Molecular structure and binding energies of monosubstituted hexacarbonyls of chromium, molybdenum, and tungsten: Relativistic density functional study

— —< <

Molecular Structure and BindingEnergies of MonosubstitutedHexacarbonyls of Chromium,Molybdenum, and Tungsten:Relativistic Density Functional Study

¨CHRISTOPH VAN WULLENLehrstuhl fur Theoretische Chemie, Ruhr-Universitat, D-47780 Bochum, Germany¨ ¨

Received 28 January 1997; accepted 17 July 1997

ABSTRACT: Relativistic density functional calculations have been carried outŽ . Žfor the group VI transition metal carbonyls M CO L M s Cr, Mo, W; L s OH ,5 2

Ž . q.NH , PH , PMe , N , CO, OC isocarbonyl , CS, CH , CF , CCl , NO . The3 3 3 2 2 2 2optimized molecular structures and M—L bond dissociation energies, as well asthe metal]carbonyl bond energy of the trans CO group, have been calculated.Besides the marked dependence of the trans M—CO bond length on the type ofligand L, such an effect on the that bond energy is also observed. For thechromium compounds, the trans Cr—CO bond length varies from 184 to 199pm and its bond energy from 242 to 150 kJrmol. For the molybdenumcompounds, the range is 197 to 216 pm and 253 to 128 kJrmol and, for tungsten,198 to 214 pm and 293 to 159 kJrmol. The observed trends can be explainedwith the p acceptor strength of the L ligand. Q 1997 John Wiley & Sons, Inc.J Comput Chem 18: 1985]1992, 1997

Keywords: density functional; relativistic; calculation; transition metal;carbonyl; trans effect

Introduction

he hexacarbonyls of chromium, molybde-T num, and tungsten may be regarded as pro-totypes of low-valent donor]acceptor complexesformed by these metals. For the oxidation state 0,

Correspondence to: C. van Wullen¨

Žthe 18-electron rule requires for monodentate lig-.ands a sixfold coordination of the central metal

atom, which then usually has an octahedral envi-ronment. In the same way as the hexacarbonylscan be viewed as prototypes of a large class oftransition metal compounds, the substitution reac-tion:

Ž . Ž .M CO q L ª M CO L q CO6 5

( )Journal of Computational Chemistry, Vol. 18, No. 16, 1985]1992 1997Q 1997 John Wiley & Sons, Inc. CCC 0192-8651 / 97 / 161985-08

¨VAN WULLEN

might be considered as an important model forchemical reactions involving such molecules.Therefore, the molecular structure and the stabilityof the monosubstituted hexacarbonyls deservesome attention. Moreover, these data depend sen-sitively on the type of ligand L and can thus giveinformation on the properties of the ligand.

The structure and the first metal]carbonyl bonddissociation energy of the hexacarbonyls ofchromium, molybdenum, and tungsten are wellknown from experimentation. However, few suchdata are available for the monosubstituted hex-acarbonyls of this study. In the case of the pen-tacarbonyl thiocarbonyls, bond dissociation ener-gies obtained by mass spectroscopy are available,1

whereas the attempts to determine molecularstructure by X-ray diffraction have failed due to

Žstructural disorder between the CO and CS. 2groups in the solid state. In the case of

Ž .Cr CO PH , a complete X-ray analysis was also5 3prevented by the same problem.3 For the trimeth-ylphosphines, experimental structural data areavailable.4 ] 6 Some of the dinitrogen complexescovered in the present work have been studied inlow-temperature matrices.7, 8 Combining UV pho-tolysis and vibrational spectroscopy, it was possi-ble to elucidate general patterns of the molecularstructure, but no information on bond lengthscould be obtained this way. For a related com-pound, the dinitrogen binding energy could beestimated9 through the kinetics of the decay of thematrix-isolated species.

In recent years, quantum chemical calculationsfor transition metal compounds have reached alevel of accuracy that enables them to provideuseful data. It is possible to determine molecularstructure and binding energies in cases where ex-

Žperimental numbers are difficult or even practi-.cally impossible to get. Morever, the results of an

a priori calculation of the structure and stability ofyet unknown compounds is valuable informationonly theory can provide.

