Symmetry-separated (σ+π) vs Bent-bond (Ω) Models of First-row Transition-metal Methylene Cations

Abstract

This is the first of two papers providing a modern valence bond explanation, within the framework of spin-coupled theory, of the general tendencies and characteristics of chemical bonding in the monocationic molecular systems MCH2,+ (M=Sc-Co). The present paper concentrates on a general study of two alternative representations of the metal-ligand double bond, namely the σ+π and bent (Ω) bond descriptions. The close equivalence of these two models is established both within the valence and core parts of the wavefunction. Our results show that the spin degrees of freedom influence the preference for a bonding model to a much greater extent than do different choices for the core orbitals or variations in orbital flexibility. The presence of nonbonding electrons on the metal is found to reduce the size of the spin space for the ω bond wavefunction relative to its σ+π alternative; this favours energetically the σ+π construction. In contrast, systems deprived of nonbonding electrons, such as ScCH+,2, TiCH2+,2 and VCH3+,2, prefer the bent bond model. Extension of the active space is found to advantage the σ+π representation as a consequence of an escalation of the difference between the spin flexibilities of the two models, and this can lead to an inversion of the hierarchy of the two descriptions (as in the case of ScCH2,+). We find that modest variations of the molecular geometry do not modify the main conclusion of this survey: The classical σ+π bond model offers the more appropriate description of the metal-methylene interaction. We discuss also the triplet character of the metal-carbon π bond, which is found to stem from the presence of unpaired nonbonding electrons on the metal and can be viewed as the result of a compromise between the preservation of some metal d-d exchange energy and the formation of a strong purely singlet-coupled covalent bond.