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 unsatura ted low-valent transition metal complexes have been computed using gradient -corrected density functional theory. The metal complexes studied are d(8) CpM(CO) (M = Rk, Ir) and trans-M(PH3)(2)X (M = Rh, Ir X = H, Cl) and d(10) ML2 (M = Pd, Pt; L = PH3, L-2 = H2PCH2CH2PH2, dpe). Reactions with the d(8) species are characterized by the formation of strongly bound ammine comple xes from which computed activation energies for oxidative addition are in e xcess of 16 kcal mol(-1). Computed reaction enthalpies are all exothermic w ith these complexes. With d(10) M(PH3)(2) species computed ammine adducts a re weak, activation barriers are in excess of 23 kcal mol(-1), and the over all reaction is endothermic for both M = lad and Pt. The introduction of th e 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 he exothermic, Comparison of the computed reac tion profiles for analogous second- and third-row complexes shows the NH3 o xidative addition reaction to be more favorable with the third-row species, which exhibit more strongly; bound ammine adducts, lower activation barrie rs, and more exothermic reactions. Of the species studied the most premisin g unsaturated fragments for effecting NH3 oxidative addition are CpIr(CO), trans-Ir(PH3)(2)X (X = H, Cl), and Pt(dpe). The more favorable thermodynami cs computed with these third-row species arise from higher M-NH2 and M-H ho molytic bond strengths in the hydridoamido products, M-NH2 bonds are comput ed 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 co ngeners. For complexes exhibiting no N -->M pi -donation M-NH2 bonds are co mputed to be up to 26 kcal mol(-1) weaker than M-H bonds. N -->M pi -donati on reduces this differential, and in Ir(PH3)(2)(H)(2)(NH2) the Ir-NH2 and I r-H bonds are calculated to have equal homolytic bond strengths, Computed a ctivation energies for NH3 oxidative addition do not appear to be related t o the strength of the ammine adduct, and for metal complexes of the same ro w 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 reorient ation is energetically costly, For metal fragments with lower Lewis acidity NH3 reorientation is more facile, but the subsequent oxidative addition re mains difficult, These ideas are supported by the accessibility of eta (1)- H and eta (3)-H,H,H NH3 adducts formed with Pt(dpe), while with Ir(PH3)(2)C l only a high-energy eta (1)-H species was located.