A critical evaluation of the results of Kohn]Sham density functional calculations shows thatthis method is generally able to yield accuratemolecular structures, and to predict binding ener-gies at least semiquantitatively. The necessary pre-requisites are the use of a gradient-correctedŽ .‘‘nonlocal’’ exchange-correlation functional andthe consideration of relativistic effects, which be-come increasingly important if 4d or 5d transitionmetal atoms are present in the complex. For re-views see refs. 10 and 11.

( )FIGURE 1. Cis and trans CO ligands in M CO L.5

In this study, results from Kohn]Sham densityŽfunctional calculations with relativistic corrections

12 .as proposed recently are presented for a seriesof monosubstituted hexacarbonyls of the group VImetals Cr, Mo, and W. The results include the

Ž .molecular structures of compounds M CO L for5various ligands L, namely for OH , NH , PH ,2 3 3

Ž .PMe , N , CO, OC isocarbonyl , CS, CH , CF ,3 2 2 2CCl , and NOq, as well as for unsaturated 16-elec-2tron pentacarbonyls. The M—L binding energy isevaluated as well as the binding energy of the

Ž . Ž .trans axial metal]carbonyl bond see Fig. 1 ,the latter depending strongly on the type of theligand L.

Some of these data, namely the M—L bondlength and the M—L bond energy for the diatomicligands N , CO, CS, and NOq, have been pub-2lished elsewhere13 and have been compared withcalculations performed by other investigators or,where possible, with experimental results. Thesecomparisons have established that the perfor-mance of the present method is satisfactory. In thiswork, such additional comparisons will be madefor the phosphine complexes, which have beenwell studied experimentally, and for the methy-lene complexes, for which some recent computa-tional results can be found in the literature.

Computational Method

The calculations have been performed with thepresent investigator’s implementation of aKohn]Sham density functional program,14 in

Žwhich first-order relativistic corrections by direct. 12perturbation theory have been built in. Compu-

tational details can be found in ref. 13. Briefly,DZP basis sets have been used for the ligandatoms, while the transition metal atoms carry fairlylarge basis sets. As compared to ref. 13, the basis

VOL. 18, NO. 161986

MONOSUBSTITUTED HEXACARBONYLS

sets for molybdenum and tungsten have been aug-mented by an additional set of p functions, takingthe next exponent of the well-tempered series. Thisaffected the results only marginally. The gradient-

Ž . 15corrected ‘‘nonlocal’’ density functional of Beckeand Perdew16 has been used in all calculations. Animportant feature of the program is its high nu-merical accuracy because the electrostatic Coulombenergy is evaluated analytically and high-qualitygrids have been used for the numerical integrationof the exchange-correlation energy.

The binding energies have been corrected forŽ .the basis set superposition effect BSSE . The major

contribution to the BSSE is the energy lowering ofthe dissociating ligand due to the basis functionslocated at the larger fragment. The BSSE correctionis important as it can affect the bond dissociationenergy by as much as 30 kJrmol in the case of theCF ligand, which is an extreme case in this re-2spect; typical values of the BSSE correction areabout 15 kJrmol for ligand basis sets used in thisstudy. Differences in zero-point vibrational energyand thermal corrections to the binding energy tendto cancel each other17 and have not been evalu-ated.

The hexacarbonyls have been optimized withinO symmetry. A C symmetry has been assumedh 4v

Ž .for the pentacarbonyls and the M CO L com-5plexes with diatomic ligands. For the unsaturated

pentacarbonyls, one might also consider a D3hŽ .trigonal bipyramidal structure and, for the dini-trogen complexes, side-on coordination might becompetitive. In these two cases, however, a C4v

structure has been deduced experimentally7, 18

through the analysis of the vibrational spectra ofthe matrix-isolated compounds. C symmetry re-sstrictions have been imposed for the optimizationof the structure of the phosphine and ammonia

Žcomplexes, and staggered C structures implying2 vw x .equivalent equatorial cis carbonyl ligands have

Žbeen assumed for the carbene complexes L s.CH , CF , CCl and the water complex. For all2 2 2

molecules studied, the angles between themetal]ligand bonds were close to those found in a

Ž .regular octahedron. In the case of Cr CO CH ,5 2the effect of a rotation of the methylene group hasbeen looked at by performing a calculation for the

Ž .eclipsed C structure as well see Table I . There2 vŽ .is almost no effect on the average equatorial

metal]carbonyl bond lengths. The other bondlengths change slightly, and binding energies arehardly affected by methylene rotation.

