Electronic and Structural Effects of Low-Hapticity Coordination of Arene Rings to Transition Metals

A DFT computational
study and a structural analysis of the coordination
of arenes to transition metals in low -hapticity (η1 and η2) modes have been developed, including a
pseudosymmetry analysis of the molecular orbitals and the introduction
of a hapticity map that makes evident the different degrees of intermediate
hapticities. Calculations on [PtIIL3(C6H6)] model complexes reveal a preference for the η2 mode, while the η1 coordination is found
to be a low-energy transition state for a haptotropic shift. The attachment
of the arene to a side group that is coordinated to the metal introduces
geometrical constraints, which result in hapticities intermediate
between one and two. Comparison of the η1 arene complexes
with benzonium cations shows that in the former case the bonding to
the metal involves essentially the π system of the arene, affecting
only slightly the delocalized nature of the carbon–carbon bonds.
This behavior is in sharp contrast with the frequently found η1 coordination of Cp that involves σ bonding and full
dearomatization of the ring.