Magnesium reduction of cis,cis,trans-RuCl2(CO)(2)L(2) (L = P(t)Bu(2)Me) yields isolable Ru(CO)(2)L(2), shown by solution spectroscopies and X-ray diffraction to have trans phosphines but cis carbonyls, in a nonplanar structure which resembles a trigonal bipyramid with one equatorial ligand missing. This unusual geometric structure is traced by ab initio (MP2) study to enhanced back-donation to CO by zero-valent Ru. This molecule reacts in time of mixing to add CO, MeNC, O-2, CS2, C2H4, or PhC=CPh. Rapid oxidative addition occurs with H-2, HCl, Cl-2, and PhC=CH. Oxidative addition is slower with MeCl, Me(3)SiH, and MeOH, which leads to more complicated reaction schemes. Reaction with PPh(2)H gives not oxidative addition but addition and displacement, yielding Ru(CO)(2)-(PPh(2)H)(2)(P(t)Bu(2)Me) and equimolar free P(t)Bu(2)Me. Magnesium reduction of cis,cis,trans-RuCl2(CO)(2)L’(2) proceeds analogously for L’ = (PPr3)-Pr-t, but for L’ = PPh(3), decomposition and ligand scavenging give Ru(CO)(2)(PPh(3))(3). Reduction of cis,trans-RuCl2(CO)(CNMe)L(2) gives the product of oxidative addition of a (t)Bu C-H bond : RuH(CO)(CNMe)-[eta(2)-P(CMe(2)CH(2))(t)BuMe]L, showing the influence of electron density at unsaturated Ru(O) on its persistence.