TRANS INFLUENCE AND MUTUAL INFLUENCE OF LIGANDS COORDINATED TO A CENTRAL ATOM

The consequences of sharing a central-atom orbital by two or more ligand orbitals is examined in general by using a simple perturbation approach. If only the ML bonding partner of this interaction is occupied, then sharing of the central-atom orbital is not favored. If only the ML antibonding partner is occupied, then the reverse is true. The results are applied to an analysis of irarts-L(MXn)L systems where L is a single-faced it donor or acceptor (e.g., Mo(diphos)2(C2H4)2). When applied to an octahedrally based series of low-spin d6 M(CO)x species, the ML Π-bond stabilization energy, Σ(Π), fits a simple equation: Σ(Π) = A-[ncis + 2ntrans]B. Here ncis and ntrans are the number of CO ligands cis and trans, respectively, to the one being considered (A and B are constants). This equation shows why CO-stretching force constants increase on increasing substitution and that trans ligands are more effective than cis ligands. Timney's equation, an experimentally derived relationship between CO force constants and structure, is of the same form. When applied to two different ligands, trans to each other in a metal complex, it quantifies previous ideas concerning the trans influence of ligands. It is shown that this need not be the only mechanism operating in these systems. Another effect is discussed wherein unoccupied orbitals of nonequivalent symmetry are allowed to mix into the occupied ones as the symmetry of the complex is lowered. This creates a bond weakening or strengthening in the M-L bond trans to the site of perturbation. © 1979, American Chemical Society. All rights reserved.