First direct structural comparison of complexes of the same metal fragmentto ketenes in both C,C- and C,O-bonding modes

Using a series of Ir(I) and Rh(I) ketene complexes, conclusions about the s tructure and bonding of complexes of the fundamentally important ketene lig and class are reached. In a unique comparison of X-ray structures of the sa me metal fragment to ketenes in both the eta (2)-(C,C) and the eta (2)-(C,O ) binding mode, the Ir-Cl bond distances in complexes of trans-Cl(Ir)[P(i-P r)(3)](2) to phenylketene [4, eta (2)-(C,C)] and diphenylketene [2a, eta (2 )-(C,O)] are 2.371(3) and 2.285(2) Angstrom, respectively. This would be co nsistent with greater trans influence of a ketene ligand bound to a metal t hrough its C double bondC bond than one connected by its C double bondO bon d. Back-bonding of Ir(I) and Rh(I) to diphenylketene was assessed using tra ns-CI(M)[P(i-Pr)(3)](2)[eta (2)-(C,O)-diphenylketene] (2a and 2d). Most bon d lengths and angles are identical, but slightly greater back-bonding by Ir (l) is suggested by the somewhat greater deformation of the ketene C double bondC double bondO system [C-C-O angles are 136.6(4) and 138.9(4) in the I r and Rh cases 2a and 2d, respectively]. Syntheses of new labeled ketenes P h2C double bond C-13 double bondO and Ph2C double bondC double bond O-18 an d their Ir(I) and Rh(I) complexes are reported, along with the generation o f an Ir(l) complex of PhCH double bond C-13 double bondO. The effects of is otopic substitution on infrared absorption data for ketene complexes are pr esented for the first time. Preliminary normal coordinate mode analysis all owed definitive assignment of absorptions ascribed to the C-O stretching fr equencies of coordinated ketenes, which are near the absorptions for aromat ic ring systems commonly found as substituents on ketenes. For free dipheny lketene and four of its complexes and a phenylketene complex characterized by X-ray diffraction, the magnitude of the C-13-C-13 coupling between the t wo ketene carbons is correlated to carbon-carbon bond distance.