EPR AND NMR-SPECTRA AS PROBES OF SPIN-DENSITY DISTRIBUTION IN HETEROCYCLIC LIGANDS COORDINATED IN TRANS-[L(IM)(NH3)(4)RU-III] - IMPLICATIONS FOR LONG-RANGE ELECTRON-TRANSFER - CRYSTAL-STRUCTURE OF TRANS-[(IM)(2)(NH3)(4)RU]CL-3-CENTER-DOT-H2O

Spectroscopic studies of trans-[(L)(Im)(NH3)(4)Ru-III], where Lm = imi dazole and L = isonicotinamide (Isn), pyridine (Py), Im, NH3, Cl-, and SO42-, indicate that pi-bonding by the trans ligand significantly aff ects 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 s pin-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-I II charge transfer transitions correlate linearly with the pi-donor/ac ceptor ability of L so that a correlation is also evident between thes e properties and the ligand field splitting of the t(2g) manifold, whi ch leads to an unsuspected correlation between the difference between the two largest g values, Delta g(12), and E degrees. Electronic pertu rbations appear to be transmitted to C5 on the imidazole ring, which i s 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 al so affect the superexchange coupling involved in electron transfer. tr ans-[(Im)(2)(NH3)(4)Ru-III]Cl-3 . H2O crystallizes in the monoclinic s pace group, P2(1)/n (No. 14), with cell parameters a = 18.111(9) Angst rom, b = 7.187(2) Angstrom, c = 14.352(7) Angstrom, beta = 113.26(4)de grees, and Z = 4 and exhibits an eclipsed conformation of the imidazol e rings. MM2 and IEHT calculations suggest why the eclipsed conformati on is slightly favored over the staggered and that the imidazole rings freely rotate in solution.