A series of 20 ruthenium(II) complex salts, known to exhibit large molecular static first hyperpolarizabilities beta(0)[HRS] from hyper-Rayleigh scattering measurements on acetonitrile solutions at 298 K (Coe, B. J.; et al. Inorg. Chem. 1997, 36, 3284; 1998, 37, 3391; J. Chem. Soc., Dalton Trans. 1999, 3617), have been studied by using electroabsorption spectroscopy in butyronitrile glasses at 77 K. The salts are of the form trans-[Ru-II(NH3)(4)(L-D)(L-A)][PF6](3) [L-D = NH3 and L-A = N-methyl-4,4'-bipyridinium (MeQ(+)) (1), N-phenyl-4,4'-bipyridinium (PhQ(+)) (2), N-(4-acetylphenyl)-4,4'-bipyridinium (4-AcPhQ(+)) (3), N-(2,4-dinitrophenyl)-4,4'-bipyridinium (2,4-DNPhQ(+)) (4), N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe(+)) (5). N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe(+)) (6), or N-methyl-2,7-diazapyrenium (Medap(+)) (7); L-D = 1-methylimidazole and L-A = MeQ(+) (8), PhQ(+) (9), 4-AcPhQ(+) (10), N-(2-pyrimidyl)-4,4'-bipyridinium (2-PymQ(+)) (11), Mebpe(+) (12), or Phbpe(+) (13); L-D = 4-(dimethylamino)pyridine and L-A = MeQ(+) (14), PhQ(+) (15) or 4-AcPhQ(+) (16); L-D = pyridine and L-A = 2-PymQ(+) (17), Mebpe(+) (18) or Phbpe(+) (19); L-D = 4-(dimethylamino)benzonitrile and L-A = MeQ(+) (20)]. The electroabsorption spectra afford dipole moment changes Deltamu(12) for the visible metal-to-ligand charge-transfer (MLCT) excitations of the dipolar complexes. The transition dipole moments mu(12) cover a range of ca. 4-7 D and generally increase with the electron accepting strength of L-A, most notably on replacing an N-methyl with a N-phenyl substituent. The Deltamu(12) values are large (ca. 14-21 D) and generally increase with the size of L-A. Comparison of experimental and calculated diabatic dipole moment changes Deltamu(ab) suggests that the orbital(s) that receive the MLCT electron are delocalized only over the first two aryl rings. ZINDO calculations on the pentaammine complexes do not accurately reproduce the MLCT energies or beta(0)[HRS] values, but they can predict the dipole properties with reasonable accuracy and also indicate that the low lying pi* orbitals span only the first two rings of L-A. beta(0)[Stark] values of 1-20 calculated according to the two-state model by using beta(0) = 3Deltamu(mu(12))(2)/(E-max)(2)/(E-max = MLCT energy maximum) are mostly in good agreement with beta(0)[HRS]. The beta(0)[Stark] values are the first meaningful first hyperpolarizabilities for 18 and 20. The electroabsorption results confirm the unusually large magnitudes of beta(0) in 1-20 and also that N-arylation enhances beta(0), the latter effect being most significant in the 4,4'-bipyridinium-based complexes. Increases in beta(0) are generally associated with decreases in E-max and increases in both mu(12) and Deltamu(12), and insertion of a trans-CH=CH bridge into the 4,4'-bipyridinium unit of L-A enhances beta(0) by ca. 35-50%.