Cyclopentadienyl Complexes Revisited Most effective interaction is between the s/p on E and occupied a 1 The interaction with p orbitals on E (e 1 ) is secondary The C 5v 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 Cp 2 Si – two structures in the solid state Divalent state is accessible for Ge and Sn and Pb – so directly form Cp 2 E in the gas phase discrete monomers with the bent structure Solid state structure of Cp 2 Pb - polymeric generally air sensitive but bulky groups offer kinetic stability bulkier R groups will give monomeric solid state structures Cp 2 Pb compounds can react with Lewis bases such as bipyridine or TMEDA to make adducts – structure.

Reactivity of Group 14 Alkyl Halides Me 3 SiCl + NaCp give the Cp-SiMe 3 Similarly, Cp* 2 SiBr 2 can be prepared reduction with K/anthracene leads to Cp* 2 Si Structure: two isomers in the solid state one with parallel rings Si-C range from Å one with centroid-Si-centroid angle of degrees

Cyclopentadienyl Complexes Revisited General MO diagram for Cp 2 E Two possible structures presented – D 5d (parallel rings) and C 2v (bent) For a 14 electron system all of these orbitals are filled. In both D 5d and C 2v the primary interaction is stabilization of three pairs of electrons. Note that the difference is 2a 1g and 2a 1 is an important difference between the two structures (due to the interaction of the element p orbital) Note that the 2a 1 is the lone pair in a valence model

Cyclopentadienyl Complexes Revisited General MO diagram for Cp 2 E Two possible structures presented – D 5d (parallel rings) and C 2v (bent) For a 14 electron system all of these orbitals are filled. The energy of the 2 a1g (D 5d ) 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 (Ph 5 C 5 ) 2 M all are bent compounds (30-50 degrees) in gas phase and solution Stereochemically active lone pair

Stannocene Stannocene, Cp 2 Sn prepared from LiCp and SnCl 2 – air sensitive, white solid m.p. 105C Structure - 46 degree angle between the centroids Cp * 2 Sn from Cp * Br and Sn or LiCp * and SnCl 2 (36 degree angle) (Ph 5 C 5 ) 2 Sn has co-planar rings, bright yellow, low basicity suggests delocalized electrons Reactivity - Cp 2 Sn + MgCp 2 → 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(C 6 F 5 ) 4 - has been used as co-catalyst) - these cations can coordinate nitrogen bases like pyridine - Cp 2 Sn Lewis base adducts with BBr 3, AlCl 3 and AlBr 3

Catenation in Heavier Group 14 Compounds Organocompounds with (R 3 E) 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  -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 R 2 EX 2

Catenation in Heavier Group 14 Compounds Reactivity – source of silylene – “R 2 Si”- and disilene – R 2 Si=SiR 2 The ratio of products depends on the identity of R

Catenation in Heavier Group 14 Compounds Polycyclic and cluster compounds

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

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

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. tripletsinglet

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.