Formation of Bridging Acylium and Nitrilium Complexes by Reaction of CO and CNC(CH3)3 with a Bridging Diiron Methylidyne Complex. Evidence for Strong Elelctron Donation from the Fe2C Core onto the μ-CHCO and μ-CHCNR Ligands

Article abstract of DOI:10.1021/ja00226a023

The reaction of the μ-methylidyne complex 2(μ-CO)(μ-CH)⁺PF6^- (1) with CO gave the cationic 1:1 adduct 2(μ-CO)(μ-CHCO)⁺PF6^- (2) in 90 percent yield.The structure, spectra, and chemical properties of 2 suggest that the bonding of the μ-CHCO ligand in 2 should be regarded as analogous to that in organic acylium cations, with a contributing formulation as a two-electron three-center bound-bridging ketene. 2 reacted with nucleophiles at the acylium carbon; reaction with Et4N⁺HFe(CO)4^- gave the aldehyde 2(μ-CO)(μ-CHCHO) (3), water gave the carboxylic acid 2(μ-CO)(μ-CHCO2H) (4), and ammonia gave the amide 2(μ-CO)(μ-CHCONH2) (5). 1 also reacted with CNC(CH3)3 to give the 1:1 adduct 2(μ-CO)<μ-CHCNC(CH3)3>⁺PF6^- (6), the structure of which was determined by X-ray crystallography; 6 crystallizes in the monoclinic space group P2₁/c, with unit cell constants a = 10.771 (2) Angstroem, b = 11.420 (2) Angstroem, c = 18.092 (3) Angstroem, β = 106.09 (1) degree, and Z = 4.The μ-ethylidyne complex 2(μ-CO)(μ-CCH3)⁺BF4^- (7) underwent a similar reaction with CNC(CH3)3 to afford the adduct 2(μ-CO)<μ-C(CH3)CNC(CH3)3>⁺BF4^- (8), which was isolated as a mixture of two cis and one trans Cp isomers.The structure of 6, together with the spectroscopic properties of 6 and 8, suggests an analogous bonding mode to that in 2.Fenske-Hall molecular orbital calculations performed on 2 and 6 provide further support for the importance of a ketene-like formulation that contributes to the overall structure of these complexes.