Chemistry of Highly Electrophilic Binuclear Cations. 1. Oxidation Reactions of [M₂(η⁵-C₅H₅)₂(CO)₄(μ-Ph₂PCH₂PPh₂)] (M = Mo, W) with [FeCp₂]X (X = BF₄, PF₆)

Oxidation of the title compounds with 2 equiv of [FeCp2]BF4 in dichloromethane leads to the tetracarbonylic fluoro complexes [M2Cp2(mu-F)(CO)(4)(mu-dppm)]BF4 in high yields (dppm = Ph2PCH2PPh2). By contrast, the analogous reaction with [FeCp2]PF6 gives the tricarbonylic fluoro derivatives [M2Cp2(mu-F)(mu-CO)(CO)(2)(mu-dppm)]PF6. Separate experiments revealed that the latter cations cannot be obtained through decarbonylation of the former fluoro complexes. By carrying out the [FeCp2]PF6 oxidations in the presence of halide ions X- (X = Cl, Br, I), the corresponding halo derivatives [M2Cp2(mu-X)(mu-CO)(CO)(2)(mu-dppm)]PF6 are formed in good yields. All above species are derived from the unsaturated dications [M2Cp2(mu-CO)(2)(CO)(2)-(mu-dppm)](2+), which are the initial products of the removal of two electrons from the title compounds. Despite its high reactivity, the tungsten dication can be isolated as a solid, thanks to its low solubility in dichloromethane, and has been shown to react with other donor molecules such as acetate ions or methanol, to give tricarbonylic derivatives [W2Cp2(mu-Y)-(mu-CO)(CO)(2)(mu-dppm)]PF6 (Y = O2CMe, OMe), which display structures comparable to those of the corresponding halogeno complexes. The structure of both tungsten fluoro complexes has been determined by X-ray diffraction methods, that on the tetracarbonylic compound revealing the presence of a weak II-bonding interaction between the BF4- counterion and a methylenic hydrogen in the diphosphine ligand. The reaction pathways likely operative in the oxidation of the title compounds are analyzed in light of the experimental findings and the critical role played by the BF4- or PF6- counterions.