Quadratic Hyperpolarizability Enhancement ofpara-Substituted Pyridines upon Coordination to Organometallic Moieties: The Ambivalent Donor or Acceptor Role of the Metal

Abstract

Coordination of para-substituted pyridines 4-X-C5H4N (where X = NMe2, CMe3, H, COMe, CN) to metal carbonyl moieties such as “cis-M(CO)2Cl” (M = Rh(I), Ir(I)) and “fac-Os(CO)3Cl2” produces an enhancement up to about 2 orders of magnitude of the quadratic hyperpolarizability βλ of the free pyridine. This effect is due either to a red shift of the intraligand charge-transfer (ILCT) transition upon coordination (when X is a strong electron donor) or to a significant effect on the metal-to-ligand charge-transfer (MLCT) transitions (when X is a strong electron acceptor). In this latter case the quadratic hyperpolarizability may assume a negative sign, due to the negative value of Δμeg upon excitation. Therefore we confirmed that as already shown by metal carbonyl moieties such as “M(CO)5” (M = Cr, W), a soft metal center displays, from the point of view of the perturbation of the quadratic hyperpolarizability of pyridines, an ambivalent acceptor or donor role. The quadratic hyperpolarizability of complexes of m...