Cyclopentadienyl Complexes Revisited

1. Cyclopentadienyl Complexes Revisited General MO diagram for CpEY Y = coordinate bond for groups 1 and 2, a nonbonding pair in CpAl or CpSn+ 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)

2. 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.

3. 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

4. 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

5. 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

6. 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

7. Stannocene Stannocene, 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(h3-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

8. 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: Polymers show extensive s-delocalization

9. 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

10. Catenation in Heavier Group 14 Compounds Rings and Cages: Cyclotrisilanes, germanes, stannanes and mixed species By reduction of R2EX2

11. 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

12. Catenation in Heavier Group 14 Compounds Polycyclic and cluster compounds

13. Catenation in Heavier Group 14 Compounds “Catorcanes” Organometallics 1996, 15, 429.

14. Catenation in Heavier Group 14 Compounds J.Organomet. Chem. 2003, 686 379.

15. triplet singlet Carbenes and Analogues Carbenes are 6-electron carbon centers, typically s-bonded to two other groups, and with two lone electrons. These can have ground states that are singlet or triplet.

16. 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.