Theoretical study of the oxidative addition of ammonia to various unsaturated low-valent transition metal species

Reaction profiles for the oxidative addition of NH₃ to a number of unsaturated low-valent transition metal complexes have been computed using gradient-corrected density functional theory. The metal complexes studied are d⁸ CpM(CO) (M = Rh, Ir) and trans-M(PH₃)₂X (M = Rh, Ir; X = H, Cl) and d¹⁰ ML₂ (M = Pd, Pt; L = PH₃, L₂ = H₂PCH₂CH₂PH₂, dpe). Reactions with the d⁸ species are characterized by the formation of strongly bound ammine complexes from which computed activation energies for oxidative addition are in excess of 16 kcal mol^-1. Computed reaction enthalpies are all exothermic with these complexes. With d¹⁰ M(PH₃)₂ species computed ammine adducts are weak, activation barriers are in excess of 23 kcal mol^-1, and the overall reaction is endothermic for both M = Pd and Pt. The introduction of the chelating dpe ligand results in stronger ammine adducts but only slightly reduced computed activation barriers. Of the d¹⁰ species only the reaction with Pt(dpe) is computed to be exothermic. Comparison of the computed reaction profiles for analogous second- and third-row complexes shows the NH₃ oxidative addition reaction to be more favorable with the third-row species, which exhibit more strongly bound ammine adducts, lower activation barriers, and more exothermic reactions. Of the species studied the most promising unsaturated fragments for effecting NH₃ oxidative addition are CpIr(CO), trans-Ir(PH₃)₂X (X = H, Cl), and Pt(dpe). The more favorable thermodynamics computed with these third-row species arise from higher M-NH₂ and M-H homolytic bond strengths in the hydrido-amido products. M-NH₂ bonds are computed to be between 6 and 13 kcal mol^-1 and M-H bonds between 5 and 14 kcal mol^-1 stronger in the third-row complexes compared to their second-row congeners. For complexes exhibiting no N?M p-donation M-NH₂ bonds are computed to be up to 26 kcal mol^-1 weaker than M-H bonds. N?M p-donation reduces this differential, and in Ir(PH₃)₂(H)₂(NH₂) the Ir-NH₂ and Ir-H bonds are calculated to have equal homolytic bond strengths. Computed activation energies for NH₃ oxidative addition do not appear to be related to the strength of the ammine adduct, and for metal complexes of the same row the computed activation energy is relatively insensitive to the nature of the unsaturated fragment. These findings are discussed in terms of an NH₃ reorientation/N-H bond activation model for the oxidative addition reaction. Although strongly Lewis acidic metal fragments usually promote oxidative addition, with NH₃ these form strong ammine adducts from which NH₃ reorientation is energetically costly. For metal fragments with lower Lewis acidity NH₃ reorientation is more facile, but the subsequent oxidative addition remains difficult. These ideas are supported by the accessibility of ?¹-H and ?³-H,H,H NH₃ adducts formed with Pt(dpe), while with Ir(PH₃)₂Cl only a high-energy ?¹-H species was located.