The syntheses of the alkynyl complexes M(4-CdropCC(6)H(4)NO(2))(dppe)(eta-C5H5) [M = Fe (1), Ru (2), Os (3)], Os(4-CdropCC(6)H(4)NO(2))(PPh3)(2)(eta-C5H5) (4) and Ru(4-CdropCC(6)H(4)NO(2))(CO)(2)(eta-C5H5) (5) are reported. Structural studies reveal a decrease in Ru-C(1) distance on proceeding from 5 to 2, consistent with greater back-donation of electron density to the alkynyl ligand from the more electron-rich metal center in 2. Electrochemical data show that the M-II/III. couple for the dicarbonyl complex 5 is at a significantly more positive potential than that of the related diphosphine complex 2, consistent with ligand variation modifying the electron richness and hence donor strength of the metal center. Time-dependent density functional calculations on model complexes M(4.CdropCC(6)H(4)NO(2))(PH3)(2)(eta-C5H5) (M = Fe, Ru, Os) have been employed to assign the intense low-energy optical transition in these complexes as MLCT in character, the higher energy band being phenyl-phenyl* in nature. Molecular quadratic optical nonlinearities have been measured using the hyper-Rayleigh scattering procedure at 1064 nm. beta values vary as Fe less than or equal to Ru less than or equal to Os for metal variation and CO < phosphines for co-ligand variation, the latter consistent with the variation in donor strength of the metal center inferred from electrochemical and crystallographic data. The observed trend in beta on metal variation follows the trend in backbonding energies calculated by DFT. (C) 2003 Elsevier Science B.V. All rights reserved.