Structural effects of carbon monoxide coordination to carbon centers.: π and σ bindings in aliphatic acyl versus aromatic aroyl cations

The binding of carbon monoxide to carbon centers has been examined with two series of aromatic and aliphatic oxocarbonium ions that are successfully isolated as crystalline and highly reactive ( hygroscopic) aroylium and acylium salts with poorly coordinating counteranions. X-Ray crystallographic analyses at - 150 degreesC afford precise structural parameters for the characteristic linear carbonyl bond (r(CO)) and the bond to the carbon centers (r(Calpha)). The correlations of these structural parameters evaluated for alkyl ( Me, Et and i-Pr) and aryl (p-Me, 2,4,6-trimethyl, p-MeO and p-fluorophenyl) oxocarbonium ions with the corresponding carbonyl stretching frequencies in the solid-state ( reflectance) IR spectra yield valuable insight into the binding mode of carbon monoxide. Most noteworthy is the synergic (pi-sigma) bonding in aroylium structures in contrast to the mainly sigma bonding in acylium structures that are organic mimics for carbon monoxide bonding in classical and nonclassical metal carbonyls, respectively.