Theoretical Study of the Oxidative Addition of Ammonia to Various Unsaturated Low-Valent Transition Metal Species

Reaction profiles for the oxidative addition of NH 3 to a number of unsaturated low-valent transition metal complexes have been computed using gradient-corrected density functional theory. The metal complexes studied are d 8 CpM(CO) (M = Rh, Ir) and trans- M(PH 3 ) 2 X (M = Rh, Ir; X = H, Cl) and d 10 ML 2 (M = Pd, Pt; L = PH 3, L 2 = H 2 PCH 2 CH 2 PH 2, dpe). Reactions with the d 8 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 d 10 M(PH 3 ) 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 d 10 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 NH 3 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 NH 3 oxidative addition are CpIr(CO), trans -Ir(PH 3 ) 2 X (X = H, Cl), and Pt(dpe). The more favorable thermodynamics computed with these third-row species arise from higher M−NH 2 and M−H homolytic bond strengths in the hydrido-amido products. M-NH 2 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 π-donation M−NH 2 bonds are computed to be up to 26 kcal mol - 1 weaker than M−H bonds. N→M π-donation reduces this differential, and in Ir(PH 3 ) 2 (H) 2 (NH 2 ) the Ir−NH 2 and Ir−H bonds are calculated to have equal homolytic bond strengths. Computed activation energies for NH 3 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 NH 3 reorientation/N−H bond activation model for the oxidative addition reaction. Although strongly Lewis acidic metal fragments usually promote oxidative addition, with NH 3 these form strong ammine adducts from which NH 3 reorientation is energetically costly. For metal fragments with lower Lewis acidity NH 3 reorientation is more facile, but the subsequent oxidative addition remains difficult. These ideas are supported by the accessibility of η 1 -H and η 3 -H,H,H NH 3 adducts formed with Pt(dpe), while with Ir(PH 3 ) 2 Cl only a high-energy η 1 -H species was located.