IS PI-BACK-BONDING IMPORTANT FOR SIGMA-BOUND ALDEHYDE AND KETONE COMPLEXES - SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF AROMATIC ALDEHYDECOMPLEXES OF THE [CPFE(CO)(2)](+) CATION

The synthesis and characterization of a series of organometallic compl exes, [CpFe(CO)(2)-(O=C(H)Ar)]PF6 (Ar = Ph (1), p-MeC(6)H(4) (2), p-NM e(2)C(6)H(4) (3), p-OMeC(6)H(4) (4), p-CF3C6H4 (5), p-CIC6H4 (6), p-(d imethylamino)cinnamaldehyde (7); Cp = eta(5)-cyclopentadienyl) are pre sented. Oxidation of [CpFe(CO)(2)](2) with AgPF6 or [Cp(2)Fe]PF6 in th e presence of a 5-fold excess of aromatic aldehyde led to the formatio n of 1-7 in good to excellent yields (especially with the latter oxida nt). Compounds 1-7 were analyzed by a combination of H-1 NMR (solution ) and FTIR spectroscopic (solid and solution state) methods. Single-cr ystal X-ray diffraction studies on 6 and [CpFe(CO)(2)(O=C(H)-p-OMeC(6) H(4))]SbF6 (8) were also performed. Binding of these aldehydes to [CpF e(CO)(2)](+) in solution and the solid state occurs exclusively throug h oxygen a-donor interactions. Structural evidence for pi-back-bonding in sigma-bound aldehyde complexes is presented. These solid-state eff ects, however, do not manifest themselves in the solution equilibrium binding constants for this class of substrates.