Chemical reduction of [Mo 2 Cp 2 (µ-Cl)(µ-PA 2 )-(CO) 2 ] (A ) Cy, Ph, OEt) gives the corresponding alkaline metal salts of the triply bonded anions [Mo 2 -Cp 2 (µ-PA 2 )(µ-CO) 2 ] -, which exhibit both molybdenum and oxygen nucleophilic sites. The PCy 2 anion reacts easily with NH 4 + , [AuCl(PR 3 )], or MeI to give unsaturated dicarbonyls [Mo 2 Cp 2 (µ-X)(µ-PA 2 )(CO) 2 ] (X ) H, AuPR 3 , Me), while [Me 3 O]BF 4 gives the methoxycarbyne [Mo 2 Cp 2 (µ-COMe)(µ-PCy 2 )(µ-CO)] and allyl chloride rearranges to give the unsaturated alkenyl complex [Mo 2 -Cp 2 (µ-PCy 2 )(µ-CMeCH 2 )(CO) 2 ].

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)].

Tetrahydrofuran solutions of the 30-electron anions [Mo(2)Cp(2)(mu-PA(2))(mu-CO)(2)](-) (A = Cy, Et, Ph, OEt) are conveniently prepared through a two-step approach. In the first step, [Mo(2)Cp(2)(CO)(6)] is treated with the chlorophosphines ClPR(2) (R = Cy, Et, Ph) or the chlorophosphite ClP(OEt)(2), in refluxing toluene or diglyme respectively, to give the corresponding 32-electron chloro-complexes [Mo(2)Cp(2)(mu-Cl)(mu-PA(2))(CO)(2)] as major products. In the second step, these air-sensitive intermediates are treated in tetrahydrofuran solution at room temperature with one of several reducing agents such as Li[BHEt(3)], Li(Hg), Na(Hg) or K[BH(s)Bu(3)] to give red solutions of the corresponding alkali-metal salts of the anions, which display significant ion pairing involving one or both oxygen atoms of the bridging carbonyl ligands, depending on the cation. All these triply bonded species are quite air-sensitive and could not be isolated as pure solids, but they can be easily protonated using a weak acid such as [NH(4)]PF(6) to give with good yield the corresponding unsaturated hydrides [Mo(2)Cp(2)(mu-H)(mu-PA(2))(CO)(2)], which are species of low to moderate sensitiveness to air, and also formally containing an intermetallic triple bond. The reactivity of the dicyclohexylphosphide-bridged anion (mainly as its Li(+) salt) towards different hydrocarbon halides RX was studied in detail. These reactions were found to be rather complex, critically depending on the reagent used, and generally resulting in the formation of several products, of which four types were identified: (a) the known agostic products [Mo(2)Cp(2)(mu-PCy(2))(mu-R)(CO)(2)] (R = Me, CH(2)Ph), (b) the new alkoxycarbyne products [Mo(2)Cp(2)(mu-COR)(mu-PCy(2))(mu-CO)] [R = Me, Et, C(O)Ph, (i)Pr, Cy], which could be conveniently isolated as pure solids, (c) the iodoxycarbyne complex [Mo(2)Cp(2)(mu-COI)(mu-PCy(2))(mu-CO)], a very unstable species formed in the reaction with EtI, and (d) the halide complexes [Mo(2)Cp(2)(mu-PCy(2))(mu-X)(CO)(2)] [X = Cl, Br, I], which were more conveniently prepared by the direct reaction of the anion with the pertinent halogen (X = Br, I). The analysis of the above results suggests that at least three primary reaction pathways are in operation: (a) nucleophilic attack of the anion through its dimetal centre, (b) nucleophilic attack of the anion through the oxygen atoms of its bridging carbonyls and (c) electron-transfer with the reagent, this being the main path to the halo-complexes [Mo(2)Cp(2)(mu-PCy(2))(mu-X)(CO)(2)].

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.

The unsaturated anion [Mo(2)Cp(2)(mu-PPh(2))(mu-CO)(2)](-) (1) (Na(+) salt) reacts with the nitrosyl complexes [MCp'(CO)(2)(NO)]BF(4) (M = Mn, Re; Cp' = eta(5)-C(5)H(4)Me) rapidly at about 193 K. Upon warming of the resulting mixtures up to 243 K orange solutions are obtained, shown to contain the corresponding oxo- and nitride-bridged tetracarbonyl complexes [Mo(2)MCp(2)Cp'(mu-N)(mu-O)(mu-PPh(2))(CO)(4)] as the major product, which could be isolated only for M = Re. Above 253 K, however, these compounds experience spontaneous decarbonylation to yield the unsaturated tricarbonyl derivatives [Mo(2)MCp(2)Cp'(mu-N)(mu-O)(mu-PPh(2))(CO)(3)] (Mo-Mo = 2.840 A for the Mn compound, according to density functional theory (DFT) calculations). These complexes in turn react rapidly with air to give the corresponding dioxodicarbonyl derivatives [Mo(2)MCp(2)Cp'(mu-N)(mu-O)(mu-PPh(2))(O)(CO)(2)] almost quantitatively. The structure of the latter product (M = Re) was determined by X-ray diffraction methods (Mo-Mo = 2.763(1) A). In contrast with the N-O bond cleavage easily taking place in the above reactions, the direct nitrosylation of 1 with N-methyl-N-nitroso-p-toluenesulfonamide induces no bond cleavage process in the nitrosyl ligand, but just gives the electron-precise tricarbonyl derivative [Mo(2)Cp(2)(mu-PPh(2))(CO)(3)(NO)] or, in the presence of CN(t)Bu, a mixture of the new isocyanide complexes [Mo(2)Cp(2)(mu-PPh(2))(CN(t)Bu)(CO)(2)(NO)] and [Mo(2)Cp(2)(mu-PPh(2))(mu-eta(1):eta(2)-CN(t)Bu)(CO)(NO)]. Separate experiments indicated that these isocyanide complexes cannot be converted one into each other, nor can they be obtained through thermal substitution reactions on the above tricarbonyl product.