Side-on binding of the nitrogen molecule to first-row transition-metal dimers

Abstract

Calculations including electron correlation have been performed for the systems M₂N₂, where M are the transition metals Ti, Cr, Fe, Co, and Ni. Rather surprisingly, it is found that these systems are well described by a single determinant. The spin is very high with a maximum of 11 for both Fe₂N₂ and Cr₂N₂. The optimal geometric structure of these side‐on bonded structures is planar with N₂ perpendicularly bridging the metal–metal bond. The electronic structure is best described as two M⁺ ionically bound to N²⁻₂, with some back‐donation occurring. With this simple picture, it is possible to explain the very irregular trend of the calculated dissociation energies with respect to N₂ and free metal atoms. For example, the calculated binding energy difference between Ti₂N₂ and Cr₂N₂ is as large as 78.3 kcal/mol at the self‐consistent‐field level, whereas the simple prediction gives 79.2 kcal/mol. The implications of the present results for the catalytic dissociation of N₂ by metal surfaces is discussed. A new explanation for the large effect on the N₂ dissociation by alkali doping is suggested.