The reactions of tcc-Ru(IMes)2(AsPh3)(CO)H 2 (1, IMes = bis(l,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene)) with HX substrates (X = OH, OEt, SH, SnPr) 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, PH2, CH3, and SiH3), involving initial AsPh3/HX substitution followed by H-transfer to hydride and H2 loss. With X = SH, H-transfer in both tcc-Ru(IMes) 2(CO)(H2S)(H)2 and ttt-Ru(IMes) 2-(CO)(H2S)(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 > NH2 > CH3 and Cl > SH > PH2 < SiH3, 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 ? M p-donation in determining trends in M - X bond dissociation energies in unsaturated systems. © 2006 American Chemical Society.