NO-binding in {Ru(NO)2}8-type [Ru(NO)2(PR3)2X]BF4 compounds

Abstract

Two different structure types were found for a series of mononuclear dinitrosyl complexes of the general formula [RuL₂(NO)₂X]BF₄ (L = monodentate phosphane, X = Cl, Br, I). The {Ru(NO)₂}⁸-type target compounds were prepared by the reduction of the respective {RuNO}⁶ precursors and subsequent oxidative addition of (NO)BF₄. About one half of the new compounds share their molecular structure with the hitherto only representative of this class of dinitrosyls, Pierpont and Eisenberg's [RuCl(NO)₂(PPh₃)₂]PF₆·C₆H₆ (Inorg. Chem., 1972, 11, 1088–1094). The C_s-symmetric cations exhibit both a linear and a bent Ru–N–O fragment, in line with a formal 6 + 2 split of the {Ru(NO)₂}⁸ electron sum in the sense of a [Ru^II(NO⁺)(¹NO⁻)]²⁺ bonding. The coordination entity's configuration in this subgroup is described by IUPAC's polyhedral symbol SPY-5. Continuous shape measures (CShM) as defined by Alvarez et al. (Coord. Chem. Rev., 2005, 249, 1693–1708) reveal a uniform deviation from the L–M–L angles expected for SPY-5, in a narrower sense, towards a vacant octahedron (vOC-5). DFT calculations confirmed that Enemark and Feltham's analysis (Coord. Chem. Rev., 1974, 13, 339–406) of the electronic situation of the {Ru(NO)₂}⁸ group remains adequate. The same holds for the second subclass of new compounds the existence of which had been predicted in the same paper by Enemark and Feltham, namely C_2v-symmetric, TBPY-5-type cations with two almost equally bonded nitrosyl ligands. In agreement with an 8 + 0 distribution of the relevant electrons, the formal [Ru⁰(NO⁺)₂]²⁺ entities are found for L/X couples that donate more electron density on the central metal. Two solid compounds (8a/b, 12a/b) were found in both structures including the special case of the PⁱPr₃/Br couple 12a/b, which led to crystals that contained both structure types in the same solid. Conversely, four compounds showed a single form in the solid but both forms in dichloromethane solution in terms of the solutions’ IR spectra. The irradiation of crystalline 12 with blue laser light resulted in the photoisomerisation of, mainly, the bent ¹NO⁻ ligand in terms of low-temperature IR spectroscopy.