Unexpected differences in reactivity between tin and lead organyl chlorides - crystal structures of their organylphosphonium salts

Pentacoordinated tin is known since the late 1950s but little is known about the ability of lead to form similar structures. Originally we investigated the reaction between a number of tetraorganylphosphonium chlorides PR₄]⁺Cl⁻ (R=Me, Buⁿ, and Ph) and several diorganyltin dichlorides SnR^′₂Cl₂ (R^′=Me, Et, Prⁿ, Buⁿ, Ph, o-, m-, p-Tol) between 100 and 240 °C. Novel pentacoordinated tin complexes, tetraorganylphosphonium diorganyltrichlorostannates PR₄]SnR^′₂Cl₃] (1-19), were formed in good to excellent yields. In a second step, this synthetic approach was extended to include the reaction of diphenyllead dichloride Ph₂PbCl₂ with PR₄]⁺Cl⁻ (R=Buⁿ, Ph). Surprisingly, a two chloride transfer was observed to form the hexacoordinated lead species PBuⁿ₄]₂PbPh₂Cl₄] (20). Under similar conditions, the pentacoordinated PPh₄]PbPh₃Cl₂] (21) was obtained by a phenyl transfer. Complexes 20 and 21 were characterised by NMR (¹H, ¹³C, ³¹P, and ²⁰⁷Pb), IR, MS, and X-ray crystallography. The anion of 20 assumes a lightly distorted octahedral geometry with the phenyl substituents in trans-positions. In the anion of 21 the phenyl substituents occupy the equatorial positions of a lightly distorted trigonal bipyramid. A thorough spectroscopical investigation of the tin complexes 1-19, including X-ray structural studies, which were possible for complexes with R^′=aryl, revealed that these complexes are monomeric with a distorted trigonal bipyramidal SnR^′₂Cl₃]⁻ anion. Both aryl groups occupy equatorial positions.