The new complexes [W2Cp2(CO)4(μ-L2)] (Cp = η5-C5H5; L2 = Ph2PCH2PPh2 (dppm), Me2PCH2PMe2 (dmpm)) have been prepared from [W2Cp2(CO)4] and the corresponding diphosphine at room temperature. Decarbonylation of the dppm complex in refluxing n-octane gives the triply bonded compound [W2Cp2(CO)2(μ-dppm)] as the major product, along with a small amount of the oxo complex [W2Cp2(μ-CH2PPh2)(O)(μ-PPh2)(CO)], which results from a P−C(sp3) cleavage in the dppm ligand. Both compounds (as their dibenzene and toluene solvates, respectively) have been characterized through single-crystal X-ray studies. The analysis of the dicarbonyl complex, carried out at 200 K, reveals the presence of linear semibridging carbonyls experiencing an incipient dynamic disorder that at higher temperatures might be observed as a fully developed disorder of each carbonyl between its two asymmetric dispositions. The latter is consistent with a structure determination carried out previously on the same compound at 291 K and with its dynamic behavior in solution. This air-sensitive complex adds oxygen readily to give the W(I)−W(V) oxo derivative [W2Cp2(O)2(CO)2(η1-dppm)]. Photochemical decarbonylation of [W2Cp2(CO)4(μ-L2)] proceeds via the hydrido cyclopentadienylidene complexes [W2(μ-η1:η5-C5H4)Cp(μ-H)(CO)3(μ-L2)] to finally give the corresponding dicarbonyls [W2Cp2(CO)2(μ-L2)]. The latter react readily with tBuNC at room temperature or below to give [W2Cp2(μ-η1:η2-CNtBu)(CO)2(μ-L2)], which, when L2 = dppm, isomerizes at room temperature to give [W2(μ-η1:η5-C5H4)Cp(μ-H)(CNtBu)(CO)2(μ-dppm)].

Decarbonylation of the dimolybdenum complexes [Mo2(η5-C5H4R)2(CO)4(μ-dppm)] (R = H, Me, dppm = Ph2PCH2PPh2) occurs readily upon heating in tetrahydrofuran or toluene solution to afford with good yield the phosphido complexes [Mo2(η5-C5H4R)2(μ-CH2PPh2)(μ-PPh2)(CO)2], which arise from an irreversible P−C(sp3) bond cleavage in the backbone of the dppm ligand. Other minor products in these reactions are the oxo complexes [Mo2(η5-C5H4R)2(μ-CH2PPh2)(O)(μ-PPh2)(CO)], which are formed by the action of oxygen on the former dicarbonyl compounds, and the triply-bonded complexes [Mo2(η5-C5H4R)2(CO)2(μ-dppm)], in which the dppm ligand remains intact. By contrast, photochemical decarbonylation of the parent tetracarbonyl complexes at 10 °C yields the triply-bonded dicarbonyls as major products, along with a small amount of the monocarbonyl complexes [Mo2(η5-C5H4R)2(μ-CH2PPh2)(μ-PPh2)(μ-CO)]. Separate experiments show that the latter compounds are formed from [Mo2(η5-C5H4R)2(μ-CH2PPh2)(μ-PPh2)(CO)2] under photolytic conditions, this reaction being reversible. Thus it is concluded that the P−C(sp3) cleavage of the dppm ligand is fairly well suppressed at ambient temperatures or below. The reactions of all the above unsaturated species with CNtBu proceed rapidly at room temperature. In this way, the new isocyanide derivatives [Mo2(η5-C5H4Me)2(μ-CH2PPh2)(μ-PPh2)(CNtBu)(μ-CO)(CO)], [Mo2(η5-C5H5)2(μ-CH2PPh2)(μ-PPh2)(CNtBu)(CO)], and [Mo2(η5-C5H4R)2(μ-η1,η2-CNtBu)(CO)2(μ-dppm)] have been prepared. All of them are formed in good yields as single isomers but have a rather low stability. Reaction of the monocarbonyl derivative with atmospheric oxygen gives the oxo complex [Mo2(η5-C5H5)2(μ-CH2PPh2)(μ-O)(μ-OPPh2)(CNtBu)(CO)], which is also obtained as a single isomer. In marked contrast to their ditungsten analogues, the isocyanide-bridged compounds [Mo2(η5-C5H4R)2(μ-η1,η2-CNtBu)(CO)2(μ-dppm)] do not experience C−H bond cleavages in their cyclopentadienylic rings to a significant extent.

Oxidation of the title compounds with 2 equiv of [FeCp2]BF4 in dichloromethane leads to the tetracarbonylic fluoro complexes [M2Cp2(μ-F)(CO)4(μ-dppm)]BF4 in high yields (dppm = Ph2PCH2PPh2). By contrast, the analogous reaction with [FeCp2]PF6 gives the tricarbonylic fluoro derivatives [M2Cp2(μ-F)(μ-CO)(CO)2(μ-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(μ-X)(μ-CO)(CO)2(μ-dppm)]PF6 are formed in good yields. All above species are derived from the unsaturated dications [M2Cp2(μ-CO)2(CO)2(μ-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(μ-Y)(μ-CO)(CO)2(μ-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 H-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.

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