The compounds Cp*RuLX (L = P(i)Pr2Ph, PCy3; X = Cl, Br, I, OCH2CF3, OSiPh3, OSiMe2Ph, NHPh), as well as their CO adducts, have been studied by NMR, IR, and X-ray diffraction in order to understand the nature of the Ru-X bond. Adducts were also formed with C2H4. Cp*Ru(P(i)Pr2Ph)(OCH2CF3) reacts with MeI (but not PhI) to give Cp*Ru(P(i)Pr2Ph)I and MeOCH2CF3. The compounds Cp*RuLX have a mirror-symmetric structure with Ru, L, X, and the Cp* center coplanar. This is understood, using extended Huckel calculations, to originate in the presence of a Ru-X partial pi bond which raises the LUMO. This increases the HOMO-LUMO gap and thus disfavors pyramidalization at the metal center. Calculations of the behavior of systems with X = pure sigma donor, pi donor, and pi acceptor support this analysis. The LUMO of Cp*RuLX still lies low enough to give visible color and to allow rapid addition of (small) Lewis bases. The Nu(CO) values of the CO adducts show the X ligand donor power to vary in the order OSiMe2Ph > NHPh > OSiPh3 > OCH2CF3 >> Cl > Br > I. Comparison of the Ru-X bond lengthening upon addition of CO is also consistent with this ranking. The lack of facile beta-hydrogen migration when X = OCH2CF3 is discussed. Crystallographic data : for Cp*Ru(PCy3)(OSiPh3) at -160-degrees-C, a = 12.131(2) angstrom, b = 17.563(3) angstrom, c = 10.703(2) angstrom, alpha = 102.59(1)-degree, beta = 110.02(1)-degree, gamma = 75.87(1)-degree with Z = 2 in space group P1BAR; for Cp*Ru(PCy3)CO(OSiPh3) at -178-degrees-C, a = 11.331(3) angstrom, b = 17.840(4) angstrom, c = 21.813(5) angstrom, and beta = 97.53(1)-degree with Z = 4 in space group P2(1/n); for Cp*RuI(P(i)Pr2Ph) at -172-degrees-C, a = 8.907(1) angstrom, b = 15.205(2) angstrom, c = 17.065(2) angstrom, and beta = 100.62(1)-degree with Z = 4 in space group P2(1/n); for Cp*Ru(PCy3)(OCH2CF3) at -173-degrees-C, a = 12.640(3) angstrom, b = 14.140(3) angstrom, c = 17.271(4) angstrom, and beta = 101.79(1)-degree with Z = 4 in space group P2(1/c); for Cp*Ru(PCy3)CO(OCH2CF3) at -172-degrees-C, a = 9.607(2) angstrom, b = 19.704(3) angstrom, c = 16.370(3) angstrom, and beta = 96.55(1)-degree with Z = 4 in space group P2(1/c).