Results and Discussion

Tables I to III contain the results of the calcula-tions on chromium, the molybdenum, and tung-

TABLE I.( )Results for Chromium Complexes Cr CO L.5

( ) ( ) ( ) ( ) ( )L R Cr—L R Cr—C R Cr—C D Cr—L D Cr—COax eq e e a x[ ] [ ] [ ] [ ] [ ]pm pm pm kJ / mol kJ / mol

e — 183.0 190.3 — —OH 218.5 184.4 189.9 70 2422OC 214.0 184.9 190.5 30 230NH 220.0 185.5 189.6 121 2333PH 235.9 186.9 189.5 129 2133PMe 239.1 187.0 188.9 164 2143N 196.1 187.7 190.7 94 2052CO 190.5 190.5 190.5 183 183

aCF 193.7 191.1 190.3 211 1772aCCl 194.2 192.0 190.7 223 1662

CS 186.9 193.0 190.7 249 167a bCH 191.4 193.5 190.6 344 1642c bCH 192.1 193.1 190.7 341 1662

+NO 174.9 199.4 194.8 431 150

a Staggered C conformation.2vb This bond energy refers to the dissociation of singlet methylene. The singlet]triplet splitting of methylene calculated by thepresent method, 72 kJ / mol, has to be subtracted to yield the energy of the dissociation of triplet methylene.c Eclipsed C conformation.2v

JOURNAL OF COMPUTATIONAL CHEMISTRY 1987

¨VAN WULLEN

TABLE II.( )Results for Molybdenum Complexes Mo CO L.5

( ) ( ) ( ) ( ) ( )L R Mo—L R Mo—C R Mo—C D Mo—L D Mo—COax eq e e ax[ ] [ ] [ ] [ ] [ ]pm pm pm kJ / mol kJ / mol


aCF 209.1 207.5 206.3 191 1532aCCl 209.0 208.4 206.7 205 1422

CS 202.4 209.5 206.8 228 142a bCH 205.0 211.7 206.8 321 1282

+NO 188.8 215.7 210.2 426 140

a Staggered C conformation.2vb ( )Dissociation to singlet methylene see footnote b in Table I .

Ž .sten compounds. The first line in each table L s e

reports the molecular structure of the unsaturatedpentacarbonyl. Whereas the equatorial metal]car-bon bond length of the three pentacarbonyls issimilar to the metal]carbon bond length in the

Žcorresponding hexacarbonyl see L s CO in Tables. Ž .I]III , the trans axial bond length is substantially

shorter, by 7 to 10 pm. This effect has previouslyŽbeen documented from MP2 second-order

. 17Moller]Plesset calculations. As an aside, the ne-

glect of the geometry relaxation of the pentacar-bonyl fragment was the source of a systematicerror in bond energy calculations from the lastdecade,19 when full geometry optimizations at arelativistic level of theory, which are routinenow,12, 20 were not yet possible.

Now, if one looks at the molecular structure ofthe monosubstituted hexacarbonyls, one finds thesame pattern: Whereas the equatorial cismetal]carbonyl bond length is hardly affected

TABLE III.( )Results for Tungsten Complexes W CO L.5

( ) ( ) ( ) ( ) ( )L R W—L R W—C R W—C D W—L D W—COax eq e e a x[ ] [ ] [ ] [ ] [ ]pm pm pm kJ / mol kJ / mol


aCF 209.3 206.6 205.9 224 1842aCCl 209.1 207.4 206.2 239 1732

CS 202.5 208.5 206.3 264 173a bCH 205.4 210.0 206.3 362 1592

+NO 188.7 214.4 209.3 456 165

a Staggered C conformation.2vb ( )Dissociation to singlet methylene see footnote b in Table I .

VOL. 18, NO. 161988


Župon substitution with the exception of L sq. Ž .NO , the axial trans bond length depends

strongly on the substituting ligand L. The axialbond length varies from 184 to 199 pm for thechromium compounds, from 197 to 216 pm formolybdenum, and from 198 to 214 pm for tung-sten. Ehlers and coworkers21 have also found asimilar effect in their MP2 calculations, and theyestablished a correlation between the axialmetal]carbonyl bond length and the amount ofp-backbonding toward the ligand L, which theyevaluated using charge decomposition analysisŽ . 22CDA . This ‘‘trans effect’’ had actually beenwell known23, 24; a good p acceptor reduces theamount of backbonding to the other ligands, andthis effect is stronger on the trans ligand because apair of trans ligands share two metal d orbitals ofappropriate symmetry, whereas only one such or-bital is shared by a pair of cis ligands. A significanteffect of the substituting ligand L on the cis met-al]carbonyl bond length can only be observed inthe case of the nitrosyl cation ligand. This may notonly be attributed to the p acidity of this ligand,but also to the fact that it is positively charged.

