Structural studies of mercuric halide complexes.

Previous crystaIlographic and spectroscopic data compiled for complexes of the type (L)HgX[2] and (L)[2]HgX[2] (where L= neutral unidentate ligand; X=CI,Br or I) have been critically reviewed and indicate that gross assumptions, often invalid, have been made concerning the relationships between crystal structure and vibrational spectra. In an attempt to provide a sound basis from which meaningful structure-spectra relationships could be drawn and also to improve our knowledge of the stereochemistry of mercuric halide complexes, the following crystal structures have been determined. (1) (PPh[3])HgCI[2] (Br[a], l[a]). (2) (1,2,5-triphenyIphosphoIe)HgCI[2] (Br[a]). (3) alpha-(PBu[3n])HgCI[2]. (4) (PEt[3])HgCI[2]. (5) (PMe[3])HgCI[2](Br[a]). (6) (2,4-dimethyIpyridine)HgBr[2] (Cl[a]). (7) (PEt[3])[2]HgCI[2]. (8) (PEtMe[2])[2]HgBr[2]. (9) partial analysis of (PBu[3n])[2]HgCl[2]. For the (L)HgX[2] complexes a variety of structures were observed ranging from discrete dimers (1 and 2), to a discrete tetramer (3) to varied extended structures (4,5 and 6). Discrete four-coordinate monomers, with varying degrees of distortion from a regular tetrahedral geometry, were observed for the (L)[2]HgX[2] complexes (7,8 and 9). The donor stength (which is related to both steric and electronic characteristics of the ligand) and the small size of the donor ligand appear to favour extended (L)HgX[2] structures, whilst the same parameters explain the extreme distortion of the (L)[2]HgX[2] monomers. Examination of the solid-state vibrational spectra of the above complexes and their halo-derivatives have, in spite of problems caused by internal Iigand mode interference, indicated certain structure/spectra relationships. This equivalent halo complex is isostructuraI on the basis of single crystal X-ray photographs. These relationships have been applied to a number of related complexes of unknown structure.It was significant that the ?(HgX) modes found for (L)[2]HgX[2] discrete monomers fell within the region associated with (HgX)[b] modes observed for (L)HgX[2] associated structures. Consequently, it has been shown that low wavenumber (HgX) modes need not necessarily indicate the presence of Hg-X bridge bonds.Finally, the ionic structure of (PEtMe[2])[3](HgCI[2])[2] consisting of a continuous zig-zag chain of [(PEtfMe[2])[2]HgCI][+] and [PEtMe[2]HgC[3][-] ions is the first report of a structure of this stoicheiometry.