The influence of the ring substituent, R, on the substitution of carbonyl groups by phosphine ligands in [Co₂{μ-PPh₂CHCRC(O)}(μ-PPh₂)(CO) ₄] (R = H, Ph or SiMe₃)

The room-temperature reaction of the complexes [Co₂{μ-PPh₂CHCRC(O)}(μ-PPh₂)(CO) ₄] (R = H 1a, Ph 1b or SiMe₃ 1c) with 1 equivalent of PPhMe₂ gave a combination of phosphine substituted products which depends on the nature of R. When R = H 1a, two isomeric monosubstituted complexes of formula [Co₂{μ-PPh₂CHCHC(O)}(μ-PPh₂)(CO) ₃(PPhMe₂)] 2a and 3a are afforded in a ratio of approximately 1:1 in which the phosphine co-ordinates either at the cobalt atom π-co-ordinated by the carbon-carbon double bond (2a) or at the cobalt to which the PPh₂ and CO groups of the metallacyclic ligand are σ bonded (3a). When R = Ph 1b, the corresponding monosubstituted derivatives, [Co₂{μ-PPh₂CHCPhC(O)}(μ-PPh₂)(CO) ₃(PPhMe₂)] 2b and 3b are formed together with traces of the disubstituted complex [Co₂{μ-PPh₂CHCPhC(O)}(μ-PPh₂)(CO) ₂(PPhMe₂)₂] 4b. In the case of R = SiMe₃ 1c, only the monosubstituted isomer [Co₂{μ-PPh₂CHC(SiMe₃)C(O)}(μ-PPh ₂)(CO)₃(PPhMe₂)] 3c is obtained. Conversion of the monosubstituted complexes 2a, 2b and 3a, 3b to the disubstituted complexes [Co₂{μ-PPh₂CHCRC(O)}(μ-PPh₂)(CO) ₂(PPhMe₂)₂] (R = H 4a or Ph 4b) takes place on thermal reaction of respectively 2a, 3a and 2b, 3b with a further equivalent of PPhMe₂. An X-ray crystal structural determination has been performed on complex 4a and reveals a pseudo-axial/cis arrangement of the phosphine groups which are located one on each cobalt atom. The complex [Co₂{μ-PPh₂CHCPhC(O)}(μ-PPh₂)(CO) ₄] 1b reacted with the secondary phosphine PPh₂H initially in the same way as with PPhMe₂ to give isomeric species of formula [Co₂{μ-PPh₂CHCPhC(O)}(μ-PPh₂)(CO) ₃(PPh₂H)] 2d and 3d. Thermolysis of complex 2d resulted in 42% conversion to the isomeric 3d along with traces of 1b and [Co₂{μ-PPh₂CHCPhC(O)}(μ-PPh₂)(CO) ₂(PPh₂H)₂] 4d.