The relative electron-donating ability of X in the new five-coordinate RuHX(CO)(PtBu2Me)2 (X = I, Br, Cl, F, OPh, OH, OCH2CF3, OEt, OCPh3, OB(Mesityl)2, OSiR3, NHPh, SPh, C2Ph) has been evaluated based on the CO stretching frequency. In all cases, the CO frequency is lower than that of the free CO and the reduction increases in the order I < Br < Cl < F < alkoxide, with the ethoxide inducing the largest shift. NHPh behaves as OPh and SPh appears at a higher frequency than OPh. Similar measurements have been conducted on the six-coordinate pyridine adducts and a similar ranking of CO frequencies is obtained, but all frequencies are shifted lower. Hydride is the weakest of all donors. There can be no conventional (two-center) Ru-X pi-bonding in these pyridine adducts because the Ru(d)/X(p(pi)) interactions are two-orbital/four-electron ones and thus are net destabilizing. However, the CO pi* orbitals interact to stabilize the Ru-X pi* orbital, thereby retaining some degree of net Ru-X pi bonding. The structure of RuH(OSiPh3)(CO)(PtBu2Me)2 is shown to be square pyramidal with H at the apical site of the pyramid and the siloxy group trans to CO. EHT and core-potential ab initio calculations (full optimization at the HF and partial optimization at the MP2 level) have been performed to determine the structure of RuHX(CO)(PH3)2 (X = F, Cl, OH, OCH3, OSiH3 with d orbitals on the Si atom). The square pyramid is preferred for all X. This structure permits optimal push-pull effects between the pi-donating X group and the pi* of CO. The CO frequency has been calculated at the MP2 level, and the ranking is identical to the experimental ones. Pi effects are shown to be larger for alkoxy than for halide, but the variation in pi-effects alone is not sufficient to account for the ranking of the CO frequencies down a column of the periodic table; the sigma effect is also involved. In particular, it is shown that strongly ionic Ru-X bonds lead to lower CO frequency. This last effect causes large changes in the F-19 chemical shift for RuH(F)(CO)(PtBu2Me)2 species upon adding a sixth ligand to Ru. It also causes strong hydrogen bonding of fluoride to added alcohol. In spite of Ru/X multiple bonding in the five-coordinate species, they are Lewis acidic toward amines and phosphines. Crystal data for RuH(OSiPh3)(CO)(PtBu2Me)2 at -160-degrees-C : a = 11.263(1) angstrom, b = 31.713(4) angstrom, c = 22.311(3) angstrom, and beta = 100.07(0)degrees with Z = 4 in space group P2(1)/n.