Solvent-dependent dihydrogen/dihydride stability for [Mo(CO)(Cp)H <inf>2</inf>(PMe<inf>3</inf>)?] ± [BF<inf>4</inf>]- Determined by multiple sorvent⋯anion⋯cation non-covalent interactions

Low-temperature (200 K) protonation of [Mo(CO)(Cp*)H(PMe 3)2] (1) by Et2O-HBF4 gives a different result depending on a subtle solvent change: The dihydrogen complex [Mo(CO)(Cp*)(η2-H2)(PMe3)2] + (2) is obtained in THF, whereas the tautomeric classical dihydride [Mo(CO)(Cp*)(H)2(PMe3)2] (3) is the only observable product in dichloromethane. Both products were fully characterised (vco IR; 1H, 31P, 13C NMR spectroscopies) at low temperature; they lose H2 upon warming to 230 K at approximately the same rate (ca. 10-3 s-1), with no detection of the non-classical form in CD2Cl2, to generate [Mo(CO)-(Cp*)(FBF3)(PMe3)2] (4). The latter also slowly decomposes at ambient temperature. One of the decomposition products was crystallised and identified by X-ray crystallography as [Mo(CO)(Cp*)(FH-FBF3)(PMe3)2] (5), which features a neutral HF ligand coordinated to the transition metal through the F atom and to the BF4 anion through a hydrogen bond. The reason for the switch in relative stability between 2 and 3 was probed by DFT calculations based on the B3LYP and M05-2X functional, with inclusion of anion and solvent effects by the conductor-like polarisable continuum model and by explicit consideration of the solvent molecules. Calculations at the MP4(SDQ) and CCSD(T) levels were also carried out for calibration. The calculations reveal the key role of non-covalent anion-solvent interactions, which modulate the anion-cation interaction ultimately altering the energetic balance between the two isomeric forms. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.