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 , IPr , SIPr ). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes . Reduction of and [Ru(IMes)(PPh3)(CO)HCl] () with NaBH4 yields the isolable borohydride complexes [Ru(NHC)(L)(CO)H(eta2-BH4)] (, L=NHC, PPh3). Both the bis-IMes complex and the IMes-PPh3 species react with CO at low temperature to give the eta1-borohydride species [Ru(IMes)(L)(CO)2H(eta1-BH4)] (L=IMes , 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=PPh3) and, ultimately, [Ru(IMes)(L)(CO)3] (L=IMes , L=PPh3). Both and lose BH3 on addition of PMe2Ph to give [Ru(IMes)(L)(L')(CO)H2](L=L'=PMe2Ph; L=PPh3, L'=PMe2Ph). Compounds and have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of (i)PrOH and H2. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr>IMes>>SIMes.

The reactions of tcc-Ru(IMes) 2 (AsPh 3 )(CO)H 2 (1, IMes ) bis (1,3-(2,4,6-trimethylphenyl)imidazol-2ylidene)) with HX substrates (X ) OH, OEt, SH, S n Pr) have been reinvestigated and shown to lead directly to the formation of the 16-electron species Ru(IMes) 2 (CO)(X)H (4-X). The fluoro analogue Ru-(IMes) 2 (CO)(F)H (4-F) has also been synthesized, and X-ray and neutron diffraction studies show that this exhibits a square-pyramidal geometry with hydride in the axial site. Density functional calculations have been performed on one possible mechanism for the formation of 4-X from 1 with various HX (X ) F, Cl, OH, SH, NH 2 , PH 2 , CH 3 , and SiH 3 ), involving initial AsPh 3 /HX substitution followed by H-transfer to hydride and H 2 loss. With X ) SH, H-transfer in both tcc-Ru(IMes) 2 (CO)(H 2 S)(H) 2 and ttt-Ru(IMes) 2 -(CO)(H 2 S)(H) 2 was considered and shown to be kinetically accessible and thermodynamically favored, suggesting that such dihydrides should not be stable with respect to this step. The calculations indicate that the ease of formation of 4-X becomes more kinetically and thermodynamically favored according to the trends F > OH > NH 2 > CH 3 and Cl > SH > PH 2 < SiH 3 , with the reactions of second-row HX substrates being more favored than the first-row analogues. Calculated reaction exothermicities allow the derivation of relative Ru-X bond strengths in 4-X, and comparison with experimentally determined M-X relative bond strengths in the literature highlights the importance of X f M π-donation in determining trends in M-X bond dissociation energies in unsaturated systems.

Addition of 1 atm of H(2)S to [Ru(IMes)(2)(CO)(EtOH)H(2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) affords the air-stable hydrogen sulfide dihydride complex, [Ru(IMes)(2)(CO)(H(2)S)H(2)] 2. Treatment of 2 with excess H(2)S leads to formation of the 16-electron bis-hydrosulfido complex, [Ru(IMes)(2)(CO)(SH)(2)] 3. Preliminary studies show that 3 reacts with both H(2) and CO in solution as well as in the solid state. Both 2 and 3 have been structurally characterized