Since the discovery of nonlinear effects, both the theory and measurement techniques have been developed significantly, especially since the invention of the laser. However, structure-NLO property relationships for organometallics as well as their NLO mechanisms are far less explored than those of organic molecules and inorganic salts. The greater flexibility and exceptionally large NLO responses of organometallic compounds attracts chemists to this field. Modification of coordinated co-ligands in organometallic systems has influence on the NLO merit by introducing new electronic charge-transfer transitions, oxidation state and coordination sphere of the metal centers. In this work, the donor sets of the ruthenium complexes were modified from the most investigated (P-P)(P-P) to (N-N)(P-P) and (N-P)2 and a series of ruthenium complexes were synthesized and characterized. In Chapter 2, the study of the Ru(N-N)(P-P) complexes is detailed. Three bidentate diphosphine ligands (dppe, dppb and dppf) and one diimine ligand (tert-Bu-bpy) were selected for this study as the diphosphine and diimine ligands, respectively. Ruthenium halide and mono-alkynyl complexes were obtained successfully. Their optical, electrochemical and spectroelectrochemical properties were examined and are discussed. The formation of eta3- and eta1-butenynyl complexes was confirmed by single-crystal X-ray diffraction. Attempts towards bis-alkynyl complexes were made, but no conclusive evidence could be obtained to confirm the successful synthesis of this species. In Chapter 3, the focus of the work is the study of ruthenium complexes with a (N-P)2 donor set. Two iminophosphine ligands, 2-(diphenylphosphino)pyridine (PPh2py) and 8-(diphenylphosphino)quinoline (PPh2qn), were selected for this study. The cis-RuCl2(N-P) complexes and the corresponding dimers were synthesized and characterized. Their optical and electrochemical properties were measured and are discussed. In Chapter 4, the quadratic and cubic nonlinear optical properties of organometallic complexes were explored by hyper-Rayleigh scattering (HRS) and frequency-depentdent Z-scan techniques, respectively. The first hyperpolarizabilities of the ruthenium halide and mono-alkynyl complexes described in Chapter 2 were determined by HRS measurements. The second hyperpolarizabilities of some organometallic complexes synthesized by the Humphrey group and the collaborators were measured by the Z-scan technique.