On the Fluxional Behavior of Monocoordinated Diimines and Azines

Intramolecular mechanisms for the haptotropic shift of transition-metal centers in monocoordinate complexes with potentially bidentate diimines and azines have been analyzed by means of qualitative molecular orbital studies, and the factors controlling the height of the barriers for such shifts are discussed. Related processes are the tautomerism in protonated formamidinates or triazenides, as well as the haptotropic shifts in main group metal complexes. A common feature for all these systems is the loss of overlap between the acceptor orbital and the diimine's lone pairs along the reaction path, while a unique feature of the transition-metal complexes is the appearance of a four-electron repulsion between the lone pairs and one of the metal's “t_2g” orbitals. For octahedral complexes, the height of the potential energy barrier is expected to be in the order Cr < Mn < Fe > Co, and the presence of a π-acceptor ligand in the trans position is expected to lower the barrier. On the ligand side, the nitrogen-nitrogen separation and the lone pair orientation are the main factors determining the height of the barrier, while mixing with orbitals of the intervening carbon skeleton accounts for smaller changes in the barrier. According to the orbital analysis presented, redox reactions should allow interconversion of monodentate and bidentate compounds, while irradiation with light of a wavelength corresponding to a ligand field transition should produce an important lowering of the barrier for the fluxional process.