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Thermolysis of [Ru(AsPh3)3(CO)H2] with the N-aryl heterocyclic carbenes (NHCs) IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) or the adduct SIPr·(C6F5)H (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene), followed by addition of CH2Cl2, affords the coordinatively unsaturated ruthenium hydride chloride complexes [Ru(NHC)2(CO)HCl] (NHC = IMes 1, IPr 2, SIPr3). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes 4–6. Reduction of 1–3 and [Ru(IMes)(PPh3)(CO)HCl] (7) with NaBH4 yields the isolable borohydride complexes [Ru(NHC)(L)(CO)H(η2-BH4)] (8–11, L = NHC, PPh3). Both the bis-IMes complex 8 and the IMes–PPh3 species 11 react with CO at low temperature to give the η1-borohydride species [Ru(IMes)(L)(CO)2H(η1-BH4)] (L = IMes 12, PPh3), which can be spectroscopically characterised. Upon warming to room temperature, further reaction with CO takes place to afford initially [Ru(IMes)(L)(CO)2H2] (L = IMes, L = PPh314) and, ultimately, [Ru(IMes)(L)(CO)3] (L = IMes 13, L = PPh315). Both 8 and 11 lose BH3 on addition of PMe2Ph to give [Ru(IMes)(L)(L′)(CO)H2](L = L′ = PMe2Ph; L = PPh3, L′ = PMe2Ph). Compounds 1–4 and 8–11 have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of iPrOH and H2. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr > IMes >> SIMes.