Cyclopentadienyl Complexes Revisited

Most effective interaction is between the s/p on E and occupied a1

The interaction with p orbitals on E (e1) is secondary

The C5v structure represents a minimum for most compounds including:CpLi, CpBe+, CpE (E = B, Al, Ga, In, Tl) and CpE+ (E = Ge, Sn)

General MO diagram for CpEY

Y = coordinate bond for groups 1 and 2, a nonbonding pair in CpAl or CpSn+

Cyclopentadienyl Compounds

Cp2Si – two structures in the solid state

Divalent state is accessible for Ge and Sn and Pb – so directly form Cp2E

in the gas phase discrete monomers with the bent structure

Solid state structure of Cp2Pb - polymeric

generally air sensitive but bulky groups offer kinetic stability

bulkier R groups will give monomeric solid state structures

Cp2Pb compounds can react with Lewis bases such as bipyridine or TMEDA to make adducts – structure.

(5-C5R5)2Ge + 2NaX (X = Cl, Br)GeX2 + 2NaC5R5

Reactivity of Group 14 Alkyl Halides

Me3SiCl + NaCp give the Cp-SiMe3

Similarly, Cp*2SiBr2 can be prepared

reduction with K/anthracene leads to Cp*2Si

Structure: two isomers in the solid state

one with parallel rings Si-C range from 2.41-2.44Å

one with centroid-Si-centroid angle of 167.4 degrees

Cyclopentadienyl Complexes Revisited

General MO diagram for Cp2E

Two possible structures presented – D5d (parallel rings) and C2v (bent)

For a 14 electron system all of these orbitals are filled. In both D5d and C2v the primary interaction is stabilization of three pairs of electrons.

Note that the difference is 2a1g and 2a1 is an important difference between the two structures (due to the interaction of the element p orbital) Note that the 2a1 is the lone pair in a valence model

Cyclopentadienyl Complexes Revisited

General MO diagram for Cp2E

Two possible structures presented – D5d (parallel rings) and C2v (bent)

For a 14 electron system all of these orbitals are filled. The energy of the 2a1g (D5d) is sensitive to the E-Cp distances – it becomes more antibonding as the Cp’s get closer to E

The bent structure represents a minimum but < 10 kJ mole-1

Heavier Cp Compounds

With the exception of (Ph5C5)2M all are bent compounds (30-50 degrees) in gas phase and solution

Stereochemically active lone pair

StannoceneStannocene, Cp2Sn prepared from LiCp and SnCl2 – air sensitive, white solid m.p. 105C

Structure - 46 degree angle between the centroids

Cp*2Sn from Cp*Br and Sn or LiCp* and SnCl2 (36 degree angle)

(Ph5C5)2Sn has co-planar rings, bright yellow, low basicity suggests delocalized electrons

Reactivity

- Cp2Sn + MgCp2 → Sn(3-Cp)3(Mg(THF)6)

- protic acids cleave the Sn-Cp bond to give CpSnX

- non-coordinating borate or triflate – Cp*Sn+ cation (Cp*Sn+B(C6F5)4

- has been used as co-catalyst)

- these cations can coordinate nitrogen bases like pyridine

- Cp2Sn Lewis base adducts with BBr3, AlCl3 and AlBr3

Catenation in Heavier Group 14 Compounds

Organocompounds with (R3E)2 are quite common. In fact they are known for essentially all combinations of C, Si, Ge, Sn, and Pb.

Some longer chain and cluster compounds are known.

Polymers:

R

E

R

**

R

E

*

**

nn

Polymers show extensive -delocalization

Catenation in Heavier Group 14 Compounds

Polymers show characteristics of sigma delocalization

Near UV absorption σ --> σ* moves to longer wavelengths as polymer increases in length

Can form delocalized anions

Catenation in Heavier Group 14 Compounds

Rings and Cages:

Cyclotrisilanes, germanes, stannanes and mixed species

By reduction of R2EX2

3 R2SiCl2 + 6 Li(C10H8)Si

Si Si6 LiCl + 6C10H8

RR

RR R

R

Catenation in Heavier Group 14 Compounds

Reactivity –

source of silylene – “R2Si”- and disilene – R2Si=SiR2

The ratio of products depends on the identity of R

Si

Si Si

RR

RR R

R

Si

Si Si

RR

RR R

R

h R2Si: + R2Si=SiR2

LiLi

SiSi

SiLi

Catenation in Heavier Group 14 Compounds

Polycyclic and cluster compounds

E

E E

E

E

E E

E EE

E

E

E E

R

R

R

R

R

R

R

R

R

R

R

RR

R

RSn SnR

SnR

RSn

RSn

RSn SnR

SnR

RSn

RSn

Catenation in Heavier Group 14 Compounds

“Catorcanes”

Organometallics 1996, 15, 429.

Catenation in Heavier Group 14 Compounds

J.Organomet. Chem. 2003, 686 379.

Carbenes and Analogues

Carbenes are 6-electron carbon centers, typically -bonded to two other groups, and with two lone electrons.

These can have ground states that are singlet or triplet.

triplet singlet

C C

R

R

R

R

Heavier Group 14 Analogues

In the singlet state these compounds can be Lewis bases or Lewis acids.

When R is a “small” group these species oligomerize.

If R is large or is there is a site for intermolecular coordination to the E center (increases the coordination number up to three or four) then these species can be stabilized.

ERR