ph2PCH2CH2PPh2) with CF3CH2OH (TFE) as proton donor. Theresults are compared with those of adduct [Cp2NbH3]·TFE fea-turing a “pure” dihydrogen bond, and classical hydrogenbonds in pyridine·TFE and Me3N·TFE. Deviation of the H···H�Afragment from linearity is shown to originate from the metal

participation in dihydrogen bonding. The latter is confirmedby the electronic parameters obtained by NBO and AIM analy-sis. Considered together, orbital interaction energies and hy-drogen bond ellipticity are salient indicators of this effect andallow the MH···HA interaction to be described as a bifurcatehydrogen bond. The impact of the M···HA interaction is shownto increase on descending the group, and this explains the ex-perimental trends in mechanisms of proton-transfer reactionsvia MH···HA intermediates. Strengthening of the M···H interac-tion in the case of electron-rich 5d metal hydrides leads todirect proton transfer to the metal atom.

ChemPhysChem 0000, 00, 1 – 12 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1&

These are not the final page numbers! ��

ly, on the protonation reaction pathway is a very importantissue. However, despite the large number of computations onDHB complexes, participation of a d-block metal in interactionwith a proton has been analyzed, if ever, only on the basis ofstructural data.[21, 26] Interestingly short metal–proton distanceand significant Mo···HF overlap population were found in oneof the first computational works on DHB[27] but were barely dis-cussed. Only the intramolecular interaction between a pendantNH group of an aminocyclopentadienyl ligand and the Ru�Hbond of an aminocyclopentadienyl ruthenium hydride complexhas been analyzed,[28] although the use of electronic parame-ters to estimate the metal-atom participation in the dihydro-gen bond suffered from incorrect application of the atoms inmolecules (AIM) approach to a transition metal with its coreelectrons described by an effective core potential (ECP).[29]

Herein, we report a systematic study aimed at analyzingtransition-metal involvement in dihydrogen bonds. To this end,both structural and electronic parameters computed for hydro-gen-bonded adducts of [(PP3)MH2] and [Cp*MH(dppe)] (M = Fe,Ru, Os; PP3 =k4-P(CH2CH2PPh2)3, dppe =k2-Ph2PCH2CH2PPh2)with the same proton donor (CF3CH2OH = TFE) are analyzedand compared with those of a “pure” dihydrogen bond, inwhich no involvement of metal is possible, and a “pure” transi-tion-metal hydrogen bond, without hydride participation.[Cp2NbH3] featuring d0 metal configuration is taken as exampleof “hydride-only” DHB. In addition the same calculations wereperformed on classical pyridine·TFE and Me3N·TFE hydrogen-bonded complexes. For all of the hydride complexes underconsideration, hydrogen bonding and proton transfer havebeen studied by us experimentally.[9, 22, 25, 30–34]

2. Results

2.1. Models and Tools for Analysis

Density functional B3PW91 calculations were performed onmodel compounds in which phenyl rings of PP3 and dppe li-gands are substituted by hydrogen atoms (PPH

3 and dpe li-gands, respectively). Earlier, we showed that the electroniceffect of phenyl rings on hydrogen-bond energy is small(tenths of a kilocalorie per mole).[33] Thus we considered modelcomplexes in the present work in order to maintain successionto preceding studies. However, previous theoretical studieswere limited mainly to the description of hydrogen-bond geo-metries and were not obtained in a unified way.[9, 22, 25, 30, 33]

Therefore, NBO and topological (atoms in molecules, AIM)analyses are applied here in addition to new geometry optimi-zation, allowing deeper understanding of the dihydrogen-bond phenomenon and experimental trends. Energies of DHBcomplex formation (DEZPE) obtained in this work are quite simi-lar to those previously reported. So, herein they are given forreference only, since additional multiple interactions betweenthe proton donor and other sites of these large molecules (e.g.O···HC, F···HC) further contribute to complexation energy.[35, 36]

For this reason we focused on the parameters of AH···B moiet-ies (B = H, M, N) characterizing the hydrogen bond itself.

Several theoretical approaches were successfully employedfor characterization of hydrogen bonding.[5, 37–39] Among them,only AIM provides (in principle) the possibility to comparecomputed with experimental data. Herein we use Wibergbond indices (WBI, derived from NBO analysis) and electron de-localization indices (DI, derived from AIM analysis) to character-ize hydrogen bonding in terms of the number of electronsshared between the atoms involved in the interaction.[6, 39–41] Akey issue we would like to analyze is the relative contributionof the M�H bond and metal lone pairs in charge transfer tothe proton donor. To this end, the electron-density shift ac-companying dihydrogen-bond formation was analyzed byusing the relative contribution of the sMH to s�OH donationenergy estimated from second-order perturbative analysis ofdonor–acceptor interactions, as implemented in NBO.[42, 43] Toquantify this contribution we devised a new parameter fromthe NBO scheme, called MH% [Eq. (1)]:

MH% ¼ 100�EsMH!s*

OHPEsMH ;LPM ;CRM!s*

OH

ð1Þ

where S stands for the total stabilization energy associatedwith delocalization of electrons from the M�H bond (EsMH

) andmetal orbitals (lone pairs, ELPM

; core electrons, ECRM) into the s�OH

orbital revealed by NBO analysis.

To analyze the factors that contribute to the bondingenergy, we carried out an energy decomposition analysis (EDA)using the ADF energy decomposition scheme, which is basedon the methods of Morokuma[44, 45] and Ziegler and Rauk.[46–48]

The stabilization energy due to formation of an HB or DHB(DE) can be divided into preparation energy (DEprep), which isneeded to bring the fragments to the geometry that theyadopt in the HB or DHB complex, and interaction energy(DEint). For each hydrogen-bonded adduct, the interactionenergy between the proton-donor and proton-acceptor moiet-ies is decomposed into three terms: Pauli repulsive orbital in-teractions (DEPauli), classical electrostatic interaction (DVelst), andattractive orbital interactions (DEoi) [Eq. (2)]:

DE int ¼ DEPauli þ DVelst þ DEoi ð2Þ

The term DEPauli, which is called exchange repulsion or Paulirepulsion, takes into account the destabilizing interactions be-tween occupied orbitals of both fragments. DVelst describes theclassical Coulomb interaction between the fragments, which isattractive in most cases. The last term in Equation (2), DEoi,characterizes the stabilizing orbital interactions between occu-pied and virtual orbitals of the two fragments.

2.2. Reference System: [Cp2NbH3]·TFE

Having three hydride ligands in the same plane, [Cp2NbH3] (1)gives dihydrogen-bonded adducts with lateral (HL) or central(HC) hydride ligand (Figure 1). The latter was shown being en-ergetically slightly preferred.[30] In agreement with this smallpreference, all parameters of these dihydrogen-bonded com-plexes (Table 1) are only slightly different for central and lateral

&2& www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 12

�� These are not the final page numbers!

A. Lledos, E. S. Shubina et al.

interaction and will be further discussed together without dis-crimination. Interestingly, all geometrical and electronic param-eters of dihydrogen bonds in 1·TFE are close to those in pyridi-ne·TFE and Me3N·TFE adducts, despite much higher formationenergy of these organic adducts (Table 1).

