EPR and NMR Spectra as Probes of Spin-Density Distribution in Heterocyclic Ligands Coordinated in trans-[L(Im)(NH₃)₄Ru^III]: Implications for Long-Range Electron Transfer. Crystal Structure of trans-[(Im)₂(NH₃)₄Ru]Cl₃·H₂O

Spectroscopic studies of trans-[(L)(Im)(NH₃)₄R^III], where Im = imidazole and L = isonicotinamide (Isn), pyridine (Py), Im, NH₃, Cl^-, and SO₄^2-, indicate that π-bonding by the trans ligand significantly affects mixing of the dπ—π (imidazole) orbitals. Analysis of the EPR spectra provides a description of the frontier dπ orbitals involved in electron transfer and estimates of Δ and V (the tetragonal and rhombic distortion parameters, respectively), all of which vary with the π-donor abilities of L. As Δ and V are of the same magnitude as the the spin—orbit coupling parameter, λ, there is extensive spin-orbit mixing of the d_xz and d_yz and (to a lesser extent) the d_xyorbitals. Reduction potentials and energies of imidazole → Ru^III charge transfer transitions correlate linearly with the π-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, Δ_g12, and E°. 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° for the R^III/IIcouple can also affect the superexchange coupling involved in electron transfer. trans-[(Im)₂(NH₃)₄Ru^III]Cl₃·H₂O crystallizes in the monoclinic space group, P2₁/n (No. 14), with cell parameters a = 18.111(9) Å, b = 7.187(2) Å, c = 14.352(7) Å, β = 113.26(4)°, 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.