Water π-Donation in trans-Tetraammineruthenium(II): Effect on Coordinated-Water Properties Induced by a Trans NO Ligand

The complex trans -[Ru(NH 3 ) 4 NO(H 2 O)]Cl 3 ·H 2 O has been isolated as a decomposition product of the dimeric cation [{Ru(NH 3 ) 4 NO} 2 (μ-S 2 )] 6+ . The elemental analysis and electronic, infrared, X-ray, and ESR spectroscopies fit well with the formulation trans -[Ru(NH 3 ) 4 NO(H 2 O)]Cl 3 ·H 2 O. The ν NO (1912 cm - 1 ) observed and the ∠(Ru−N−O) = 178.1°(5) are consistent with the nitrosonium character of the NO ligand. Cyclic voltammetry showed only one redox process in the range −0.5 to +1.2 V, which was attributed to the reaction trans -[(H 2 O)(NH 3 ) 4 Ru II (NO + )] 3+ + e - ↔ trans -[(H 2 O)(NH 3 ) 4 Ru II (NO 0 )] 2+ . The p K a values 3.1 ± 0.1 and 7.7 ± 0.1 (μ = 0.10 M, NaCl) have been measured for the reaction trans -[Ru(NH 3 ) 4 L(H 2 O)] n + + H 2 O ↔ trans -[Ru(NH 3 ) 4 L(OH)] ( n - 1)+ + H 3 O +, where L = NO + and CO, respectively. The substitution of the coordinated water molecule in trans -[Ru(NH 3 ) 4 (H 2 O)NO] 3+ by chloride ions proceeds about 30-fold times slower than in [Ru(NH 3 ) 5 (H 2 O)] 3+ ( k Cl _ = 8.7 × 10 - 5 M - 1 s - 1 and 3.7 × 10 - 6 M - 1 s - 1, respectively; 40 °C, μ = 2.0 NaCl, [H + ] = 1.0 × 10 - 2 mol L - 1 ). Quantum mechanical DFT calculations show that the mixing between the lone pair of the oxygen, π in character, and the d xz orbital of the metal is linearly related to the p K a of the water ligand and to the water lability. The calculations have also shown that the π−d mixing is strongly dependent on the trans ligand L. The electronic spectra of the trans -[Ru(NH 3 ) 4 (H 2 O)L] n + (L = CO and NO + ) species are discussed on the basis of DFT and ZINDO/S calculations.