- Kinetic and mechanistic studies of cis-Mo(CO)4L2 derivatives (L=phosphine or phosphite) are presented for the substitution reaction of one of the ligands (L) by carbon monoxide. These processes were observed to proceed by a dissociative mechanism with the rate of substitution being greatly enhanced as the size of the phosphorus ligand increases within a series of phosphine or phosphite derivatives. The cis-Mo(CO)4L2 species were found to react stereospecifically with 13CO to afford cis-Mo(CO)4(13CO)L derivatives. In addition, the [Mo(CO)4L] intermediate produced by phosphine loss from a stereospecifically 13CO-labeled cis-Mo(CO)4L2 derivative was observed to be nonfluxional during its solution lifetime. Rate studies of phosphine dissociation in trans-Mo(CO)4(PPh3)2 are reported which demonstrate this process to be less facile when compared with the analogous process in cis-Mo(CO)4(PPh3)2. Observations on the cis trans isomerization reaction of Mo(CO)4(PPh3)2 resulting from dissociative phosphine loss are also discussed. 1979, American Chemical Society. All rights reserved.