Spectroscopic studies of trans-[(L)(Im)(NH3)(4)Ru-III], where Lm = imidazole and L = isonicotinamide (Isn), pyridine (Py), Im, NH3, Cl-, and SO42-, indicate that pi-bonding by the trans ligand significantly affects mixing of the d(pi)-pi (imidazole) orbitals. Analysis of the EPR spectra provides a description of the frontier d(pi) orbitals involved in electron transfer and estimates of Delta and V (the tetragonal and rhombic distortion parameters, respectively), all of which vary with the pi-donor abilities of L. As Delta and V are of the same magnitude as the the spin-orbit coupling parameter, lambda, there is extensive spin-orbit mixing of the d(xz) and d(yz) and (to a lesser extent) the d(xy) orbitals. Reduction potentials and energies of imidazole --> Ru-III charge transfer transitions correlate linearly with the pi-donor/acceptor ability of L so that a correlation is also evident between these properties and the ligand field splitting of the t(2g) manifold, which leads to an unsuspected correlation between the difference between the two largest g values, Delta g(12), and E degrees. Electronic perturbations appear to be transmitted to C5 on the imidazole ring, which is the site linked to Ru-modified proteins used as probes of long-range electron transfer. This implies that variations of the ligand in the trans position to modify the E degrees for the Ru-III/II couple can also affect the superexchange coupling involved in electron transfer. trans-[(Im)(2)(NH3)(4)Ru-III]Cl-3 . H2O crystallizes in the monoclinic space group, P2(1)/n (No. 14), with cell parameters a = 18.111(9) Angstrom, b = 7.187(2) Angstrom, c = 14.352(7) Angstrom, beta = 113.26(4)degrees, and Z = 4 and exhibits an eclipsed conformation of the imidazole rings. MM2 and IEHT calculations suggest why the eclipsed conformation is slightly favored over the staggered and that the imidazole rings freely rotate in solution.