No abstract

We have argued1 that potential ir-donor ligands, X, when attached to a 15-electron fragment ML" can confer stability by forming not only an M-X tr-bond but also an X-*-M ir-bond via lone pairs on X. We have offered structural evidence of this for the d6 species Ir(H)2(ORf)(PCy3)2' and Cp*Ru(ORf)(PCy3)2 (Cp* = CsMes, Rf = CH2CF3). Both molecules nevertheless rapidly add nucleophiles under the mildest of conditions, so that any M/X ir-donation (and thus multiple bonding) still leaves them operationally unsaturated.We now report a new example of this "ir-stabilized unsaturated" phenomenon in the class of molecules RuH(X)(CO)P2 (P = Pl-Bu2Me). These are synthesized from RuH(C1)(CO)P23 by metathesis using Nal, KOPh, LiNHPh, KOH, KOCH2CF3, KO-SiPh3> KOSiMe2Ph, and KOSiMe3. The structure4 of the OSiPh3 representative (Figure 1) shows it to have a square-pyramidal geometry with the bulky phosphines nearly linearly arrayed, but the CO and siloxide ligands bent toward one side of the basal plane, presumably away from the hydride ligand.The CO ligand trans to the group X serves as a spectroscopic indicator of the availability of electron density at the RuH(X)P2 fragment: greater back-bonding, from a more electron-rich metal center, will lower the CO stretching frequency in the infrared spectrum.5 These data (Table I) are generally low (< 1900 cm-1) and reveal Nand O-based ligands to be better donors than halide. Note also that the donor power of the groups OSiPhxMe3_x increases as x decreases. The CH2CF3 group is less electron

and give RuH(C2Ph)(CO)P2. The CO stretching frequency of this product shows evidence of ir donation from acetylide. The large downfield 13C chemical shift of Ca in this compound may have the same cause. A labeling study shows that the proton eliminated in HX is that of the PhC2H. Reaction of RuHF(CO)P2 with HSiMe3 yields FSiMe3 and RuH"(SiMe3)(CO)P2 (n = 1 and 3). Reaction of RuH(C2Ph)(CO)P2 with equimolar PhC2H gives products of Ru-H addition with both regiochemistries. Reaction with excess PhC2H gives Ru(C2Ph)2(CO)P2, whose structure is proposed on the basis of variable-temperature 31P and 13C NMR studies. Reaction of these Ru-H bonds with D2 or alkyne must occur cis to H (i.e., between H and X). Such attack stereochemistry is made easier when the opening of the HRuX angle is energetically facile. Ab initio SCF calculations show that the opening of the HRuX angle is easier for a heavier halide, which accounts for the higher reactivity observed for the iodide derivative. An internal competition experiment shows that PhC2H reacts faster with RuHX(CO)P2 when X = C2Ph than when X = Cl. Underlying this complex reactivity is the fact that RuHX(CO)P2 is a multifunctional reagent.

The crystal structures of Cu(PCy3)2I and [Cu(PCy3)I]2 are reported in order to furnish data to help understand the variation of Cu/Cu distances with changing halide in planar structures of the type LnCuKjt-X^Cu'L* (n * 1, 2). It is found that iodide shows a shorter Cu/Cu separation (2.89 A) than does chloride (3.07 A) in [Cu(PCy3)X]2 species. An extended Huckel analysis of the bonding in these molecules indicates the Cu* 1 11s and p orbitals to be most important in bonding and shows larger Cu/Cu overlap populations for the stronger donor (iodide over chloride). This is traced to weak a and tt Cu/Cu bonding interactions. Such bonding interaction is diminished when the terminal phosphine ligand is replaced by a T-donor (halide), in agreement with literature data for CU2X42" species.Analogous weakening is effected by addition of another terminal ligand to each copper (i.e., I^Cu^-X^Cu'I^).The geometry within the Cu(m-X)2Cu rhombus is shown to exist in a broad potential energy well, and diminished Cu/Cu interactions (i.e., longer Cu/Cu distances) are compensated by improved interaction of halide orbitals with in-plane out-of-phase Cu/Cu orbitals. This explains the surprisingly large in-plane distortions observed for Cu2X42species in solids containing various cations and in L2Cu(m-X)2CuL2. Crystallographic data: for Cu(PCy3)2l (at -174 °C), a = 9.634 (2) A, b = 22.975 (5) A, c = 9.058 (2) A, a = 97.38 (1)°, 0 = 114.49 (1)°, and y * 93.40 (1) °with Z = 2 in space group Pi;for [Cu(PCy3)I]2 (at -172 °C), a = 9.757 (2) A, b = 12.780 (3)_A, c -8.808

For the species RuHXP2(CO), IrH2XP2, and Cp*RuXP (P = bulky phosphine, X = halide or pseudohalide), bothM]'X) are found to be quite rapid. In addition, hydride exchange occurs for RuHCl-(CO)P2 and RuDC1(CO)P'2, as well as for IrH2Cl(P'Bu2Ph)2 and IrD2Cl(PtBu2Me)2. Exchange is generally faster for halides than for hydrides yet is much slower for the groups phenoxide, OSiPh3, and C2Ph. These equilibria favor the better donating halide being bonded to the less electron-rich metal center. A variable-temperature NMR study of the degenerate exchange Cp*Ru(P*Bu2Me)Cl + Cp*Ru'(P*Bu2Me)Br <=t Cp*Ru(P'Bu2Me)Br + Cp*Ru'-(PtBu2Me)Cl establishes a second-order rate law with AH* = 8.6 ± 0.8 kcal/mol and AS* = -20 ± 3 cal/(mol K). These results clearly indicate the transient existence in solution of halideand/or hydride-bridged dimers of monomeric metal complexes.