The effect of halogen atom on the molecular quadratic nonlinearity of half sandwich complexes in the presence of an acceptor

Half sandwich complexes of the type [CpM(CO)nX] {X = Cl, Br, I; If, M = Fe, Ru; n = 2 and if M = Mo; n = 3} and [CpNiPPh3X] {X = Cl, Br, I} have been synthesized and their second order molecular nonlinearity (β) measured at 1064 nm in CHCl3 by the hyper-Rayleigh scattering technique. Iron complexes consistently display larger β values than ruthenium complexes while nickel complexes have marginally larger β values than iron complexes. In the presence of an acceptor ligand such as CO or PPh3, the role of the halogen atom is that of a π donor. The better overlap of Cl orbitals with Fe and Ni metal centres make Cl a better π donor than Br or I in the respective complexes. Consequently, M–π interaction is stronger in Fe/Ni–Cl complexes. The value of β decreases as one goes down the halogen group. For the complexes of 4d metal ions where the metal–ligand distance is larger, the influence of π orbital overlap appears to be less important, resulting in moderate changes in β as a function of halogen substitution.