It should be noted that this effect manifestsitself in the CsO bond lengths as well: A reduc-tion of backbonding to the trans carbonyl ligand

Ž .strengthens and shortens its CsO bond. In thepresent calculations, the axial CsO bond lengthfor the pentacarbonyl phosphines, pentacarbonylisocarbonyls, and unsaturated pentacarbonyls was0.5 to 1 pm longer than in the corresponding hex-acarbonyl, whereas this bond length was 1.5 pmshorter in the case of the nitrosyl cation complexes,where it was even slightly shorter than in freecarbon monoxide. Furthermore, the trans effect hasan impact on the interaction of the CsO stretchingmodes in metal carbonyls because stretching a COligand makes it a better p acceptor. These ruleshave already been formulated in Kraihanzel andCotton’s classical study 25 on the analysis of vibra-tional spectra of metal carbonyls. For the samereason, the A CsO stretching mode of the hex-1 gacarbonyls, where all CsO bonds stretch simulta-neously, has the highest frequency.26

The isocarbonyl complexes in Tables I to III aresomewhat hypothetical, but this ligand has beenincluded because pentacarbonyl isocarbonyl spe-cies have been discussed—but experimentally ex-cluded—as a possible product after photolysis ofthe corresponding hexacarbonyl in a low-tempera-ture matrix.18, 27 The present calculations predictthat the isocarbonyl binding energy is much lowerthan for the dinitrogen ligand. Because very low

Ž .temperature matrices at ; 20 K were required toobserve the dinitrogen complexes,7 the computa-tional result is consistent with the fact that noisocarbonyl species could be observed. Themetal]ligand bond energy for water, which hasthe lowest p acidity of all ligands studied here, isalso predicted to be lower than for the dinitrogenligand.

In addition to the trans effect on bond lengths,there is also such an effect on the trans metal]car-bonyl bond energies. This problem has only beenaddressed rarely from the theoretical perspec-tive.24, 28 The binding energies in the last columnsof Tables I to III have been obtained after a com-

Ž .plete geometry optimization of the M CO L frag-4Žments with the four carbonyls in the basal posi-

.tions of a square pyramid and corrected for theBSSE. Note that all entries within these columnsrefer to a metal]carbonyl binding energy, so thevariation is quite large: chromium]carbonyl bind-ing energies range from 242 down to 150 kJrmol,and the range is 253 to 128 kJrmol for the molyb-denum and 293 to 159 kJrmol for the tungstencompounds. The trend in the axial metal]carbonylbinding energy is not unexpected and follows thetrend for the the bond length: ligands L, which arestrong p acceptors, tend to decrease the backbond-ing to the trans carbonyl ligand. Therefore, thetrans metal]carbonyl bond length increases and itsbond energy decreases. For weak p acceptors, thesituation is reversed. For molybdenum and tung-sten, to remove the trans CO group is more diffi-

Ž . q Ž .cult for M CO NO than for M CO CH , al-5 5 2though the trans metal]carbon bond length ismuch longer for the nitrosyl complexes. This mightbe caused by NOq being a charged ligand, leadingto an extra destabilization of the unsaturated frag-

Ž . qment M CO NO .4Many experimental data are available for pen-

tacarbonyl phosphine complexes. Table IV con-tains a compilation of the present results togetherwith experimental data for pentacarbonyl phos-

Ž .phines where available and pentacarbonyltrimethyl phosphines. A complete set of data isavailable for the trimethyl phosphines, whereas, in

Ž .the case of parent phosphines, an X-ray structureis available only for the chromium compound,3

and even this one suffers from structural disorder.The metal]phosphorus bond lengths obtained inthis work are longer than for the unsubstitutedphosphines and are too long, by 2 to 4 pm, com-pared with the experimental values. If the calcula-tion is repeated with two sets of polarization func-

Žtions on the phosphorus atoms e.g., with expo-


¨VAN WULLEN

TABLE IV.Present Results and Experimental Data for Phosphine Complexes.

