The enthalpies of binding of a number of N-donor ligands to the complex Mo((PPr3)-Pr-i)(2)(CO)(3) in toluene have been determined by solution calorimetry and equilibrium measurements. The measured binding enthalpies span a range of similar to 10 kcal mol(-1): Delta H-binding=-8.8 +/- 11.2 (N-2-Mo((PPr3)-Pr-i)(2)(CO)(3)); -10.3 +/- 0.8 (N-2); -11.2 +/- 0.4 (AdN(3) (Ad=1-adamantyl)); -13.8 +/-0.5(N2CHSiMe3); -14.9 +/- 0.9 (pyrazine=pz); -14.8 +/- 0.6 (2,6-Me(2)PZ); -15.5 +/- 1.8 (Me2NCN); -16.6 +/- 0.4 (CH3CN); -17.0 +/-0.4 (pyridine); -17.5 +/- 0.8 ([4-CH(3)pz][PF6] (in tetrahydrofuran)); -17.6 +/- 0.4 (C6H5CN); -18.6 +/- 1.8 (N2CHC (=O)OEt); and -19.3 +/-2.5 kcal mol(-1) (pz)Mo((PPr3)-Pr-i)(2)(CO)(3)). The value for the isonitrile AdNC (-29.0 +/- 0.3) is 12.3 kcal mol(-1) more exothermic than that of the nitrile AdCN (-16.7 +/- 0.6 kcal mol(-1)). The enthalpies of binding of a range of arene nitrile ligands were also studied, and remarkably, most nitrile complexes were clustered within a 1 kcal mol(-1) range despite dramatic color changes and variation of v(CN). Computed structural and spectroscopic parameters for the complexes Mo ((PPr3)-Pr-i)(2)(CO)(3)L are in good agreement with experimental data. Computed binding enthalpies for Mo((PPr3)-Pr-i)(2)(CO)(3)L exhibit considerable scatter and are generally smaller compared to the experimental values, but relative agreement is reasonable. Computed enthalpies of binding using a larger basis set for Mo(PMe3)(2)(CO)(3)L show a better fit to experimental data than that for Mo((PPr3)-Pr-i)(2)(CO)(3)L using a smaller basis set. Crystal structures of Mo((PPr3)-Pr-i)(2)(CO)(3)(AdCN), W((PPr3)-Pr-i)(2)(CO)(3)(Me2NCN), W ((PPr3)-Pr-i)(2)(CO)(3)(2,6-F2C6H3CN), W((PPr3)-Pr-i)(2)(CO)(3)(2,4,6-Me3C6H2CN), W((PPr3)-Pr-i)(2)(CO)(3)(2,6-Me(2)pz), W((PPr3)-Pr-i)(2)(CO)(3)(AdCN), Mo((PPr3)-Pr-i)(2)(CO)(3)(AdNC), and W((PPr3)-Pr-i)(2)(CO)(3)(AdNC) are reported.