Molecular and Electronic Structure of (L-Histidinato)pentammineruthenium(III) Chloride Monohydrate, (NH3)5Ru^m(his)Cl3*H₂0: X-ray Structure, Single-Crystal Polarized Charge-Transfer Spectra, and ab Initio and Semiempirical Molecular Orbital Calculations

The synthesis, X-ray structure, and polarized single-crystal optical spectra are presented for the title complex. Crystals ofruc1₃0₂N₈C₆H₂₄.H₂0 are orthorhombic, space group P2i2_{2\\\\\\\\_twith a = 7.113 (2) A, b = 9.990 (1) A, c = 25.717 (3) A, V = 1827.2 (9) A³, Z = 4, and R_F(i?_wF) = 0.061 (0.063) for 1339 reflections. The structure contains (NH₃)₅Ru(his)³⁺cations linked to lattice chloride ions and water molecules by an extensive network of hydrogen bonds. Five ammine and one histidine N atoms provide slightly distorted octahedral coordination for Ru. The Ru-N(his) bond length [2,020 (8) A] is substantially shorter than the average Ru—N(ammine) distance [2.09 (2) A] and the corresponding Ru-N(heterocycle) distances in several related pentammineruthenium(III) complexes. The histidine imidazole ring is staggered with respect to the four equatorial NH₃ligands and is approximately perpendicular to a. Polarized single-crystal spectra of the title complex obtained at 80 K reveal that the low-energy LMCT absorption at 22200 cm^-1(450 nm) is strongly polarized along the Ru-N(his) direction and suggest that the higher energy LMCT band at 33 000 cm^-1(303 nm) may exhibit similar polarization. Ab initio molecular orbital calculations were performed on (NH₃)₅Ru^m-imidazole with use of effective core potentials for the inner atomic electrons and valence basis sets of double-Equality. The ground state has been established as²d_xz, where d_xzis the Ru(4d) orbital capable of interaction with the imidazole 7r-system; the unpaired electron is fully localized in this orbital. The a-donation from imidazole (0,36e) is twice as large as that from any ammine (0.17e), but the imidazole 7r-donation is quite weak (0.06e). Geometry optimization yields good agreement between calculated and experimental Ru-N bond lengths. The relevant Ru parameters required for the semiempirical INDO/S method have been developed from atomic spectral data. Excited state calculations on (NH₃)₅Ruⁱⁿ-5-methylimidazole provide assignments for the optical charge-transfer spectra as 7t₃— dx_zand 7t2-d_xz, where 7rj and 7r₂are the two high-lying occupied imidazole 7r-orbitals. These two LMCT transitions are calculated at 21 200 cm^-1(472 nm) and 35 000 cm^-1(286 nm), respectively, and both have their largest transition moment components along the Ru-N (heterocycle) axis.