Model studies of π-bonded organometallic systems Mn-C₂H₂ and Mn-C₂H₄

Abstract

An improved understanding of the structural and energetic aspects of transition metal-carbon bonds is essential to progress in surface chemistry, catalysis, and organometallic chemistry. As a possible route to this goal, ab initio electronic structure theory has been applied to two very simple systems, Mn-C₂H₂ and Mn-C₂H₄. Single configuration self-consistent-field theory was used in conjunction with a better than double zeta basis set of contracted gaussian functions. For the manganese-acetylene system, potential energy curves were predicted for electronic states arising from Mn⁺(4s3d⁵) and Mn(4s²3d⁵, 4s4p3d⁵, and 4s3d⁶) interacting with the ¹Σ _g ⁺ ground state of C₂H₂. For the first three dissociation limits, the present results are consistent with those based on the earlier studied Be-C₂H₂ model system. The latter model assumes that metal 3d orbitals play a relatively passive role in the bonding. The 4s3d⁶ dissociation limit has no counterpart for the Be-C₂H₂ system, and the predicted potential energy curves are very sensitive to basis set. As one approaches the Hartree-Fock limit the latter curves become quite flat. The lowest of these, the ⁶ B ₂ state, appears unlikely to be bound by more than 10 kcal/mole. Thus it seems that additional ligands (e.g. CO) will play an essential role in the binding of unsaturated hydrocarbons to the Mn atom. Alternatively, the building up of a cluster or surface of Mn atoms might serve to provide a more appealing resting place for acetylene. Less detailed calculations on Mn-C₂H₄ suggest that the same interpretation is applicable.

Work performed under the auspices of the U.S. Energy Research and Development Administration.

Work performed under the auspices of the U.S. Energy Research and Development Administration.

Notes

Work performed under the auspices of the U.S. Energy Research and Development Administration.