Sigma trans promotion effect in transition metal complexes:: a manifestation of the composite nature of binding energy

It is well known that intra-fragment electronic reorganization is one among several important components of the binding energy (bond dissociation energy) for two fragments forming an adduct. An interesting manifestation of this notion is proposed: a counterintuitive trend in binding energies that shall be commonly observed for a series of particular open shell transition metal containing complexes binding a small molecule at an empty coordination site. The prediction is that stronger sigma interaction between the metal and the ligand trans to the empty site will result in a larger magnitude of binding energy and, possibly, a lower substrate coordination barrier. This trend, which we call the 'trans promotion effect' (TPE), is exemplified with a case study of dinitrogen binding by tri- and four-coordinated Mo(III) complexes. It is demonstrated that a trans amine ligand significantly increases the magnitude of dinitrogen binding energy. Orbital analysis and decomposition of the binding energy show that substrate coordination is favoured by stronger interaction of Mo with the trans ligand due to the d(z)2 orbital of Mo that is occupied in the complex but vacant in the dinitrogen adduct.