Five-co-ordinate complexes in transition metals. Crystal and molecular structures of two forms of dicyanotris-(5-methyl-5H-dibenzophosphole)nickel(II) and of dicyanotris-(5-ethyl-5H-dibenzophosphole)nickel-(II)

Abstract

Dicyanotris-(5-methyl-5H-dibenzophosphole)nickel(II) may be isolated as two allogon isomers by crystallization from different solvents. One form (Me4) has tetragonal-pyramidal nickel bonds with one dibenzophosphole ligand at the apex. The axial nikel–phosphorus bond (2·32 Å) is significantly longer than the basal nickel–phosphorus bonds (2·18 Å). Cyanide groups, in trans-positions, are bent away from the apical ligand, the NC–Ni–CN angle being 141°. Another form (Me3) has trigonal-bipyramidal nickel bonds with axial cyanide groups, the NC–Ni–CN angle being 176°. One equatorial nickel–phosphorus bond (2·29 Å) is slightly longer than the other two (2·22 and 2·23 Å). A molecule of methanol of crystallization in this form interacts weakly with one of the cyanide groups. Dicyanotris-(5-ethyl-5H-dibenzophosphole)nickel(II), Et3, has trigonal-bipyramidal nickel bonds similar to Me3. In all three molecules two of the nearly planar dibenzophosphole ligands are arranged close to each other in a plane-to-plane arrangement which is not necessitated by steric factors. The nickel coordination is discussed in relation to other five-co-ordinate nickel complexes.