From .sigma.- to .pi.-bonding: crystal structures of (pentamethylcyclopentadienyl)triiodostannane and dichloro(pentamethylcyclopentadienyl)stibine

Abstract

Cyclopentadienyl compounds of the tetravalent group 14 elements and of the trivalent group 15 elements are characterized by their dynamic behavior due to migration processes of the relevant ElR, fragment. As shown by experiments and calculations, the energy difference between the [sigma]-ground state and the [pi]-transition state in these molecules becomes smaller on going to the heavier elements and on exchanging halogen atoms for organic ligands. Thus, Cp compounds of tetravalent tin or lead and of trivalent antimony or bismuth bearing halogen ligands should be suitable candidates for the realization of a [pi]-ground state. In this context, we have investigated the solid-state structure of the title compounds Me5C5SnI3 (1) and Me5C5SbCl2 (2) by X-ray crystallography. It is shown that in the tin compound 1 the main-group fragment is still [sigma]-bonded, whereas [pi]-bonding is observed for the antimony compound 2. The bonding parameters for 1 and 2 are discussed with those of other tin, lead, antimony, or bismuth Cp compounds from the literature. Crystal data with Mo K[alpha]([lambda] = 0.71073 A) radiation are as follows: 1, a = 8.044 (2) A, b = 15.094 (2) A, c = 13.907 (2) A, [beta] = 102.43 (2)°, Z = 4, monoclinic, space group P21/n, Rw = 0.050; 2, a = 10.479 (2) A, b = 8.384 (2) A, c = 14.101 (3) A, [beta] = 94.50 (2)°, Z = 4, monoclinic, space group P21/a, Rw = 0.068.