The homolytic bond dissociation energies of a series of five and six-coordinate mono-and dihydride complexes of the type HM-(diphosphine)2 and [H2M(diphosphine)(2)](+) (where M = Co, Rh, and Ir) are calculated and compared with experimental values. This work probes the relationship between the homolytic bond dissociation energies (HMBDEs) of these complexes in these two different coordination environments and formal oxidation states. The results of these calculations and previous experimental observations suggest that for M = Rh the HMBDE of the five coordinate HM(diphosphine)2 species are 0-2 kcal/mol larger than the HMBDE of the corresponding six-coordinate [H,M(diphosphine),](+) species. For M = Ir the bond energies of the five-and six-coordinate complexes are nearly the same and for M = Co the six-coordinate species are 1-5 kcal/mol less than the corresponding five-coordinate species. Simplified models of large and complicated ligands seem to capture the essential trends and give very good estimates of these thermodynamic properties compared with experimentally available obtain.