Polytopal rearrangement of [Ni(acac)2(py)]: a new square pyramid<==>trigonal bipyramid twist mechanism.

The interconversion mechanisms between three idealized polytopal forms (a square pyramid and two trigonal bipyramids) of [M(bidentate)(2)(unidentate)] were investigated by an original combination of molecular mechanics (MM) and density functional theory (DFT) approaches. MM was used to model the mechanistic rearrangement path, and DFT to study selected points along this path. The test case was a five-coordinate [Ni(acac)(2)(py)] species. In the case of [Ni(acac)(2)(py)] it was confirmed (both by MM and by DFT) that the three polytopal forms do indeed represent shallow local minima, of which the square pyramid (SQP) is more stable than the other two. Small energy barriers that separate the three minima prevent spontaneous rearrangement among the polytopal forms in geometry-optimization simulations. The driving force for MM simulation of the polytopal rearrangements was supplied through the L-M-L angle bending terms. MM results for relative energies and geometries are fully supported by DFT. Finally, the implication of the present results to explain some racemization mechanisms of octahedral complexes (namely, the intramolecular bond rupture of tris(chelate) species, and intermolecular dissociation of bis(bidentate) species) is briefly discussed.