Coordination of para-substituted pyridines 4-X-C 5 H 4 N (where X ) NMe 2 , CMe 3 , H, COMe, CN) to metal carbonyl moieties such as "cis-M(CO) 2 Cl" (M ) Rh(I), Ir(I)) and "fac-Os(CO) 3 Cl 2 " produces an enhancement up to about 2 orders of magnitude of the quadratic hyperpolarizability β λ 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 ∆µ eg 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 more π-delocalized para-substituted pyridine ligands (where X ) trans-or trans,trans-(CHdCH) n C 6 H 4 -4′-NMe 2 with n ) 1 or 2) involving the same carbonyl Rh(I), Ir(I), and Os(II) moieties corresponds to an enhancement with respect to the free ligands of about 1.5-2.3 times that when n ) 1 and 1.5-1.7 times that when n ) 2. This enhancement, which is relatively low when compared to simple pyridines, is dependent upon the length of the π-delocalized bridge (it decreases with increasing n) and upon the nature of the metal center (oxidation state and ligand coordination sphere). Some of the complexes investigated show significant values (between 400 × 10 -30 and 600 × 10 -30 D cm 5 esu -1 ) of the product µβ 0 .

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