The geometry of 1·TFE adducts shows the expected almostlinear disposition of the H···H�O unit, typical of intermolecularhydrogen bonds. Small deviations (up to 10–128) from fullylinear arrangements are not rare in classical hydrogen bonds[49]

and are found again here for pyridine·TFE and Me3N·TFE(Table 1). Stronger deviations of protons from the hydrogen-

bond axis (line between heavy atoms in A�H···B) for intermo-lecular hydrogen bonds are usually related to bifurcated inter-actions or lower hydrogen-bond strength.[50–52]

In accord with our expectations, formation of theNbH···HORF bond induces Md+�Hd� polarization of the metal–hydride bond due to the increased positive charge on themetal atom and the negative charge on the hydride ligand. Inagreement with high polarizability of M�H bonds, the in-creased negative charge on HM on dihydrogen-bond formationis 2–3 times higher than that on the basic nitrogen atom inclassical hydrogen bonds. The MH%, values, calculated accord-ing to Equation (1), show that in this system the electron den-sity shift from the sMH orbital of the base to the s�OH orbital ofthe proton donor is alone responsible for the HNb···HO bondpopulation, with 96–98 % impact on this interaction. The restcomes from the donation of the metal core electrons (4p and4s) to the s�OH orbital and is purely residuary. A similar patternis found for nN: to s�OH donation, with 2 % participation of 1score electrons in classical hydrogen bonding.

Both WBI and DI show nonzero electron occupancy for theNb···HO contact, despite the lack of d lone pairs on niobium,probably due to involvement of the metal d electrons in Nb�Hbond formation. The values obtained for HNb···HO contacts aresubstantially higher and are typical for medium-strength hy-drogen bonds.[53, 54]

Critical points (3, �1) are found for dihydrogen bondshaving the characteristics typical for this kind of bonding (seeSupporting Information for details). The H···H bond ellipticityindices e are close to zero and thus indicate linear bondsformed by overlapping hydrogen s shells.

2.3. [(PP3)MH2]·TFE

Formation of dihydrogen bonds to the axial (Hax, trans to theapical phosphorus atom) and to the equatorial hydride (Heq) li-gands of [(PP3)MH2] (2 ; Figure 2) was shown theoretically andexperimentally. However, only the former interaction precedesproton transfer and h2-H2 complex formation.[9]

Relative energies (Eax�Eeq) of the axial and equatorial ad-ducts 2·HOR depend on the proton donor. The equatorial com-plex is preferred when HOR is a weak proton donor (meth-anol), but the opposite is found for stronger alcohols such asCF3OH[9] and CF3CH2OH (this work). The Eax�Eeq energy differ-ence depends also on the metal, and is 2.7, 1.5, and 0.7 kcalmol�1 in favor of the axial TFE adduct for Fe, Ru, and Os, re-spectively. Parameters of the 2·TFE adducts are gathered inTable 2.

In both axial and equatorial complexes the M···HO distanceonly slightly increases on descending the group, in agreementwith the increase in metal radii, whereas the H···H distance in-creases significantly from Fe to Os (Table 2). Note this increaseis not regular for axial adducts, for which the (Ru)H···H distanceis the longest in the series. The H···H�O angle is systematicallysmaller than the M···H�O angle in axial complexes. The twoangles are very close to each other in equatorial adducts, andthe case of Ru is again the exception.

Figure 1. Optimized structures of hydrogen-bonded adducts at central (top)or lateral (bottom) hydride sites of [Cp2NbH3] (1).

Table 1. Main geometrical and electronic parameters of hydrogenbonded complexes of TFE with [Cp2NbH3] and organic bases (pyridine,Me3N).[a]

NbHC···HO NbHL···HO Py···HO Me3N···HO

DEZPE [kcal mol�1] �6.4 �5.8 �9.8 �10.7r(X···HO) [�] 1.604 1.620 1.794 1.733r(Nb···HO) [�] 3.156 3.252 – –ff(X···H�O) [8] 169.1 169.3 172.7 172.4ff(Nb···H�O) [8] 167.9 156.9 – –Dq(X)[b] �0.089 �0.097 �0.054 �0.036Dq(HO)[b] 0.019 0.017 0.018 0.018Dq(Nb)[b] 0.029 0.037 – –WBI(X···HO)[c] 0.029 0.029 0.086 0.099WBI(Nb···HO)[c] 0.011 0.011 – –DI(X···HO)[d] 0.070 0.071 0.121 0.139DI(Nb···HO)[d] 0.020 0.020 – –eHB

[e] 0.038 0.033 0.028 0.015MH% 98 96 98 98EHB [kcal mol�1][f] �4.9 �4.7 �9.3 �12.3

[a] X = hydride ligand (HNb) or N; HO = oxygen bound proton of TFE.[b] NBO charge. [c] Wiberg bond index. [d] Electron delocalization index.[e] Hydrogen-bond ellipticity. [f] AIM interaction energy, EHB = 1/2 V(1)

ChemPhysChem 0000, 00, 1 – 12 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org &3&

These are not the final page numbers! ��

Directionality of Dihydrogen Bonds

Dihydrogen-bond formation increases the negative chargeon the interacting hydride ligand in all of the 2·TFE complexes,whereas positive charge on the metal atom significantly in-creases only in the case of the 2-Fe adduct. The change of themetal charge becomes smaller on descending the group. The

electron-density shift from the sMH orbital of the base to thes�OH orbital of the proton donor (MH%) accounts for more than90 % of the acid–base interaction energy, but presence ofmetal d-electrons together with a suitable orientation of theOH group relative to the metal atom allows 8–9 % contributionof nM:!s�OH interaction in TFE complexes with 2-Ru and 2-Os.Critical points (3, �1) are found for 2·TFE as well, but the ellip-ticity index is comparable to that of the NbH···H(O) bond onlyin the case of the equatorial Fe adduct. Dihydrogen bond pathbending is higher (eHB = 0.114–0.173) in the rest of the com-plexes and reaches a value of 0.232 (typical of p bonds) in theaxial ruthenium adduct. All of these trends reflect participationof the metal d electrons in dihydrogen-bond formation, whichincreases evenly down the group in equatorial complexes, andin the order Fe<Os<Ru in axial complexes. The H···H energiescalculated from AIM data are in accord with these geometricaland electronic characteristics. Notably, this approach showspreference for equatorial adducts in agreement with experi-mental data for weakly donating alcohols.[24]

2.4. [Cp*MH(dppe)]·TFE

The hydrides [Cp*MH(dppe)] present a unique opportunity tocompare hydride- and metal-bonded H complexes and werestudied quite thoroughly.[22, 25, 33] However, neither NBO nor AIManalysis of such complexes has been reported. In these three-legged piano-stool complexes there is the possibility for hydro-gen bonding with the metal atom at a position trans to the hy-dride ligand, which thus avoids hydride involvement (Figure 3).The preference for bonding at the hydride side (syn) was firstshown by a B3LYP study on different models of the iron con-gener.[33] This preference is preserved in the case of rutheni-um[25] and osmium,[22] as found by B3PW91 calculations givingEsyn�Eanti of �4.3, �3.7, and �3.8 kcal mol�1 for Fe, Ru, and Os,respectively. Hydrogen-bonded complexes in which the protondonor attacks the metal atom at the side opposite to the hy-dride ligand (anti) have not been found experimentally andwill be described here for the sake of comparison (Table 3).

Our calculations on classical hydrogen-bonded complexesPy·TFE and Me3N·TFE show that small deviation of the O�H···Nangle from 1808 (up to 88 in the minimum structure) givesvery little energy gain (ca. 0.2 kcal mol�1), whereas further dis-tortion leads to continuous destabilization of the complex(Figure 4). The same analysis for the example of syn-3-Os·TFEstarting at linear O�H···HM angle gives a strikingly differentresult. Though the energy profile is similarly shallow (Support-ing Information Figure S1) it reaches a minimum at anO�H···HM angle of 144.78 when the O�H···M angle is 171.68(Figure 4). The O�H···HM angle is smaller than the O�H···Mangle in all dihydrogen-bonded complexes syn-3·TFE; the dif-ference increases from iron (D= 128) to osmium (D= 278).Such structures of syn adducts are already evidence for a bifur-cate interaction,[55] which involves the proton of the alcoholand both sites of the M�H moiety of the hydride complex(Scheme 1 a). This is in contrast to all bifurcated complexesconsidered in the literature for organic[13, 56] and even organo-

Figure 2. Optimized geometries of hydrogen-bonded adducts at the axial(top) and equatorial (bottom) hydride ligand sites of model [(PPH

3)MH2] com-plexes (2).

