The origin of structural variety of alkyne complexes of d8 metals. An example of structural isomerism

Abstract

The X-ray structures of three metal alkyne complexes are presented: Ir(PMe 2 Ph) 3(MeC 2Me) +, M(CO) 4(F 3CC 2CF 3) (M = Ru, Os). The iridium(I) complex adopts an unusual structure (“CS”) which differs notably from that expected for four-coordinate d 8 ML 4 square-planar complexes. The structure resembles that of a square pyramid, with essentially C s symmetry, with a short apical MP bond (2.236 vs 2.31 Å for the basal MP bonds), short MC (2.01 Å) and long CC 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 H 2. Extended Hückel calculations are presented to rationalize the structural and reactivity aspects of the whole family of d 8 ML 3(alkyne) species (M = Pt II, Ir I, Co I, Fe O). It is shown that the four-electron donation of an alkyne ligand is responsible for the unusual CS structure of certain of these ML 3(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 ML bonds of the ML 3 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.