The new compounds N-phenyl-4,4'-bipyridinium (PhQ(+)), N-(4-acetylphenyl)-4,4'-bipyridinium (4-AcPhQ(+)), and N-(2,4-dinitrophenyl)-4,4'-bipyridinium (2,4-DNPhQ(+)), together with the known ligand N-methyl-4,4'-bipyrihinium (MeQ(+)), have been used to prepare a series of Ru(II) complex salts trans-[Ru(NH3)(4)(L-D)(L-A)](PF6)(3) [L-D = NH3 and LA = MeQ(+) (1), PhQ+ (2), 4-AcPhQ+ (3), or 2,4-DNPhQ(+) (4); LD = 4-(dimethylamino)pyridine (dmap) and L-A = PhQ(+) (7) or 4-AcPhQ(+) (11); L-D = 1-methylimidazole (mim) and L-A = PhQ(+) (8) or 4-AcPhQ(+) (12); L-D = 4-(dimethylamino)benzonitrile (dmabn) and L-A = PhQ(+) (9) or 4-AcPhQ(+) (13); L-D = phenothiazine (PTZ) and L-A = PhQ(+) (10) or 4-AcPhQ(+) (14)]. These complexes display intense, visible metal-to-ligand charge-transfer (MLCT) absorptions, due to d pi(Ru-II) --> pi*(LA) excitations. The MLCT energy decreases as the acceptor strength of LA increases, in the order MeQ(+) < PhQ(+) < 4-AcPhQ(+) < 2,4-DNPhQ(+), and/or as the donor strength of LD increases, in the order PTZ ( dmabn ( NH3 ( mim ( dmap. X-ray crystal structure determinations have been carried out for [PhQ(+)]Cl . 2H(2)O and trans-[Ru(NH3)(4)(PhQ(+))(PTZ)] (PF6)(3). Et2O (10 . Et2O). [PhQ(+)]Cl . 2H(2)O, chemical formula C16H17ClN2O2, crystallizes in the triclinic system, space group Pi, with a = 7.675(2) Angstrom, b = 9.895(2) Angstrom, c = 10.175(2) Angstrom, alpha = 96.003(1)degrees, beta = 104.74(2)degrees, gamma = 90.398(1)degrees, and Z = 2. 10 . Et2O, chemical formula C32H44F18N7OP3RuS, crystallizes in the triclinic system, space group P (1) over bar, with a = 10.310(3) Angstrom, b = 10.698(2) Angstrom, c = 20.986(4) Angstrom, alpha = 95.09(2)degrees, beta = 91.49(2)degrees, gamma = 105.53(2)degrees, and Z = 2. The dihedral angles between the two pyridyl rings of the 4,4'-bipyridinium unit are 19.8 degrees in [PhQ(+)]Cl . 2H(2)O and 2.6 degrees in 10 . Et2O. Molecular first hyperpolarizabilities beta of the complex salts, obtained from hyper-Rayleigh scattering measurements at 1064 nm, are in the range (698-1214) x 10(-30) esu. Static hyperpolarizabilities Bo derived by using the two-level model are very large, with trans-[Ru(NH3)(4)(4-AcPhQ(+))(dmap)] (PF6)(3) (11) having the largest at 410 x 10(-30) esu. When the MLCT absorption is sufficiently far from the second harmonic at 532 nm, both beta and beta(0) increase as the absorption energy decreases.