Table 2. Selected parameters of hydrogen-bonded complexes between themodel [(PPH

3)MH2] complexes and TFE.[a]

MHax···HO MHeq···HO

Fe Ru Os Fe Ru OsDEZPE [kcal mol�1] �10.5 �10.9 �10.3 �7.9 �9.1 �9.6r(HM···HO) [�][b] 1.582 1.685 1.624 1.516 1.591 1.614r(M···HO) [�] 2.571 2.623 2.743 2.707 2.712 2.740ff(HM···H�O) [8][b] 158.6 154.7 155.6 165.9 158.5 161.1ff(M···H�O) [8] 168.1 167.1 165.0 165.3 166.5 161.6Dq(HM)[b] �0.058 �0.058 �0.059 �0.064 �0.062 �0.053Dq(HO) 0.002 0.007 0.015 -0.003 0.003 0.009Dq(M) 0.043 0.015 �0.003 0.033 0.022 0.004WBI(HM···HO)[b] 0.069 0.055 0.053 0.072 0.061 0.054WBI(M···HO) 0.023 0.019 0.016 0.028 0.024 0.022DI(HM···HO)[b] 0.077 0.066 0.069 0.085 0.076 0.070DI(M···HO) 0.038 0.050 0.042 0.032 0.048 0.048eHB 0.114 0.232 0.173 0.049 0.123 0.166MH% 97 91 91 97 92 92EHB, kcal mol�1 �6.0 �4.6 �5.2 �6.8 �5.7 �5.4

[a] See footnote to Table 1. [b] Characteristics of bonded hydride.

&4& www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 12

�� These are not the final page numbers!

A. Lledos, E. S. Shubina et al.

metallic[2] systems which feature two basic centers that are notdirectly bonded (Scheme 1 b).

Perusal of structural data in Table 3 shows that the M···H(O)distance is shorter in dihydrogen-bonded syn than in metal-bonded anti adducts and essentially does not vary with themetal. In contrast, the H···H(O) distance in syn complexes in-creases rather significantly on descending the group.

The natural atomic charges (Table 3) show that there is anincrease of the negative charge on the hydride ligand, but it issmaller for Os-bound hydride than for FeH. The change inmetal charge is positive for Fe and Ru but negative for Os.Wiberg bond indices decrease significantly down the group forH···H(O) contacts and increase for M···H(O) contacts. This indi-cates weakening of the hydride–proton interaction and relativeincrease of the contribution of metal–proton interaction.

The most striking results come from the AIM analysis of thisseries. Importantly, they strongly support the change in the

balance between H···H(O) andM···H(O) interactions. Only onecritical point (3, �1) is still foundfor the syn adducts, and the hy-drogen-bond path remains be-tween the proton and the hy-dride ligand. However, the H···Hbond ellipticity eHB climbs toa value typical of a C=C doublebond for Ru and very severe de-viation from the HM···HO axisoccurs in the case of osmium(Table 3, Figure 5). Such behaviorof the electron density is typicalfor the process of bond breaking(e.g. hydrogen-bond breaking inthe formamide–formic acid com-plex)[57] or for weak multicen-tered intermolecular interac-tions.[58, 59] However, bifurcated

Figure 3. Optimized geometries of hydrogen-bonded adducts at the hydride(syn, top) and metal (anti, bottom) sites of [Cp*MH(dpe)] (3).

Table 3. Selected parameters of hydrogen-bonded complexes between [Cp*M(dpe)H] and TFE.[a]

MH···HO (syn) HM···HO (anti)

Fe Ru Os Fe Ru OsDEZPE [kcal mol�1] �9.1 �9.2 �9.5 �4.5 �5.4 �5.8r(H···HO) [�] 1.612 1.735 1.795 – – –r(M···HO) [�] 2.561 2.573 2.571 2.628 2.648 2.686ff(HM···H-O) [8] 156.8 147.6 144.7 – – –ff(M···H-O) [8] 168.5 171.4 171.6 162.8 165.1 162.5Dq(HM) �0.067 �0.060 �0.044 0.020 0.013 0.018Dq(HO) 0.008 0.012 0.013 0.005 0.008 0.006Dq(M) 0.027 0.003 �0.014 �0.009 �0.022 �0.034WBI(HM···HO) 0.056 0.038 0.031 – – –WBI(M···HO) 0.025 0.024 0.029 0.031 0.032 0.039DI(HM···HO) 0.069 0.050 0.041 – – –DI(M···HO) 0.039 0.061 0.069 0.051 0.073 0.079eHB 0.129 0.456 1.305MH% 95 86 74 – – –EHB [kcal mol�1] �5.8 �4.4 �4.1 �2.8 �2.7 �3.0

[a] See footnote to Table 1

Figure 4. Variation of hydrogen-bonded adduct energy relative to the mini-mum with the hydrogen-bond angle (O�H···X, where X = N or HOs) in hydro-gen-bonded adducts Py·TFE (g), Me3N·TFE (a), and syn-[Cp*OsH-(dpe)]·TFE (>L->).

Scheme 1. a) Bifurcate interaction involving the proton of the HA and bothsites of the M�H moiety of the hydride complex. b) Bifurcate interaction fea-turing two basic centers that are not directly bonded.

ChemPhysChem 0000, 00, 1 – 12 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org &5&

These are not the final page numbers! ��

Directionality of Dihydrogen Bonds

hydrogen bonds described in the literature, in which theproton interacts with two acceptor atoms, feature two bondcritical points with similar or different electron densities de-pending on the geometry.[39, 60] Clearly, the electron-density be-havior observed for 3·TFE originates from the increased partici-pation of the metal d orbitals in the syn hydrogen bonding.This is confirmed by the orbital analysis : the impact of sMH!s�OH interaction is 95 % for syn-3-Fe·TFE and significantly dropsdown the group to 74 % for syn-3-Os·TFE. The values of EHB, de-rived from AIM, change in the same order as DEZPE for anticomplexes but have opposite order for syn adducts. This is be-cause the main impact on EHB comes from the electron densitybetween atoms connected by bond path, and thus EHB doesnot take into account large impact of additional interactions inthe case of syn adducts. Although AIM parameters show nostraightforward relationship with complex stability DEZPE, weconsider AIM to be a useful tool to probe the properties of thefragment of interest in such weak multicentered intermolecular

interactions and to reveal the primary bonding site and impactof the secondary interactions.

2.5. Covalent versus Electrostatic Effects

Until now the analysis of the dihydrogen-bonded complexeshas been mainly based on the electron flow from the protonacceptor to the proton donor. This term accounts for thecharge-transfer interaction that can be related with the cova-lent nature of hydrogen bonds.[61] However, it was recognizedthat there is an important electrostatic component in classicalhydrogen bonds and the discussion on the electrostatic andcovalent components of hydrogen bonding is a recurrent topicin the literature.[62] The importance of electrostatic interactionin dihydrogen bonding has also been demonstrated.[63–65] Thedecomposition of the interaction energy is a useful tool to ad-dress this feature, and we performed an EDA analysis for all ofthe complexes discussed so far (Table 4). From the bonding en-ergies, all of the DHBs described therein can be categorized asmedium-strength interactions. Moreover, the influence of themetal on the bonding energy is small and leads to smallchanges of the energy terms. As found in other DHB energydecomposition studies,[64, 65] the interaction energy receives animportant part of its stabilizing character from the orbital inter-action term (DEoi). The percentage DEoi of all the attractiveforces (DEoi +DVelst) in pure DHB NbH···H (>45 %) is larger thanthe percentage of orbital interactions found for classic hydro-gen bonds (ca. 40 %). The electrostatic attractive term (DVelst) isbalanced by the positive Pauli repulsion energy (DEPauli). Hence,the orbital interaction energies are mainly responsible for thestability of the complexes. The leading role of the delocaliza-tion interaction as a driving attractive force for a wide range ofDHB interaction has been stressed in a recent thorough reviewon hydrogen bonding.[5] From our EDA results, a small de-crease in the percentage of orbital interaction (%DEoi) from 45

Figure 5. Fragment of molecular graph for [Cp*Os(dpe)H]···HOR (red: bondcritical points, yellow: ring critical points, green: cage critical point).

