High-yield preparations have been developed for 16-e square-pyramidal complexes of ruthenium(II) RuCl(CO) [2, 2 PBu t 2 ) 2 C 6 H 3 ] (1), RuHCl[1,3-(CH 2 PBu t 2 ) 2 C 6 H 4 ] (2), and RuHI[1,3-(CH 2 PBu t 2 ) 2 C 6 H 4 ] (3). NMR and X-ray studies have revealed strong η 2 CH bonding in the Ru[1,3-(CH 2 PBu t 2 ) 2 C 6 H 4 ] fragment of 2 and 3. These complexes show stretched agostic C-H bonds, elongated ipso-C-Ru distances, reduced 1 J CH couplings, and large IPR shifts in 1 H NMR. A consistent picture has emerged of a stronger agostic bonding and greater C-H bond activation in the η 2 C-H trans to chloride in 2. This finding is attributed to stronger π-donor properties of chloride compared to iodide and increased back-donation into the antibonding orbital of the η 2 C-H. Analysis of the X-ray and NMR T 1 data has also indicated the presence of an attractive interaction, dihydrogen bonding, between the agostic C-H hydrogen and neighboring hydride in 2 and 3.

Novel bis(phosphine) pincer complexes of iridium based on ferrocene and ruthenocene have been synthesized. The reaction of 1,3-bis((di-tert-butylphosphino)methyl)metallocenes [t-BuP,CH,PM] (M = Fe, 10; M = Ru, 13) with [Ir(COE)2Cl]2 in refluxing toluene leads to the corresponding chloro-hydrido pincer complexes as a mixture of H-endo- (M = Fe, 11; M = Ru, 14) and H-exo-IrH(Cl)[t-BuP,C,PM] (M = Fe, 12; M = Ru, 15) isomers. Treatment of compounds 11, 12 and 14, 15 with NaH in hot cyclooctane generates corresponding dihydrido complexes IrH2[t-BuP,C,PM] (M = Fe, 6; M = Ru, 7). Reactivity of the chelated iridium atom in hydrido-iridium pincer complexes and oxidation centered at the iron atom of ferrocene-based pincer complexes were studied. Complexes H-exo-IrH(Cl)(CO)[t-BuP,C,PFe] (20), Ir(CO)[t-BuP,C,PFe] (21), and Ir(CO)[2,6-(tBu2PO)2C6H3] (30) were characterized by X-ray crystallography. On the basis of νCO values for carbonyl iridium derivatives, electronic characteristics of the corresponding fragments {Ir[t-BuP,C,PM]} n (n = 0, +1) were elucidated. Complexes 6 and 7 reveal unprecedented catalytic activity in cyclooctane dehydrogenation in the presence of tert-butylethylene as a hydrogen acceptor:  turnover numbers 3300, 2571, and 1843 were obtained for 6, 7, and the known complex IrH2[2,6-(tBu2PO)2C6H3] (4c), respectively, at 180 °C for 8 h. The high catalytic activity for 6 and 7 and the difference in the catalytic activity of these “isostructural” complexes are discussed in terms of steric and electronic effects.

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