( ) ( ) ( )R M—PR R M—C R M—C3 ax eq[ ] [ ] [ ]Compound pm pm pm Source

( )Cr CO PH 235.9 186.9 189.5 This work5 3235.0 Ref. 3

( )Cr CO PMe 239.1 187.0 188.9 This work5 3236.6 185.0 189.3 Ref. 4

( )Mo CO PH 252.7 201.7 205.5 This work5 3( )Mo CO PMe 254.9 202.2 205.1 This work5 3

250.8 198.4 203.6 Ref. 5( )W CO PH 251.8 201.5 205.1 This work5 3( )W CO PMe 254.2 202.0 204.8 This work5 e

251.6 200.0 201.0 Ref. 6

nents such as those used by Pacchioni and28 .Bagus , the metal]phosphorus bond distance

shrinks by only 0.5 pm. The trans metal]carbonylbond lengths are also shorter than those foundexperimentally. However, the latter are sometimesdifficult to obtain experimentally. Note that the

Ž . 6experimental structure for W CO PMe reports5 3a rather small difference between the axial

Ž .and average equatorial tungsten]carbonyl bondlength, which does not match the pattern observedfor the other compounds. The investigators sug-gested that the experimental uncertainties mightbe larger than the differences in the W—C bondlengths. This seems to be the case indeed, becauseproblems in the experimental structure determina-tion are indicated by the large differences between

Žthe individual equatorial W—C bond lengths with.range from 198 to 203 pm and between the equa-Ž .torial C—O bond lengths from 113 to 119 pm .

According to the calculations, these differencesshould be much smaller. The calculated transmetal]carbonyl bond lengths indicate that the pacidity of trimethylphosphine is not so much largerthan the p acidity of the unsubstituted phosphineligand. It may be conjectured that the bond energyof the metal]trimethylphosphine bond is higherbecause it is a better s donor. The P—R bonddistances in the PR ligand are shorter in the3

wcomplex than in the free ligand PH : 144.1 pm,3Ž .M CO PH : 143.3 pm; PMe : 187.0 pm,5 3 3Ž . xM CO PMe : 185.0 pm . If a phosphine ligand5 3

approaches the metal center from an infinite dis-tance, the P—R bond length begins to shrink al-ready at a large metal]phosphorus distance. This

Ž .is probably the consequence of changing an s-typelone pair into a metal]phosphorus bond. The over-all shortening of the P—R bond therefore gives

little insight into the amount of backbonding. Onthe other hand, a small variation of the metal]phosphorus bond length around its equilibriumvalue induces an opposite variation of the P—Rbond length. This is consistent with assumingbackbonding into the P—R s U orbitals of theligand. A detailed investigation of the electronicproperties of phosphine ligands is beyond the scopeof this study and will be given elsewhere.29

Let us now have a closer look at the results forthe carbene complexes. In Table V, the metal]car-bene bond lengths, the axial metal]carbonyl bondlengths, and the metal]methylene binding ener-gies obtained in the present work and from othercalculations can be found. Density functional re-

Ž . Ž .sults for Cr CO CF and Cr CO CCl were re-5 2 5 2cently published,30 but only at the LDA level andare thus of no use for comparison with the presentdata. For the fluorocarbenes, MP2 data from Ehlersand coworkers21 are available, but the bindingenergy could not be obtained at the coupled clus-ter level at that time due to program limitations;however, an MP2 estimate using isostructural re-actions is available.

The amount of backbonding to the methyleneŽ .CH ligand in the calculation of Ehlers and co-2

workers21 was rather low for the chromium com-Ž .plex i.e., lower than in chromium hexacarbonyl .

This result is unexpected, but somehow consistentwith their finding that the axial Cr—CO bond

Ž .length in Cr CO CH should be shorter than in5 2chromium hexacarbonyl. However, intuition sug-gests that methylene is a stronger p acceptor thancarbonyl in these complexes and that the axial Cr

Ž .—CO bond length in Cr CO CH should then be5 2longer than in chromium hexacarbonyl, as is foundin the present work and in the results of Ziegler

VOL. 18, NO. 161990


TABLE V.Selected Computational Results for Carbene Complexes.