Table 4. Energy decomposition analysis and orbital interaction analysis of the dihydrogen- and hydrogen-bonded complexes.[a]

Energy decomposition [kcal mol�1] Fragment orbital overlaps Fragment orbital energy [eV]Base H-bond M DEPauli DVelstat DEPauli +DVelst DEoi DEoi

[b] DEint nM s�AH

��

� �sMH s�AH

��

� �nM sMH s�AH

[Cp*M(dpe)H]

MH···HO (syn)Fe 16.5 �13.4 3.1 �11.3 46 �8.2 0.09 0.26 �3.52 �5.93 �0.78Ru 15.8 �13.7 2.1 �11.0 45 �8.9 0.16 0.26 �3.88 �6.62 �0.79Os 15.8 �14.2 1.5 �10.8 43 �9.3 0.16 0.26 �3.82 �6.77 �0.79

HM···HO (anti)Fe 14.6 �11 3.6 �9.0 45 �5.3 0.18 0.01 �3.45 �6.08 �0.79Ru 16 �12.4 3.6 �10.3 45 �6.7 0.22 0.05 �3.72 �6.84 �0.82Os 16.7 �13.3 3.4 �10.7 45 �7.3 0.23 0.06 �3.71 �6.95 �0.83

[(PPH3)MH2]

MHax···HO

Fe 15.6 �13.9 1.6 �11.2 45 �9.6 0.09 0.36 �3.91 �5.25 �0.80Ru 15.2 �14.7 0.6 �11.1 43 �10.5 0.15 0.30

0.19[c]

�4.33 �4.81�4.98[c]

�0.81

Os 13.5 �14.3 �0.8 �9.8 41 �10.6 0.14 0.30 �4.31 �5.05 �0.81

MHeq···HOFe 17.6 �14.1 3.5 �12.2 46 �8.7 0.11 0.27 �3.90 �5.74 �0.80Ru 18.3 �15.5 2.8 �12.6 45 �9.7 0.17 0.30 �4.36 �5.55 �0.82Os 18 �15.8 2.3 �12.2 44 �10 0.14 0.31 �4.35 �5.68 �0.82

[Cp2NbH3]NbHC···HO 11.1 �9.7 1.4 �8.1 46 �6.7 – 0.19 – �5.24 �0.73NbHL···HO 12.2 �9.2 3.0 �9.4 51 �6.4 – 0.24 – �6.44 �0.72

Py Py···HO 19 �17.8 1.2 �11.3 39 �10.1 0.26 – �5.91 – �0.76Me3N Me3N···HO 21.8 �19.2 2.6 �12.9 40 �10.3 0.21 – �4.82 – �0.77

[a] Computed at the BP86/TZP level of theory. [b] %DEoi = 100·DEoi/(DEoi+DVelst).[c] Extensive mixing of [(PPH3)RuH2] HOMO�3 and HOMO�2 takes place on

RuHax·TFE complex formation due to the small energy gap (0.08 eV).

&6& www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 12

�� These are not the final page numbers!

A. Lledos, E. S. Shubina et al.

to 42 % is observed on going from Fe to Os hydrides. This be-havior could be related to the increase in the H···H distancewhen the proton approaches the metal side. The relationshipbetween the H···H distance and the energy components inDHB has been pointed out.[5] However, these results do notgive direct indication of metal involvement in the dihydrogenbond.

3. Discussion

An important feature of hydrogen-bonded adducts is that, inthe absence of steric constraints, the A�H···B moiety tends tobe linear.[66] Thorough analysis of the effects of varying thisangle by means of database studies and ab initio calculationshave shown a strong relationship between distance and angle,as well as between directionality at the donor H atom and hy-drogen-bond strength.[56] In other words, the A�H···B angle inclassical intermolecular hydrogen-bonded complexes is usuallyclose to 1808. Interactions in which the A�H···B angle is smallerthan 1408 have substantially lower stabilization energies.[56]

This is also valid for dihydrogen-bonded complexes, whereinthe A�H···HM angle ranges from 1688 to 1788.[7] An A�H bondapproaches an M�H bond in a side-on direction (M�H···HA

100–1308), so the proton is also close to the metal atom.[8] Thisengenders possible ambiguity, because metal dp electronscould, in principle, interact with H(A). Thus, there was a needto prove that there is indeed hydride–proton interaction. Thesupport was found in studies on main group element hy-drides[67] and on [Cp2NbH3] (1)[30] . The latter features a d0 metalconfiguration and thus could be regarded as an example of“pure” dihydrogen bonding with transition metal hydride.

The results described herein for d6 metal hydrides 2 and 3show that hydrogen bonding solely to core metal atom is pos-sible only when there are particular geometrical premises, as in[Cp*MH(dppe)] complexes 3 (Figure 3). Otherwise, as shownfor the two series of the iron subgroup metal hydrides, there isa simultaneous interaction with a core metal atom and hydrideligand, so it can be considered as a bifurcate hydrogen bond.To quantify the impact of M···H and H···H interactions, severalparameters were employed (Figure 6–8). Each one of them—DI, WBI, eHB, and MH%—gives its own result, but together theyprovide strong evidence that balance of M···H and H···H inter-

actions changes on descending the group, in favor of M···H.The impact of each of these interactions certainly depends onthe particular system (ligand environment). Thus, for the[(PP3)MH2] series H···H bonding dominates for all three metals,whereas for [Cp*MH(dppe)] , on going down the group, theimpact of M···H becomes nearly equal in WBI terms and evendominates in DI terms. This supports the need for comprehen-sive analysis employing different approaches/parameters.

According to the experimental data, transition metalsbecome more basic on descending the group,[68] as reflectedby the proton accepting ability (Ej) of the transition metalatom[69] [e.g. Ej Cp2Ru (0.67)<Cp2Os (0.81) and Cp*2Ru (0.85)<Cp*2Os (1.05)] .[15] We found similar a trend for the anti-3·TFEadducts : characteristics of hydrogen bond to the metal (likeWBI and especially DI) increase in the order Fe<Ru<Os, so in-creased participation of d lone pairs in the syn hydrogen bond-ing should be expected for Ru and, particularly, Os and couldbe envisaged for other electron-rich (5d) metals. Such increaseof the M···H interaction strength for 5d transition metals seemsto be a general trend. For example, similar changes in H···H/M···H balance were found for [Cp*MoH3(dppe)] and [Cp*WH3-(dppe)] interacting with proton donors and can be evoked toexplain the difference in their spectroscopic and chemical be-havior.[21, 70, 71]

The orbital component of a classical hydrogen bond is ofdonor–acceptor nature; the lone pair orbital of the base do-nates electrons to an empty s�AH orbital that has its main coeffi-

Figure 6. Variation of electron DI for H···H (solid bars) and M···H (hatchedbars) contacts in dihydrogen-bonded complexes.

