Metal-ligand bond strength affect

The nature of the central metal ion. In principle the redox potential of the Mre+1/re couple and the stabilization of the M" + 1 ion by the ligand R affect the metal-carbon bond strength. The stability of the transient complex is also affected by the rate of ligand exchange of the LmM"+1 R complex. 

Thermochemical measurements have been made in an attempt to establish )(Mo=Mo). The interpretation of the data is dependent upon assumptions of the relevant metal-ligand bond energies and, in view of these uncertainties, ranges of values have been quoted 592 196 kJ mol-1 for [Mo2(NMe2)6] and 310-395 kJ mol-1 for [Mo2(OPri)6].203 MO calculations have indicated that the strength of the Mo—Mo triple bond is affected significantly by the nature of the attached ligands and n donors, such as NH2) stabilize this bond. 

Jiao Y, Morris J, Brennessel WW, Jones WD. Kinetic and thermodynamic selectivity of the intermolecular C-H activation at [Tp Rh(PMe3)]. How does the ancillary ligand affect the metal-carbon bond strength J Am Chem Soc. 2013 135 16198-16212. Vetter AJ, Rieth RD, Jones WD. The activation of alkyl cyanides using a rhodiumtris(pyrazolyl)borate complex. Proc Natl Acad Sci USA. 2007 104 6957-6962. 

These results also suggest that, as the oxidation state is lowered, increasing numbers of electrons of the metal reside in essentially ligand orbitals which do not affect the M-N bond strength. 

Besides the dissociation of CO, the most common other ligand dissociation is that of a phosphine (PR3) or phosphite [P(OR)3]. There are two factors that influence the rate of dissociation of a phosphine—its donor ability and its cone angle. Both of these factors affect the M-P bond strength. Phosphines are better donors than phosphites because the P atom is more electron rich. For the same reason, alkyl phosphines are better donors than aryl phosphines (i.e., PMea > PPhs). Better donors create stronger Lewis acid-Lewisbase interactions with transition metals, and therefore dissociate slower. 

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