THE ORIGIN OF STRUCTURAL VARIETY OF ALKYNE COMPLEXES OF D8 METALS - AN EXAMPLE OF STRUCTURAL ISOMERISM

The X-ray structures of three metal alkyne complexes are presented: Ir(PMe2 Ph)3(MeC2Me)+, M(CO)4(F3CC2CF3) (M = Ru, Os). The iridium(I) complex adopts an unusual structure ("CS") which differs notably from that expected for four-coordinate d8 ML4 square-planar complexes. The structure resembles that of a square pyramid, with essentially Cs symmetry, with a short apical MP bond (2.236 vs 2.31 Å for the basal MP bonds), short MC (2.01 Å) and long CC bonds (1.306 Å), which indicate a strong metal-alkyne interaction. The ruthenium(O) and osmium(O) complexes adopt the expected pseudo-octahedral structure. The unusual CS structures, previously reported for cobalt(I) and iron(O) complexes, contrast with the square-planar structure of platinum(II) alkyne complexes. Fluxionality is present in the iridium complex, as well as in those of cobalt, and is attributed to a rapid intramolecular site exchange among inequivalent phosphorus ligands. The iridium complex reacts with additional ligands like alkynes and H2. Extended Hückel calculations are presented to rationalize the structural and reactivity aspects of the whole family of d8 ML3(alkyne) species (M = PtII, IrI, CoI, FeO). It is shown that the four-electron donation of an alkyne ligand is responsible for the unusual CS structure of certain of these ML3(alkyne) species. The alkyne moieties induce a four-electron destabilization in the square-planar structure which is large for diffuse d orbitals such as those of iridium(I), cobalt(I), iron(O), but not of platinum(II). The fluxionality of the complexes is shown to be associated with an easy rotation of the alkyne about the metal-alkyne midpoint, accompanied by a breathing motion of the ML bonds of the ML3 metal fragment. It is shown that unsymmetrical alkynes stabilize a conformation of the complex in which the ligand is turned by 90° with respect to the one presented here. The reactivity towards additional ligands is shown to be associated with the presence of a low-lying LUMO pointing away from the apical bond. Effects of substituents at the alkyne and of ligands at the metal are discussed. It is shown that it is residual four-electron destabilization in M(CO)4(alkyne) complexes which is responsible for the lability of CO ligands and thus for the ligand exchange process. © 1990.