Figure 7. Variation of WBI for H···H (solid bars) and M···H (hatched bars) con-tacts in dihydrogen-bonded complexes.

Figure 8. Variation of dihydrogen-bond ellipticities (solid bars) and MH%(� 10�2, hatched bars) in dihydrogen-bonded complexes.

ChemPhysChem 0000, 00, 1 – 12 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org &7&

These are not the final page numbers! ��

Directionality of Dihydrogen Bonds

cient on the protonic hydrogen atom (Figure 9 a). In otherterms it is a two-electron, two-orbital interaction between theHOMO of the proton acceptor and the antibonding orbital of

the A�H bond of the proton donor (s�AH).[72] This analysis alsoapplies to dihydrogen bonds involving main group hydrides,as shown by Hugas et al.[64] In this case, instead of the lonepair of the base, the donor orbital is of sEH nature, mainly cen-tered on the hydridic hydrogen atom (Figure 9 b).

Interestingly, according to both Gaussian/B3PW91 and ADF/BP86 results, in none of the complexes is the sMH bonding or-bital the HOMO (Figure 10). The quantitative analysis of BP86/TZP results for 2 and 3 shows that in [(PP3)FeH2] (2-Fe), theFe�Hax bond orbital is HOMO�3 with an energy of �5.30 eV,while three d lone pairs are highest in energy (HOMO toHOMO�2 with energies of �3.99 to �4.40 eV). This pattern oforbital energies is conserved in the case of 2-Os: HOMO andHOMO�3 lie at �4.36 and �5.07 eV. In 2-Ru the sRuHax

orbital isHOMO�2 (�4.90 eV), while HOMO�3 is a d lone pair(�4.98 eV; Figure 10). This orbital shift is probably a cause ofaperiodic trends observed for calculated dihydrogen-bond pa-rameters and may be responsible for aperiodic change in hy-dride reactivity.[24, 73] The orbital energy pattern found for 2 can

also explain the lowest stability of [(PP3)RuH(h2-H2)]+ in theseries due to weaker back-donation properties of the rutheni-um d lone pairs relative to Fe and Os.[74–76]

In [Cp*FeH(dppe)] (3-Fe), the metal d lone pairs are still thehighest occupied orbitals (�3.63 to �4.27 eV,) whereas thesFeH orbital is HOMO�5 with an energy of �6.04 eV(Figure 10).[77] This big energy gap (EHOMO�EHOMO�5 = 2.41 eV)could lead to more preferable bonding of the HOR proton tothe d orbital of iron in the case of [Cp*FeH(dppe)] , but 3d orbi-tals are relatively compact to compete successfully with sMH inbinding with HOR (Figure 11). Thus, it leads only to slight dis-

tortion of the H···HO interaction in syn-3-Fe·TFE in comparisonto 2-Fe·TFE or 1·TFE. A similar energetic pattern is revealed for[(PP3)OsH2]/[Cp*OsH(dppe)] , despite even lower sMH energy(�6.86 eV) and corresponding increase of the HOMO–HOMO�5 energy gap from 2.41 to 2.92 eV on going from Feto Os in [Cp*MH(dppe)] (Figure 10).

Osmium 5d orbitals are much more diffuse than Fe 3d orbi-tals, and this could be another factor which promotes thenOs:!s�OH interaction. Orbital energies of [Cp*RuH(dppe)] arevery close to those of [Cp*OsH(dppe)] (EHOMO�5 =�6.73,EHOMO�EHOMO�5 = 2.74 eV for 3-Ru), but the smaller size of the4d orbital leads to decreased impact of M···Hd+ in [Cp*Ru-(dppe)H]···HORF complex.

The results of quantitative Kohn–Sham MO analysis are inline with the NBO analysis presented above. The main parame-ters describing the interaction between the proton-donor andproton-acceptor fragments (fragment orbital energies, over-laps) are collected in Table 4. In these large organometallic hy-drides, the orbital interaction is more complex than in previ-ously analyzed main group hydrides such as LiH or BH4

� ,[64] be-cause more extensive orbital mixing occurs. Still, this analysisshows the increase of the metal lone pair–s�AH overlap on de-scending the group with approximately constant impact ofsMH. The values of DEoi are in agreement with the fragment or-

Figure 9. Schematic orbital-interaction diagram for a) classical hydrogenbond, b) dihydrogen bond with a main group hydride, and c) dihydrogenbond with a transition metal hydride.

Figure 10. Molecular orbital energy plot for 2 (HOMO–HOMO�4) and 3(HOMO–HOMO�2 and HOMO�5). sMHax

in 2 and sMH orbitals in 3 (a),sMHeq

(g), metal d lone pairs (c).

Figure 11. Molecular orbitals in 3-Fe hydrogen-bonded complexes (as isosur-face at 0.08958).

&8& www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 12

�� These are not the final page numbers!

A. Lledos, E. S. Shubina et al.

bital overlaps and energies (Table 4). Thus, in the presence ofa suitable transition metal fragment the orbital-interaction dia-gram (Figure 9 b) should be modified due to the presence ofan additional donor orbital, which is a metal lone pair. In sucha case the two-orbital, two-electron interaction turns intoa three-orbital, four-electron interaction (Figure 9 c). This occu-pied d orbital is in the same region as the M�H bond, butbent with respect to the M�H axis. In this way, simultaneousinteraction of the s�AH orbital with both occupied orbitals re-quires a nonlinear MH···HA arrangement.

4. Conclusions

The arrangement in which two basic centers—transition metalatom and hydridic hydrogen atom—are bound togethermakes transition metal hydride complexes unique proton ac-ceptors. The two centers’ working together leads to bifurcatehydrogen-bonded complexes with proton donors in which theproton interacts simultaneously with the hydride ligand andthe transition metal atom. To the best of our knowledge, this isin contrast to the hydrogen-bonding pattern of organic com-pounds, wherein bifurcate interactions involve two neighbor-ing basic centers not directly bonded but separated bya spacer atom(s) or belonging to different molecules.[13, 66]

Some of the well-established ideas on hydrogen-bonding inter-actions, such as the relationship between linearity and interac-tion strength or the bond-order concept assuming that the va-lence of the proton remains constant in the transfer processfrom donor to acceptor,[52, 78, 79] should be applied with care tometal hydride complexes, including the metal–proton interac-tion described in this paper.

Classical hydrogen bonds are described as three-center,four-electron (3c–4e) systems. From this point of view, the di-hydrogen bond is also unique phenomenon which should bedescribed as a 4c–4e interaction, but the majority of its fea-tures fulfil the hydrogen-bond definition. Application of thisterminology to bifurcate hydrogen bonds involving organicbases (Scheme 1 b) leads to 4c–6e bonding.[13] The MH···HAbonds considered in this work (Scheme 1 a) are also 4c–6e in-teractions, and this is yet another reason to call them bifurcatehydrogen bonds.

The balance of energies of d lone pairs and sMH orbitals, aswell as a diffuseness and an approachability of metal d orbitals,determine the balance of Md�···Hd+ and Hd�···Hd+ interactionsin a hydrogen-bonded complex. Increasing Md�···Hd+ interac-tion leads to distortion of the dihydrogen-bond geometry (de-viation of Hd�···Hd+�O moiety from linearity) with the bendingof the hydrogen-bond path (increase in ellipticity). Simultane-ous decrease of the Hd�···Hd+ interaction, a salient feature ofthe [Cp*MH(dppe)] series, counterbalances the increase in hy-dride basicity on descending the group. Moreover, we suggestthat the balance of the two interactions determines the natureof the kinetic product of proton transfer. Increased metal par-ticipation changes this from nonclassical M(h2-H2) for Fe andRu to classical cis-M(H)2 for Os in 3, as found both theoreticallyand experimentally.[22, 25, 33, 80] The increased energy of the

s(M�Hax) bond orbital in 2-Ru can explain the highest ability ofthis complex to undergo protonation within the series.[24]

Thus, possible consequences of the bifurcated nature of di-hydrogen bonds are greater activation of an H�A bond andlower activation barrier for proton transfer. However, the in-creased Md�···Hd+ interaction could ultimately lead to directformation of a classical protonation product having lower ki-netic and thermodynamic acidity, and thus affect the catalyticactivity of such species.

