The five-coordinate ruthenium N-heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr2Me2)4H][BArF 4] (1; IiPr2Me2 = l,3-diisopropyl4,5- dimethylimidazol-2-ylidene ; ArF = 3,5-(CF3) 2C6H3), [Ru(IEt2Me2) 4H]-[BArF4] (2; IEt2Me2 = l,3-diethyl-4,5-dimethylimidazol-2-ylidene) and [Ru(IMe4) 4H][BArF4] (3; IMe4=l,3,4,5- tetramethylimidazol-2-ylidene) have been synthesised following reaction of [Ru(PPh3)3HCl] with 4-8 equivalents of the free carbenes at ambient temperature. Complexes 1-3 have been structurally characterised and show square pyramidal geometries with apical hydride ligands. In both dichloromethane or pyridine solution, 1 and 2 display very low frequency hydride signals at about (3 -41. The tetramethyl carbene complex 3 exhibits a similar chemical shift in toluene, but shows a higher frequency signal in acetonitrile arising from the solvent adduct [Ru(IMe4)4(MeCN)H] [BArF4], 4. The reactivity of 1-3 towards H2 and N2 depends on the size of the N-substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H2 and N2 only at low temperature and incom-pletely, while 3 affords [Ru(IMe4)4(n2H2)H] [BAr F4] (7) and [Ru(IMe4)4(N 2)H][BArF4] (8) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1-3 to give [Ru(NHC) 4(CO)H][BArF4] (911). Addition of O2 to solutions of 2 and 3 leads to rapid oxidation, from which the Ru III species [Ru(NHC)4(OH)2][BAr F4] and the RuIV oxo chlorido complex [Ru(IEt2Me2)4(O)Cl][BArF 4] were isolated. DFT calculations reproduce the greater ability of 3 to bind small molecules and show relative binding strengths that follow the trend CO O2 N2 H2. 2009 Wiley-VCH Verlag GmbH Co. KGaA.