Five-co-ordinate complexes of 9-substituted 9-phosphafluorenes with certain Group VIII transition-metal halides and cyanides

Abstract

The 9-alkyl-9-phosphafluorenes (I), alternatively named 5-alkyl-5H-dibenzophospholes (Ia; R = Me, Et), form a series of five-co-ordinate complexes of the type phos₃ MX₂(M = Ni^II, X = Cl, Br, I, NCS, CN; M = Pd^II, X = Br; M = Pt,^II X = Br, SCN; M = Co^II, X = Br), which are stable in the solid state but which undergo reversible dissociation in solution in certain organic solvents. The magnetic and spectroscopic properties of the complexes are discussed and related to those of other five-co-ordinate complexes of the above metals. The nickel(II) complexes are unusual in having magnetic moments in the range from 0·65–1·5 B.M., whereas the corresponding cobalt(II) complexes are of the normal low-spin type, having moments of 1·95 B.M.

Evidence for the structure of dicyanotris(9-methyl-9-phosphafluorene)nickel(II) has been adduced by X-ray crystallographic analysis.

Four-co-ordinate complexes of the above ligands with palladium(II) and platinum(II) halides and cyanides have also been prepared, and their structures in solution examined by ¹H n.m.r. spectroscopy. The complex phos₂ PtCl₂(phos = 9-methyl-9-phosphafluorene) has the cis-configuration in trifluoroacetic acid solution, whereas the corresponding platinum and palladium cyanide complexes have the trans-configuration.

9-Phenyl-9-phosphafluorene (I; R = Ph) resembles triphenylphosphine in forming only normal four-co-ordinate complexes with the above metal halides.

It is suggested that unusual co-ordinating ability of the 9-alkyl-9-phosphafluorenes [and of 2-phenylisophosphindoline (III) which also forms a similar series of complexes] is due to a favourable combination of both steric and electronic (donor–acceptor) properties.

Preliminary experiments show that 10-methylphenoxaphosphine (VI; R = Me) also forms similar five-co-ordinate complexes, but the 10-phenyl analogue forms only four-co-ordinate complexes.