Computational Details

Calculations were performed with the Gaussian 09[81] package atthe DFT/B3PW91 level.[82–84] Phenyl rings of dppe and PP3 ligandswere replaced by hydrogen atoms in the model complexes (lead-ing to the dpe and PPH

3 ligands). This approach was successfullyused previously by us in a computational study on [Cp*(dppe)MH](M = Fe, Ru, Os) hydrogen-bonded complexes and was now ex-tended to [Cp2NbH3] and [(PP3)MH2] (M = Fe, Ru, Os).

Effective core potentials (ECP) were used to represent the inner-most electrons of the niobium, osmium, ruthenium, and ironatoms, as well as the electron core of the phosphorus atoms.[85, 86]

The basis set for the Nb, Os, Ru, Fe, and P atoms was that associat-ed with the pseudopotential,[85, 86] with a standard double-zLANL2DZ contraction,[81] supplemented in the case of P with a setof d-polarization functions.[87] The carbon and hydrogen atoms ofthe Cp, Cp* PPH

3, and dpe ligands together with the atoms of theproton donor molecules (C, F, H) that are not involved in hydrogenbonds were described with a 6-31G basis set.[88] The hydridic hy-drogen atom and the hydrogen and oxygen atoms of the protondonor molecules involved in hydrogen bonding were describedwith a 6-31G(d,p) set of basis functions.[89]

The structures of the reactants and hydrogen bonded complexeswere fully optimized with this basis set without any symmetry re-strictions. Natural atomic charges and Wiberg bond indices[90] (WBI)were calculated by using the natural bond orbital (NBO) analysis[42]

option as incorporated in Gaussian 09.

Topological analysis of the electron density distribution function1(r) was performed by using the AIMAll program package[91] basedon the wave function obtained by B3PW91 calculations. The AIMextended wave function format allows QTAIM analyses of molecu-lar systems containing heavy atoms described with ECP. Electrondelocalization index (DI) is the average number of electrons delo-calized (shared) between two atoms A and B, which becomesa bond index when A and B are connected by a bond path.[92–94]

The DI between two atoms was obtained by integration of the ex-change-correlation density over the atomic domains of theseatoms. The energy of the hydrogen-bonding interaction was esti-mated by using the correlation between the energy of the contact(Econt) and the value of the potential energy density function V(r) atthe (3, �1) critical point: Econt = 1/2 V(r).[95, 96] Hydrogen-bond ellip-ticity eHH was defined as e = (l1/l2�1), where l1 and l2 are the neg-ative eigenvalues of the Hessian of the electron density at thebond critical point ordered such that l1<l2<0.

The energy decomposition and Kohn–Sham MO analysis[97, 98] werecarried out by means of the ADF program,[99] using the BP86[83, 100]

functional and the TZP basis set.[101] Geometries of reactants andcomplexes were reoptimized at the BP86/TZP level. TZP is a basisset of Slater functions, and is of triple-x quality for all atoms withone set of polarization functions. The 1s core shells of the heavy

ChemPhysChem 0000, 00, 1 – 12 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org &9&

These are not the final page numbers! ��

Directionality of Dihydrogen Bonds

atoms were treated by the frozen-core approximation. Comparisonof B3PW91 and BP86 geometries and bonding energies for all thecomplexes studied (see Supporting Information) reveals that theyare similar, with the largest difference being about 1.7 kcal mol�1

for the energies, 0.08 � for the H···H distances, and 0.09 � for theM···H distances. This allows the results obtained by the two kindsof analysis to be compared.

Acknowledgements

Financial support from the Russian Foundation for Basic Re-search (project No. 11-03-01210-a), the Russian Federation Presi-dent grant (MK-314.2010.3,) and the Spanish MICINN (ProjectsCTQ2011-23336 and Consolider-Ingenio 2010 CSD2007-00006) isgratefully acknowledged. CESCA is acknowledged for providingcomputational resources.

Keywords: density functional calculations · hydride ligands ·hydrogen bonds · quantum chemistry · transition metals

[1] L. M. Epstein, E. S. Shubina, Coord. Chem. Rev. 2002, 231, 165 – 181.[2] L. Brammer, Dalton Trans. 2003, 3145 – 3157.[3] M. J. Calhorda, Chem. Commun. 2000, 801 – 809.[4] E. Arunan, G. R. Desiraju, R. A. Klein, J. Sadlej, S. Scheiner, I. Alkorta,

D. C. Clary, R. H. Crabtree, J. J. Dannenberg, P. Hobza, H. G. Kjaergaard,A. C. Legon, B. Mennucci, D. J. Nesbitt, Pure Appl. Chem. 2011, 83,1637 – 1641.

[5] S. J. Grabowski, Chem. Rev. 2011, 111, 2597 – 2625.[6] O. A. Filippov, A. M. Filin, V. N. Tsupreva, N. V. Belkova, A. Lled�s, G.

Ujaque, L. M. Epstein, E. S. Shubina, Inorg. Chem. 2006, 45, 3086 – 3096.[7] N. V. Belkova, E. S. Shubina, L. M. Epstein, Acc. Chem. Res. 2005, 38,

624 – 631.[8] R. H. Crabtree in Encyclopedia of Supramolecular Chemistry, Vol. 1 (Eds. :

J. L. Atwood, J. W. Steed), Marcel Dekker, New York, 2004, pp. 666 –672.

[9] N. V. Belkova, T. N. Gribanova, E. I. Gutsul, R. M. Minyaev, C. Bianchini,M. Peruzzini, F. Zanobini, E. S. Shubina, L. M. Epstein, J. Mol. Struct.2007, 844, 115 – 131.

[10] S. Rizzato, J. Berg�s, S. A. Mason, A. Albinati, J. Kozelka, Angew. Chem.2010, 122, 7602 – 7605; Angew. Chem. Int. Ed. 2010, 49, 7440 – 7443.

[11] L. R. Falvello, Angew. Chem. 2010, 122, 10243 – 10245; Angew. Chem.Int. Ed. 2010, 49, 10045 – 10047.

[12] P. Vidossich, M. A. Ortuno, G. Ujaque, A. Lledos, ChemPhysChem 2011,12, 1666 – 1668.

[13] T. Steiner, Angew. Chem. 2002, 114, 50 – 80; Angew. Chem. Int. Ed. 2002,41, 48 – 76.

[14] E. S. Shubina, A. N. Krylov, D. V. Muratov, A. A. Filchikov, L. M. Epstein,Russ. Chem. Bull. 1993, 42, 1919 – 1920.

[15] L. M. Epstein, A. N. Krylov, E. S. Shubina, J. Mol. Struct. 1994, 322, 345 –352.

[16] E. S. Shubina, A. N. Krylov, A. Z. Kreindlin, M. I. Rybinskaya, L. M. Ep-stein, J. Organomet. Chem. 1994, 465, 259 – 262.

[17] M. Besora, A. Lledos, F. Maseras, Chem. Soc. Rev. 2009, 38, 957 – 966.[18] S. S. Kristj�nsd�ttir, J. R. Norton, Transition Metal Hydrides (Ed. : A.

