The five-coordinate ruthenium N-heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr(2)Me(2))(4)H][BAr4F] (1; IiPr(2)Me(2)=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; Ar-F = 3,5-(CF3)(2)C6H3), [Ru(IEt2Me2)(4)H][BAr4F] (2; IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) and [Ru(IMe4)(4)H][BAr4F] (3; IMe4=1,3,4,5-tetramethylimidazol-2-ylidene) have been synthesised following reaction of [Ru(PPh3)(3)HCl] 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 delta -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][BAr4F], 4. The reactivity of 1-3 towards H-2 and N-2 depends on the size of the N-substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H-2 and N-2 only at low temperature and incompletely, while 3 affords [Ru(IMe4)(4)(eta H-2(2))H][BAr4F] (7) and [Ru(IMe4)(4)(N-2)H][BAr4F] (8) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1-3 to give [Ru(NHC)(4)(CO)H][BAr4F] (9-11). Addition of O-2 to solutions of 2 and 3 leads to rapid oxidation, from which the Ru-III species [Ru(NHC)(4)(OH)(2)][BAr4F] and the Ru-IV oxo chlorido complex [Ru(IEt2Me2)(4)(O)Cl][BAr4F] were isolated. DFT calculations reproduce the greater ability of 3 to bind smafl molecules and show relative binding strengths that follow the trend CO >> O-2 > N-2 > H-2.