MOLECULAR AND ELECTRONIC-STRUCTURES OF PENTAAMMINERUTHENIUM(II)-THIOETHER COMPLEXES - THE NATURE OF RU(II)-S BACK-BONDING ELUCIDATED BY STRUCTURAL, ELECTRONIC SPECTRAL, AND MOLECULAR-ORBITAL STUDIES

The nature of Ru(II)-S(thioether) bonding has been probed by a combination of structural, spectroscopic, and computational methods. The synthesis, X-ray structure, and electronic spectra are presented for the (NH3)5Ru[S-CH3)(C2Hs)].2PF6 complex which crystallizes in the monoclinic space group P2(1)/n with a = 8.3664 (5) angstrom, b = 12.337 (1) angstrom, c = 17.780 (1) angstrom, beta = 100.388 (5)-degrees, V = 1805.1 (4) angstrom3, Z = 4, and R(F)(R(wF)) = 0.042 (0.072) for 3475 reflections. The structure contains approximately octahedral (NH3)5RuS(Me)Et2+ cations separated by disordered PF6- anions. The Ru(II)-N distances span the range 2.135 (4)-2.168 (4) angstrom and average 0.043 angstrom longer than those of the Ru(III) analogue. In contrast, the Ru(II)-S distance of 2.316 (1) angstrom is 0.055 angstrom shorter than that of the Ru(III) analogue, implying substantial back bonding. Structural parameters of the coordinated thioether in both Ru(II)- and Ru(III)-thioether complexes are close to those reported for the free ligand. Ab initio molecular orbital (MO) calculations have been performed on the ground state of the (N-H3)5Ru(II)S(CH3)2 complex with full geometry optimizations carried out at the Hartree-Fock (HF) level; partial geometry optimizations were made with correlation energy corrections included via Moller-Plesset perturbation theory (MP2, MP3). Good agreement between calculated and experimental structures is obtained only at the correlated (MP2, MP3) levels; at the HF level, the Ru(II)-S distance is computed more than 0.2 angstrom too long. Electronic population analyses at both the HF and MP2 levels are used to elucidate the metal-thioether interactions, in particular with respect to the nature of Ru(II)-S back bonding. Semiempirical MO calculations (INDO/S) using singly excited configuration interaction (SECI) and time-dependent Hartree-Fock (TDHF) methods permit assignment of the electronic spectra. Two Ru-thioether MLCT transitions are located in the 35000-40000-cm-1 region; in addition, a LMCT transition occurs at slightly higher energy (approximately 45 000 cm-1).