Dedieu), VCH, New York, 1991, pp. 309 – 359.[19] E. S. Shubina, A. N. Krylov, N. V. Belkova, L. M. Epstein, A. P. Borisov,

V. D. Mahaev, J. Organomet. Chem. 1995, 493, 275 – 277.[20] S. BolaÇo, L. Gonsalvi, P. Barbaro, A. Albinati, S. Rizzato, E. Gutsul, N.

Belkova, L. Epstein, E. Shubina, M. Peruzzini, J. Organomet. Chem.2006, 691, 629 – 637.

[21] N. V. Belkova, M. Besora, M. Baya, P. A. Dub, L. M. Epstein, A. Lled�s, R.Poli, P. O. Revin, E. Shubina, Chem. Eur. J. 2008, 14, 9921 – 9934.

[22] P. A. Dub, N. V. Belkova, O. A. Filippov, G. A. Silantyev, J.-C. Daran, L. M.Epstein, R. Poli, E. S. Shubina, Eur. J. Inorg. Chem. 2010, 1489 – 1500.

[23] A. V. Iogansen, Theor. Experim. Khim. 1971, 7, 302 – 311.

[24] E. I. Gutsul, N. V. Belkova, G. M. Babakhina, L. M. Epstein, E. S. Shubina,C. Bianchini, M. Peruzzini, F. Zanobini, Russ. Chem. Bull. 2003, 52,1204 – 1206.

[25] N. V. Belkova, P. A. Dub, M. Baya, J. Houghton, Inorg. Chim. Acta 2007,360, 149 – 162.

[26] M. Jim�nez-Tenorio, M. C. Puerta, P. Valerga, S. Moncho, G. Ujaque, A.Lledos, Inorg. Chem. 2010, 49, 6035 – 6057.

[27] G. Orlova, S. Scheiner, J. Phys. Chem. A 1998, 102, 260 – 269.[28] F. Shi, Organometallics 2006, 25, 4034 – 4037.[29] F. P. Arnold, Organometallics 1999, 18, 4800 – 4809.[30] E. V. Bakhmutova, V. I. Bakhmutov, N. V. Belkova, M. Besora, L. M. Ep-

stein, A. Lled�s, G. I. Nikonov, E. S. Shubina, J. Tomas, E. V. Vorontsov,Chem. Eur. J. 2004, 10, 661 – 671.

[31] E. Gutsul, N. Belkova, M. Sverdlov, L. Epstein, E. Shubina, V. Bakhmutov,T. Gribanova, R. Minyaev, C. Bianchini, M. Peruzzini, F. Zanobini, Chem.Eur. J. 2003, 9, 2219 – 2228.

[32] N. V. Belkova, P. O. Revin, L. M. Epstein, E. V. Vorontsov, V. I. Bakhmutov,E. S. Shubina, E. Collange, R. Poli, J. Am. Chem. Soc. 2003, 125, 11106 –11115.

[33] N. V. Belkova, E. Collange, P. Dub, L. M. Epstein, D. A. Lemenovskii, A.Lled�s, O. Maresca, F. Maseras, R. Poli, P. O. Revin, E. S. Shubina, E. V.Vorontsov, Chem. Eur. J. 2005, 11, 873 – 888.

[34] P. A. Dub, N. V. Belkova, K. A. Lyssenko, G. A. Silantyev, L. M. Epstein,E. S. Shubina, J.-C. Daran, R. Poli, Organometallics 2008, 27, 3307 –3311.

[35] P. A. Dub, N. V. Belkova, O. A. Filippov, J.-C. Daran, L. M. Epstein, A.Lled�s, E. S. Shubina, R. Poli, Chem. Eur. J. 2010, 16, 189 – 201.

[36] V. A. Levina, O. A. Filippov, E. I. Gutsul, N. V. Belkova, L. M. Epstein, A.Lledos, E. S. Shubina, J. Am. Chem. Soc. 2010, 132, 11234 – 11246.

[37] P. L. A. Popelier, J. Phys. Chem. A 1998, 102, 1873 – 1878.[38] S. J. Grabowski, J. Phys. Chem. A 2001, 105, 10739 – 10746.[39] I. Rozas, PCCP 2007, 9, 2782 – 2790.[40] O. A. Filippov, V. N. Tsupreva, L. M. Golubinskaya, A. I. Krylova, V. I. Bre-

gadze, A. Lledos, L. M. Epstein, E. S. Shubina Inorg. Chem. 2009, 48,3667 – 3678.

[41] J. Poater, X. Fradera, M. Sola, M. Duran, S. Simon, Chem. Phys. Lett.2003, 369, 248 – 255.

[42] E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A.Bohman, C. Morales, F. Weinhold, NBO 5.0, Theoretical Chemistry Insti-tute, University of Wisconsin, Madison, 2001.

[43] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899 – 926.[44] K. Morokuma, J. Chem. Phys. 1971, 55, 1236 – 1244.[45] K. Morokuma, Acc Chem. Res. 1977, 10, 294 – 300.[46] T. Ziegler, A. Rauk, Theor. Chem. Acc. 1977, 46, 1 – 10.[47] T. Ziegler, A. Rauk, Inorg. Chem. 1979, 18, 1558 – 1565.[48] T. Ziegler, A. Rauk, Inorg. Chem. 1979, 18, 1755 – 1759.[49] S. Scheiner, Hydrogen Bonding, A Theoretical Perspective, Oxford Univer-

sity Press, New York, 1997.[50] J. Ireta, J. Neugebauer, M. Scheffler, J. Phys. Chem. A 2004, 108, 5692 –

5698.[51] R. Parthasarathi, V. Subramanian, N. Sathyamurthy, J. Phys. Chem. A

2006, 110, 3349 – 3351.[52] I. Majerz, I. Olovsson, Phys. Chem. Chem. Phys. 2010, 12, 5462 – 5467.[53] X. Fradera, J. Poater, S. Simon, M. Duran, M. Sol, Theor. Chem. Acc.

2002, 108, 214 – 224.[54] M. V. Vener, A. N. Egorova, D. P. Fomin, V. G. Tsirelson, J. Phys. Org.

Chem. 2009, 22, 177 – 185.[55] R. Taylor, O. Kennard, W. Versichel, J. Am. Chem. Soc. 1984, 106, 244 –

248.[56] P. A. Wood, F. H. Allen, E. Pidcock, CrystEngComm 2009, 11, 1563 –

1571.[57] L. F. Pacios, J. Comput. Chem. 2006, 27, 1641 – 1649.[58] U. Koch, P. L. A. Popelier, J. Phys. Chem. 1995, 99, 9747 – 9754.[59] J. Cioslowski, S. T. Mixon, W. D. Edwards, J. Am. Chem. Soc. 1991, 113,

1083 – 1085.[60] I. Rozas, I. Alkorta, J. Elguero, J. Phys. Chem. A 1998, 102, 9925 – 9932.[61] F. Weinhold, C. R. Landis, Valency and Bonding, A Natural Bond Orbital

Donor-Acceptor Perspective, Cambridge University Press, New York,2005.

[62] G. Gilli, P. Gilli, The Nature of the Hydrogen Bond : Outline of a Compre-hensive Hydrogen Bond Theory, Oxford University Press, Oxford, 2009.

&10& www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 12

�� These are not the final page numbers!

A. Lledos, E. S. Shubina et al.

[63] H. Cybulski, M. Pecul, J. Sadlej, T. Helgaker, J. Chem. Phys. 2003, 119,5094 – 5104.

[64] D. Hugas, S. Simon, M. Duran, C. Fonseca Guerra, F. M. Bickelhaupt,Chem. Eur. J. 2009, 15, 5814 – 5822.

[65] S. J. Grabowski, W. A. Sokalski, J. Leszczynski, Chem. Phys. 2007, 337,68 – 76.

[66] G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford UniversityPress, New York, 1997.

