The structure of five-coordinate Ru(II) complexes RuHCl(CO)(P i Pr 3) 2 , 1, RuCl 2 (CO)(P i Pr 3) 2 , 2, and Ru(Ph)Cl(CO)(P t Bu 2 Me) 2 , 12, are reported. All three of these complexes have square-based pyramid geometry with the strongest σ-donor ligand trans to the vacant site. These 16-electron complexes do not show bona fide agostic interactions. This is attributed to the strong trans influence ligand (H, CO, and Ph) and π-donation of the Cl, which is further supported by the fact that two agostic interactions are present in the Clremoval product of 12, i.e., the four-coordinate [RuPh(CO)L 2 ]BAr′ 4 (L) P t Bu 2 Me, Ar′) 3,5-C 6 H 3 (CF 3) 2), 16. Structural comparison of 16 and 12 reveals that removal of Cldoes not change the remaining ligand arrangements but creates two low-lying LUMOs for agostic interactions, which persist in solution as evidenced by IR spectroscopy. Reactions of 16 with E-H (E) B, C(sp)) bonds cleave the Ru-Ph bond and form Ru-E/H bonds by different mechanisms. The reaction with catecholborane gives [RuH(CO)L 2 ]BAr′ 4 , which further reacts with catecholborane to give [Ru(BR 2)(CO)L 2 ]BAr′ 4. However, the reaction with Me 3 SiCCH undergoes a multistep transformation to give a PhCCSiMe 3-and Me 3 SiCCH-coupled product, the mechanism of which is discussed. Reaction of RuCl 2 (CO)L 2 with 1 equiv MeLi affords RuMeCl(CO)L 2 , 5, which further reacts with MeLi forming RuMe 2 (CO)L 2 , 7. Variable-temperature 13 C{ 1 H} NMR spectra reveal the two methyls in 7 are inequivalent and exchange by overcoming an energy barrier of 6.8 kcal/mol at-30°C. The chloride of 5 can be removed to give [RuMe(CO)L 2 ]BAr′ 4 .

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In situ reduction of (bpy)Re(CO) 3 Cl, bpy = 2,2Ј-bipyridine, at a platinum cathode under CO 2 atmosphere in acetonitrile or acetone has produced a series of distinct EPR spectra which are attributed to (bpy Ϫ ) containing species and which invariably reveal the interaction of the unpaired electron with the metal nuclei ( 185,187 Re, I = 5/2). The EPR investigation of various model complexes (bpy Ϫ )Re(CO) 3 X, generated by electrochemical reduction under argon of diamagnetic precursors (bpy)Re(CO) 3 X, X = Cl Ϫ , CF 3 SO 3 Ϫ , CH 3 O Ϫ , H Ϫ , tetrahydrofuran, CH 3 CN, CO, HCO 2 Ϫ , HCO 3 Ϫ and CH 3 C(O) Ϫ has been used to interpret the results from the reactions under CO 2 atmosphere. Paper 7/02467C

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