Chemistry of highly electrophilic binuclear cations. 1. Oxidation reactions of [M-2(eta(5)-C5H5)(2)(CO)(4)(mu-Ph2PCH2PPh2)] (M = Mo, W) with [FeCp2]X(X = BF4, PF6)

Oxidation of the title compounds with 2 equiv of [FeCp2]BF4 in dichlorometh ane leads to the tetracarbonylic fluoro complexes [M2Cp2(mu-F)(CO)(4)(mu-dp pm)]BF4 in high yields (dppm = Ph2PCH2PPh2). By contrast, the analogous rea ction with [FeCp2]PF6 gives the tricarbonylic fluoro derivatives [M2Cp2(mu- F)(mu-CO)(CO)(2)(mu-dppm)]PF6. Separate experiments revealed that the latte r cations cannot be obtained through decarbonylation of the former fluoro c omplexes. By carrying out the [FeCp2]PF6 oxidations in the presence of hali de 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 ar e 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 t he title compounds. Despite its high reactivity, the tungsten dication can be isolated as a solid, thanks to its low solubility in dichloromethane, an d has been shown to react with other donor molecules such as acetate ions o r methanol, to give tricarbonylic derivatives [W2Cp2(mu-Y)-(mu-CO)(CO)(2)(m u-dppm)]PF6 (Y = O2CMe, OMe), which display structures comparable to those of the corresponding halogeno complexes. The structure of both tungsten flu oro complexes has been determined by X-ray diffraction methods, that on the tetracarbonylic compound revealing the presence of a weak II-bonding inter action between the BF4- counterion and a methylenic hydrogen in the diphosp hine 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.