( ) ( ) ( )R M—CR R M—C D M—CR2 ax e 2[ ] [ ] [ ]Compound pm pm kJ / mol Source

( )Cr CO CH 191.4 193.5 344 This work5 2a193.0 195.4 351 QR-DFT

b( )189.0 185.4 353 CCSD T / / MP2( )Cr CO CF 193.7 191.1 211 This work5 2

c190.4 185.6 180 MP2( )Cr CO CCl 194.2 192.0 223 This work5 2

( )Mo CO CH 205.0 211.7 321 This work5 2a206.1 213.6 335 QR-DFT

b( )202.8 219.6 353 CCSD T / / MP2( )Mo CO CF 209.1 207.5 191 This work5 2

c207.4 212.8 162 MP2( )Mo CO CCl 209.0 208.4 228 This work5 2

( )W CO CH 205.4 210.0 362 This work5 2a203.4 210.7 380 QR-DFT

b( )203.1 211.9 380 CCSD T / / MP2( )W CO CF 209.3 206.6 224 This work5 2

c205.7 208.3 199 MP2( )W CO CCl 209.1 207.4 239 This work5 2

a ‘‘Quasi-relativistic’’ density functional calculations, refs. 31 and 32. The methylene singlet]triplet splitting calculated in that work,66 kJ / mol, has been added to the published value, which refers to a dissociation into the pentacarbonyl and triplet methylene.b ( ) ( )MP2 geometries using quasi-relativistic core potentials for Mo and W and binding energies by the coupled cluster CCSD T

( )method from ref. 21 .c ( )MP2 geometries using quasi-relativistic core potentials for Mo and W and MP2 binding energies estimated from isostructural

( )reactions from ref. 21

31, 32 Žet al. their value for chromium hexacarbonyl20 .is 191.0 pm . The metal]ligand bond lengths of

the MP2 structures of first-row transition metalcompounds are notoriously too short,21 but in thiscase there is little doubt that the MP2 calculationsfail to reproduce correctly the trend in the bondlength.

For the molybdenum and tungsten methylenecomplexes, all calculations agree that the transmetal]carbonyl bond length should be longer thanin the corresponding hexacarbonyl. Compared withthe results of the present work and to the quasi-relativistic density functional results of ref. 32, theelongation of the trans metal]carbonyl bond in

Ž .Mo CO CH is probably overestimated in the5 2MP2 calculations. The same seems to be the case

Ž .for Mo CO CF and has also been observed for5 2Ž . q 13the nitrosyl complex Mo CO NO .5

All calculations predict similar bond dissocia-tion energies for the methylene complexes. Thevalues obtained in the present work are consis-tently lower than in the other calculations. Thismay be attributed to the BSSE correction, which

has not been considered in other work. The BSSEcorrection leads to a reduction in binding energiesby 15 to 20 kJrmol for the methylene bindingenergies, but is probably somewhat smaller if bet-ter ligand basis sets are used.33 For all three met-als, the metal—CF bond dissociation energy ob-2tained in this work is ; 30 kJrmol higher thanthe MP2 result of Ehlers et al.21 Because of thisdiscrepancy, it is suggested that these investiga-

Ž .tors soon evaluate these energies at the CCSD Tlevel.

What is the effect if one replaces methylenewith the halomethylenes CF or CCl ? Because of2 2the interaction of the halogen lone pairs with theempty p-orbital at the carbene center, the latterbecomes less available for accepting p-backbond-ing. The availability of the empty p orbital can be

Ž .monitored in the isolated singlet carbene by look-ing at the NMR chemical shift of the carbon atom.34

The absolute magnetic shieldings of the carbonatom, as calculated with the multiconfiguration

Ž . 34IGLO MC-IGLO method, is y718 ppm forŽ .singlet CH , y101 ppm for CF , and y346 ppm2 2


¨VAN WULLEN

for CCl . Therefore, the p-acceptor strength should2be CF - CCl - CH , and the same ordering is2 2 2found for the trans metal]carbonyl bond lengthŽ .and the metal]carbene bond energy for all threemetals. These trends may be helpful for the inter-pretation of results for experimentally known car-bene complexes, where the carbene center is al-ways stabilized with respect to methylene, usuallyby a heteroatom in the a position.

