Is pi-donation the only way? Unprecedented unsaturated Ru(II) species devoid of pi-donor ligands

Reaction of RuHX(CO)L-2 (L = (PBu2Me)-Bu-t) with (BuLi)-Bu-t in pentane or toluene at -40 degrees C gives Ru(H)(2)(CO)L-2 as a reactive and thermolabile square-pyramidal species with inequivalent hydrides (one apical). This molecule forms 1:1 adducts with N-2, H-2, PH2Ph, PHPh2 or PHCy2, and forms Ru(H)(2)(CO)(3)L-2 then Ru(CO)(3)L-2 with CO. Oxidative addition of the H-C bond of HC2Ph the H-Si bonds of SiPh2H2 and SiMe3H and the H-O bond of H2O occurs with elimination of H-2, to give RuH(C2Ph) (CO)L-2, RuH(SiR3) (CO)L-2 and RuH(OH)(CO)L-2, respectively. Ru(H)(2)(CO)L-2 reacts with MeI to give RuMeI(CO)L-2 and RuHI(CO)L-2. Above -40 degrees C, Ru(H)(2)(CO)L-2 hydrogenates isobutylene and subsequently metallates one Bu-t group of its phosphine, to give RuH(CO)L(P similar to C). This strained molecule reacts with arenes to give RuH(aryl) (CO)L-2. Reaction of RuHCl(CO)L-2 with PhLi provides an alternative synthesis of RuHPh(CO)L-2, which rapidly (similar to 2 h) exchanges its H and Ph groups with C6D6 or with toluene. Reaction of RuHPh(CO)L-2 with CO gives the much less reactive RuH(Ph)(CO)(2)L-2, while RuHPh(CO)L-2 reacts with MeI to give RuMeI(CO)L-2 and with EtBr to give first Ru(Et)Br(CO)L-2, then RuHBr(CO)L-2 and ethylene. N-chlorosuccinimide converts RuHPh(CO)L-2 into RuClPh(CO)L-2. On a timescale of 2 days, RuH(aryl) (CO)L-2, in arene solvent, rearranges to Ru(eta(6)-arene) (CO)L and free L. The structural and electronic properties of the family of unsaturated RuXH(CO) (PH3)(2) (X = H, SiH3, CCH, F, Cl, Br, OH, OMe) complexes have been analyzed by core potential ab initio methods at the MP2 level. The preferred structure for each member of this family is calculated to be square-pyramidal with the strongest a-donor ligand (H or SiH3) at the apical site. Powerful sigma-donating groups (i.e., ligands with a strong trans influence: H or SiR3) are found to be very efficient at compensating the electron deficiency at the metal. A pi-donating ligand occupies a basal site, trans to the CO group. Due to the lack of a low-lying empty metal d(pi) orbital (i.e., the molecule is a sigma Lewis acid), rr effects are weaker at stabilizing the unsaturation: a push-pull interaction involving the p lone pair(s) of X, the occupied d metal orbital and the pi* (CO) orbitals constitutes an additional, but secondary, stabilizing factor. This explains why Ru(H)(2)(CO)L-2 and RuH(SiR3) (CO)L-2 are both observable species. The calculated Ru-N bond dissociation energy of RuXH(NH3) (CO) (PH3)(2) confirms the dominant role of the sigma-donation of the ligands, especially that of the ligand trans to NH3. Thus, compounds of the type RuH(X) (CO) (PR3)(2) are better regarded as primarily 'sigma-stabilized' 16-electron species whose properties are then finely tuned by pi effects.