Quadratic hyperpolarizability enhancement of para-substituted pyridines upon coordination to organometallic moieties: The ambivalent donor or acceptor role of the metal

Coordination of para-substituted pyridines 4-X-C5H4N (where X = NMe2, CMe3, 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 ord ers of magnitude of the quadratic hyperpolarizability beta(lambda) of the f ree pyridine. This effect is due either to a red shift of the intraligand c harge-transfer (ILCT) transition upon coordination (when X is a strong elec tron donor) or to a significant effect on the metal-to-ligand charge-transf er (MLCT) transitions (when X is a strong electron acceptor). In this latte r case the quadratic hyperpolarizability may assume a negative sign, due to the negative value of Delta mu(eg) upon excitation. Therefore we confirmed that as already shown by metal carbonyl moieties such as "M(CO)(5)" (M = C r, W), a soft metal center displays, from the point of view of the perturba tion of the quadratic hyperpolarizability of pyridines, an ambivalent accep tor or donor role. The quadratic hyperpolarizability of complexes of more p i-delocalized para-substituted pyridine ligands (where X = trans- or trans, trans-(CH=CH)(n)C6H4-4'-NMe2 with n = 1 or 2) involving the same carbonyl R h(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 tim es that when n = 2. This enhancement, which is relatively low when compared to simple pyridines, is dependent upon the length of the pi-delocalized br idge (it decreases with increasing n) and upon the nature of the metal cent er (oxidation state and ligand coordination sphere). Some of the complexes investigated show significant values (between 400 x 10(-30) and 600 x 10(-3 0) D cm(5) esu(-1)) of the product mu beta(0).