Kinetic studies of polyene exchange and substitution in low valent octahedral metal complexes

Ashford, Neil F

Abstract

The kinetics of polyene exchange and substitution in (polyene)Cr(CO)3 complexes has been investigated. The exchange reaction involving displacement of complexed polyene for free polyene (Chapter 2) has shown two competing rate determining pathways to be operative; a Jigand independent pathway giving rise to the exchange product and under certain conditions small amounts of Cr(CO)6, and a ligand dependent pathway involving exclusive formation of the exchange product. Both reactions are postulated to originate from a common reaction intermediate. Structural interpretations of this intermediate are discussed in the light of theoretical molecular orbital calculations. Phosphite substitution of (polyene)Cr(CO)3 complexes to form in Chapter 3. In contrast to the exchange reactions, limiting rates at high ligand concentrations are apparent, indicating the presence of a ligand free intermediate. Such an intermediate is consistent with that postulated in the exchange reaction and enables interpretation of both exchange and substitution reactions in terms of a common mechanism under different limiting conditions. Formation of isomeric products fac-L3Cr(CO)3 and mer-L3Cr(CO)3 from the substitution reactions together with a study of the fac?mer isomerisation of these octahedral complexes are the subject of Chapter 4. Kinetic information indicates the isomerlc product formation to be a result of a preceeding fluxional five-coordinate intermediate, whilst the isomerisation reaction proceeds via an intramolecular mechanism involving a six-coordinate trigonal prismatic intermediate.

Citation

Ashford, N. F. (1987). Kinetic studies of polyene exchange and substitution in low valent octahedral metal complexes