Structural studies on some bivalent metal complexes.

Previous investigations of the solid-state structures of (R[3]P)[2]HgX[2] and (R[3]P)HgX[2] complexes have been extended by the determination of the following crystal structures: (1) (Ph[3]P)[2]HgCl[2] (2) (Cy[3]P)HgCl[2]. The structure of (Ph[3]P)[2]HgCl[2] is that of a discrete monomer with a distorted tetrahedral geometry. In comparison with the structures of the (R[3]P)[2]HgX[2] complexes previously determined, the degree of distortion can be related to the donor strengths of the phosphine and halogen ligands. (Cy[3]P)HgCl[2] forms a structure that contains two independent chlorine bridged dimer units in each unit cell. The structure of this complex, compared with other (R3P)HgX2 complexes is determined by the donor strength and steric properties of the donor ligands. The related (R[3]P)CdX[2] complexes have been crystallographically and spectroscopically investigated. The following crystal structures have been determined: (3) (Et[3]P)CdI[2] - discrete dimer (4) alpha-(Cy[3]P)CdCl[2] - discrete tetramer (5-7) (Me[2]PhP)CdX[2] [X = Cl, Br or i] - polymers. The solid-state vibrational spectra of several (R[3]P)CdX[2] complexes havebeen studied using the crystal structures (3-7) as a basis for interpretation. The structure/spectra correlations so determined have then been applied to a number of related complexes of unknown structure. The indication from these studies is that the less extendedstructures are formed with the stronger donating R[3]P and X ligands. The solid-state structures of (R[3]P)MX[2] complexes [M = Hg, Cd or ZnJ have been rationalised in terms of the donor strengths of the R[3]P and X ligands, and the acceptor strengths of the metal atoms.In general, the less associated structures are favoured by: (1) stronger sigma-donor properties (e.g. Et[3]P> Ph[3]P); (2) stronger covalent M-X bonding (i.e. M-I> M-Br> M-Cl);(3) stronger acceptor strength of M (i.e. Hg> Cd > Zn).