The reaction of Ru(PPh(3))(3)Cl-2 with 1,3-diaryltriazenes, (p-RC(6)H(4)N)(p-RC(6)H(4)NH)N (HRL; R = H, Me, OMe, Cl), in ethanol affords Ru-II(RL)(2)(PPh(3))(2), which is oxidized by cerium(IV) to the trivalent complex isolated as [Ru-III(RL)(2)(PPh(3))(2)]PF6. Yields are near-quantitative for both oxidation states. The X-ray structures of Ru(HL)(2)-(PPh(3))(2) and [Ru(CIL)(2)(PPh(3))(2)]PF6 . 2CH(2)Cl(2) have revealed centrosymmetric trans geometry for both RuN4P2 coordination spheres. The four-membered triazenide chelate rings are near-perfect planes, and the average Ru-N length decreases by similar to 0.04 Angstrom in going from the bivalent to the trivalent complex due to the contraction of the metal radius in the primarily sigma-bonded RuN4 frame. In contrast, the Ru-P length is similar to 0.07 Angstrom, shorter in the bivalent complex because of strong Ru-P 4d pi-3d pi back-bonding, which is lacking in the trivalent complex. Studies on the energetics of models have revealed that cis geometry is precluded by severe steric crowding of triazenide vis-a-vis phosphine aryl functions. The repulsions are very strong even in the "best" rotameric configurations of triazene aryls. The bivalent complexes are diamagnetic and display an MLCT band near 500 nm. The trivalent species have one unpaired electron (1.78-1.86 mu(B)), and they show a strong band near 1200 nm which has been assigned to LMCT excitation (a(u) --> a(g), b(g)) on the basis of model EHMO results. The E 1/2 values of the stereoretentive ruthenium(II)-ruthenium(II) couple lie in the range 0.08-0.37 V vs SCE, increasing in the order OMe < Me < H < Cl. Crystal data for the complexes are as follows : Ru(HL)(2)(PPh(3))(2), crystal system triclinic, space group P1 ($) over tilde$$; a = 10.964(6) Angstrom, b = 11.821(7) Angstrom, c = 21.183(9) Angstrom, alpha = 76.52(4)degrees, beta = 78.17(4)degrees, gamma = 71.18(4)degrees, V = 2502(2) Angstrom 3, Z = 2, r = 3.93%, R(w) = 4.32%; [Ru(ClL)(2)(PPh(3))(2)]PF6 . 2CH(2)Cl(2), crystal system triclinic; space group P1 $($) over bar$$, a = 11.093(6) Angstrom, b = 12.386(4) Angstrom, c = 13.757(6) Angstrom, alpha = 104.11(3)degrees, beta = 103.48(4)degrees, gamma = 110.57(3)degrees, V = 1607(1) Angstrom 3, Z = 1, R = 5.00%, R(w) = 5.64%.