Conclusion

Structural and energetic data for monosubsti-tuted hexacarbonyls of group VI metals have beenobtained for a variety of ligands. This data show a

Ž .correlation with electronic properties p acidity ofthe ligands. Using this correlation, the data ob-tained in this work can be used to assess the pacidity of other ligands, based on observable quan-tities such as bond lengths and bond dissociationenergies. Work along these lines is in progress.29

References

1. G. D. Michels, G. D. Flesch, and H. J. Svec, Inorg. Chem., 19,Ž .479 1980 .

2. J. Y. Saillard and D. Grandjean, Acta Crystallogr. B, 34, 3318Ž .1978 .

3. G. Huttner and S. Schelle, J. Organomet. Chem., 47, 383Ž .1973 .

Ž .4. K. J. Lee and T. L. Brown, Inorg. Chem., 31, 289 1992 .5. M. S. Davies, M. J. Aroney, I. E. Buys, and T. W. Hambley,

Ž .Inorg. Chem., 34, 330 1995 .6. F. A. Cotton, D. J. Darensbourgh, and B. W. S. Kolthammer,

Ž .Inorg. Chem., 20, 4440 1981 .7. J. K. Burdett, A. J. Downs, G. P. Gaskill, M. A. Graham, J. J.

Ž .Turner, and R. F. Turner, Inorg. Chem., 17, 523 1978 .

8. J. J. Turner, M. B. Simpson, M. Poliakoff, W. B. Maier Jr.,Ž .and M. A. Graham, Inorg. Chem., 22, 911 1983 .

9. J. J. Turner, M. B. Simpson, M. Poliakoff, and W. B. MaierŽ .Jr., J. Am. Chem. Soc., 105, 3898 1983 .

Ž .10. T. Ziegler, Chem. Rev., 91, 651 1991 .Ž .11. T. Ziegler, Can. J. Chem., 73, 743 1995 .

Ž .12. C. van Wullen, J. Chem. Phys., 103, 3589 1995 .¨Ž .13. C. van Wullen, J. Chem Phys., 105, 5485 1996 .¨Ž .14. C. van Wullen, Chem. Phys. Lett., 219, 8 1994 .¨

Ž .15. A. D. Becke, Phys. Rev. A, 38, 3098 1988 .Ž .16. J. P. Perdew, Phys. Rev. B, 33, 8822 1986 .

17. A. W. Ehlers and G. Frenking, J. Am. Chem. Soc., 116, 1514Ž .1994 .

Ž .18. R. N. Perutz and J. J. Turner, Inorg. Chem., 14, 262 1975 .19. T. Ziegler, V. Tschinke, and C. Ursenbach, J. Am. Chem.

Ž .Soc., 109, 4825 1987 .20. G. Schreckenbach, T. Ziegler, and J. Li, Int. J. Quant. Chem.,

Ž .56, 477 1995 .21. A. W. Ehlers, S. Dapprich, S. G. Vyboishchikow, and G.

Ž .Frenking, Organometallics, 15, 105 1996 .22. F. Basolo and R. G. Pearson, Progr. Inorg. Chem., 4, 381

Ž .1962 .23. J. D. Atwood, M. J. Wovkulich, and D. C. Sonnenberger,

Ž .Acc. Chem. Res., 16, 350 1983 .Ž .24. R. D. Davy and M. B. Hall, Inorg. Chem., 28, 3524 1989 .

25. C. S. Kraihanzel and F. A. Cotton, J. Am. Chem. Soc., 84,Ž .4432 1962 .

Ž .26. L. E. Orgel, Inorg. Chem., 1, 25 1962 .27. M. A. Graham, M. Poliakoff, and J. J. Turner, J. Chem. Soc.

Ž .A, 2939 1971 .Ž .28. G. Pacchioni and P. S. Bagus, Inorg. Chem., 31, 4391 1992 .

29. C. van Wullen, manuscript in preparation.¨Ž .30. H. Jacobsen and T. Ziegler, Organometallics, 14, 224 1995 .

31. H. Jacobsen, G. Schreckenbach, and T. Ziegler, J. Phys.Ž .Chem., 98, 11406 1994 .

Ž .32. H. Jacobsen and T. Ziegler, Inorg. Chem., 35, 775 1996 .33. A. Rosa, A. W. Ehlers, E. J. Baerends, J. G. Snijders, and G.

Ž .te Velde, J. Phys. Chem., 100, 5690 1996 .34. C. van Wullen and W. Kutzelnigg, J. Chem. Phys., 104, 2330¨

Ž .1996 .

VOL. 18, NO. 161992

Documents

Molecular structure and binding energies of monosubstituted hexacarbonyls of chromium, molybdenum, and tungsten: Relativistic density functional study