Agostic bonding in pincer complexes of ruthenium

High-yield preparations have been developed for 16-e square-pyramidal complexes of ruthenium(II) RuCl(CO)[2,6-(CH2PBu2t)(2)C6H3] (1), RuHCl[1,3-(CH2PBu2t)(2)C6H4] (2), and RuHI[1,3-(CH2PBu2t)(2)C6H4] (3). NMR and X-ray studies have revealed strong eta(2) CH bonding in the Ru[1,3-(CH2PBu2t)(2)C6H4] 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 H-1 NMR. A consistent picture has emerged of a stronger agostic bonding and greater C-H bond activation in the eta(2) C-H trans to chloride in 2. This finding is attributed to stronger pi-donor properties of chloride compared to iodide and increased back-donation into the antibonding orbital of the eta(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.