[67] R. H. Crabtree, P. E. M. Siegbahn, O. Eisenstein, A. L. Rheingold, Acc.Chem. Res. 1996, 29, 348.

[68] R. H. Crabtree, The Organometallic Chemistry of the Transition Metals,Wiley, 2005.

[69] E. S. Shubina, A. N. Krylov, A. Z. Kreindlin, M. I. Rybinskaya, L. M. Ep-stein, J. Mol. Struct. 1993, 301, 1 – 5.

[70] N. V. Belkova, P. O. Revin, M. Besora, M. Baya, L. M. Epstein, A. Lled�s,R. Poli, E. S. Shubina, E. V. Vorontsov, Eur. J. Inorg. Chem. 2006, 2192 –2209.

[71] J. Andrieu, N. V. Belkova, M. Besora, E. Collange, L. M. Epstein, A.Lled�s, R. Poli, P. O. Revin, E. S. Shubina, E. V. Vorontsov, Russ. Chem.Bull. 2003, 52, 2679 – 2682.

[72] A. Rauk in Orbital Interaction Theory of Organic Chemistry, 2nd ed.,Wiley, 2001, pp. 137 – 149.

[73] K. Abdur-Rashid, T. P. Fong, B. Greaves, D. G. Gusev, J. G. Hinman, S. E.Landau, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2000, 122, 9155 –9171.

[74] J. Eckert, A. Albinati, R. P. White, C. Bianchini, M. Peruzzini, Inorg. Chem.1992, 31, 4241 – 4244.

[75] C. Bianchini, E. Farnetti, M. Graziani, M. Peruzzini, A. Polo, Organome-tallics 1993, 12, 3753 – 3761.

[76] C. Bianchini, D. Masi, M. Peruzzini, M. Casarin, C. Maccato, G. A. Rizzi,Inorg. Chem. 1997, 36, 1061 – 1069.

[77] HOMO�3 and HOMO�4 are sM�Cp* orbitals.[78] M. Ramos, I. Alkorta, J. Elguero, N. S. Golubev, G. S. Denisov, H. Bene-

dict, H.-H. Limbach, J. Phys. Chem. A 1997, 101, 9791 – 9800.[79] I. Alkorta, I. Rozas, O. Mo, M. Yanez, J. Elguero, J. Phys. Chem. A 2001,

105, 7481 – 7485.[80] M. Baya, O. Maresca, R. Poli, Y. Coppel, F. Maseras, A. Lled�s, N. V. Bel-

kova, P. A. Dub, L. M. Epstein, E. S. Shubina, Inorg. Chem. 2006, 45,10248 – 10262.

[81] Gaussian 09 (Revision B.1), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.Scuseria, M. A. Robb, J. R. Cheeseman, J. Montgomery, J. A. , T. Vreven,K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakat-suji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Strat-mann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G.Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K.Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashen-ko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-

Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B.Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc. ,Wallingford, CT, 2009.

[82] J. P. Perdew in Electronic Structure of Solids (Eds. : P. Ziesche, H. Eschrig),Akademie Verlag, Berlin, 1991, pp. 11.

[83] J. P. Perdew, Phys. Rev. B 1986, 33, 8822 – 8824.[84] A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652.[85] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270 – 283.[86] W. R. Wadt, P. J. Hay, J. Chem. Phys. 1985, 82, 284 – 298.[87] A. Hçllwarth, M. Bçhme, S. Dapprich, A. Ehlers, A. Gobbi, V. Jonas, K. F.

Kohler, R. Stegmann, A. Veldkamp, G. Frenking, Chem. Phys. Lett. 1993,208, 237 – 240.

[88] W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56, 2257 –2261.

[89] P. Hariharan, J. A. Pople, Theor. Chim. Acta. 1973, 28, 213 – 222.[90] K. Wiberg, Tetrahedron 1968, 24, 1083 – 1096.[91] T. A. Keith, AIMAll (Version 11.09.18), TK Gristmill Software, Overland

Park KS, USA, 2011 (aim.tkgristmill.com).[92] R. F. W. Bader, M. E. Stephens, J. Am. Chem. Soc. 1975, 97, 7391 – 7399.[93] R. F. W. Bader, Acc. Chem. Res. 1985, 18, 9 – 15.[94] R. F. W. Bader, Chem. Rev. 1991, 91, 893 – 928.[95] E. Espinosa, I. Alkorta, I. Rozas, J. Elguero, E. Molins, Chem. Phys. Lett.

2001, 336, 457 – 461.[96] E. Espinosa, E. Molins, C. Lecomte, Chem. Phys. Lett. 1998, 285, 170 –

173.[97] F. M. Bickelhaupt, E. J. Baerends, Rev. Comput. Chem. Vol. 15 (Eds. : K. B.

Lipkowitz, D. B. Boyd), Wiley, 2007, pp. 1 – 86.[98] G. te Velde, F. M. Bickelhaupt, E. J. Baerends, C. Fonseca Guerra, S. J. A.

van Gisbergen, J. G. Snijders, T. Ziegler, J. Comput. Chem. 2001, 22,931 – 967.

[99] E. J. Baerends, T. Ziegler, J. Autschbach, D. Bashford, A. B�rces, F. M.Bickelhaupt, C. Bo, P. M. Boerrigter, L. Cavallo, D. P. Chong, L. Deng,R. M. Dickson, D. E. Ellis, M. van Faassen, L. Fan, T. H. Fischer, C. Fonse-ca Guerra, A. Ghysels, A. Giammona, S. J. A. van Gisbergen, A. W. Gçtz,J. A. Groeneveld, O. V. Gritsenko, M. Grning, S. Gusarov, F. E. Harris, P.van den Hoek, C. R. Jacob, H. Jacobsen, L. Jensen, J. W. Kaminski, G.van Kessel, F. Kootstra, A. Kovalenko, M. V. Krykunov, E. van Lenthe,D. A. McCormack, A. Michalak, M. Mitoraj, J. Neugebauer, V. P. Nicu, L.Noodleman, V. P. Osinga, S. Patchkovskii, P. H. T. Philipsen, D. Post, C. C.Pye, W. Ravenek, J. I. Rodr�guez, P. Ros, P. R. T. Schipper, G. Schrecken-bach, J. S. Seldenthuis, M. Seth, J. G. Snijders, M. Sol, M. Swart, D.Swerhone, G. te Velde, P. Vernooijs, L. Versluis, L. Visscher, O. Visser, F.Wang, T. A. Wesolowski, E. M. van Wezenbeek, G. Wiesenekker, S. K.Wolff, T. K. Woo, A. L. Yakovlev, ADF2010, SCM, Theoretical Chemistry,Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com.

[100] A. D. Becke, Phys Rev A 1988, 38, 3098 – 3100.[101] E. Van Lenthe, E. J. Baerends, J. Comput. Chem. 2003, 24, 1142 – 1156.

Received: February 3, 2012Revised: April 19, 2012Published online on && &&, 2012

ChemPhysChem 0000, 00, 1 – 12 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org &11&

These are not the final page numbers! ��

Directionality of Dihydrogen Bonds

ARTICLES

O. A. Filippov, N. V. Belkova, L. M. Epstein,A. Lledos,* E. S. Shubina*

&& –&&

Directionality of Dihydrogen Bonds:The Role of Transition Metal Atoms Classical or bifurcate? Metal involve-

ment in dihydrogen bonding (DHB) isstudied theoretically for two series ofgroup 8 metal hydride complexes incomparison to hydride-only DHB of

[Cp2NbH3] and classical H-bonds. TheM···HA interaction increases on de-scending the group, and so the MH···HAsystem can be regarded as a bifurcateH-bond (see picture).

&12& www.chemphyschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 12

�� These are not the final page numbers!