REACTIONS OF COORDINATED LIGANDS .58. THE REACTION OF DIMOLYBDENUM MU-SIGMA-ETA(2)-(4E)-VINYLIDENE COMPLEXES WITH PROTON SOURCES AND DIAZOMETHANE - SYNTHESIS AND CRYSTAL-STRUCTURE OF THE MO-MO BONDED COMPLEX [MO-2(MU-SIGMA-ETA(2)-CH2C(PH)CH2)(CO)(3)(ETA-C5H5)(2)][CF3SO3]

Protonation (CF3CO2H) of the 'side-on' bonded dinuclear vinylidene complexes [Mo-2{mu-sigma:eta(2)-(4e)-C=CR(2)}(CO)(4)(eta-C5H5)(2)] (R = H or Me) afforded the bridged mu-vinyl complexes [Mo-2{OC(O)CF3}(mu-CH=CR(2))(CO)(4)(eta C5H5)2], whereas a similar reaction of one of the isomers of [Mo-2{mu-sigma:eta(2)-(4e)-C=C(Ph)Me}(CO)(4)(eta-C5H5)(2)] afforded the isomeric complexes [Mo-2{OC(O)CF3}{mu-CH=C(Me)Ph}(CO)(4)(eta-C5H5)(2)] and [Mo-2{OC(O)CF3}{mu-CH=C(Ph)Me}(CO)(4)(eta-C5H5)(2)]. Deuterium-labeling experiments and extended Huckel molecular-orbital (EHMO) calculations suggest that these reactions involve delivery of a proton by CF3CO2H to the alpha-carbon of the vinylidene complex in a direction perpendicular to the plane containing the beta-carbon substituents. Protonation of the alpha-carbon is also implicated in the reaction of [Mo-2{mu-sigma:eta(2)-(4e)-C=CMe(2)}(CO)(4)(eta-C5H5)(2)] with HBF4.Et(2)O or CF3SO3H where the products are the unusual cationic species [Mo-2{mu-sigma:eta(2)-CH2C(Me)CH2}(CO)(3)(eta-C5H5)(2)] [X] (X = BF4 or CF3SO3). The same cations are also produced by alpha protonation of the mu-allylidene complex [mo(2){mu-sigma:eta(3)-CHC(Me)CH2}(CO)(4)(eta-C5H5)(2)]. Similarly, protonation (CF3SO3H) of [Mo-2{mu-sigma:eta(3)-CHC(Ph)CH2}(C)(4)(eta-C5H5)(2)] (formed on thermolysis of [Mo-2{mu-sigma:eta(2)-(4e)-C=C(Ph)-Me}(CO)4(eta-C5H5)(2)] afforded the X-ray crystallographically identified complex [Mo-2{mu-sigma:eta(2)-CH2C-(Ph)CH2}(CO)(3)(eta-C5H5)(2)][CF3SO3]. In the solid state the complex contains an unusual bonding mode for a dinuclear allyl complex, the CH2C(Ph)CH2 fragment being bonded via eta(2) co-ordination to one molybdenum atom and a three-centre two-electron interaction between one of the CH2 groups of the allyl and both metal atoms, which are bonded to each other by a formal triple bond. In solution this cation and the related methyl-substituted species show dynamic behaviour resulting in equivalencing on the NMR time-scale of the two anti- and two syn-protons of the allyl ligand. The reaction of diazomethane with [Mo-2{mu-sigma:eta(2)-(4e)-C=CR(2)}(CO)(4)(eta-C5H5)(2)] (R = H or Me) has also been studied. The reactions lead respectively to the formation of the mu-allene complexes [Mo-2(mu-eta(2):eta(2)-CH2=C=CR(2))(CO)(4)(eta-C5H5)(2)]. A similar reaction between CH2N2 and the unsymmetrical labile 'side-on' bonded vinylidene [Mo-2{mu-sigma:eta(2)-(4e)-C=C(Ph)H}(CO)(4)(eta-C5H5)(2)] gave [Mo-2{mu-eta(2):eta(2)-CH2=C=C(Ph)H}(CO)(4)(eta-C5H5)(2)], whereas, in contrast, [Mo-2{mu-sigma:eta(2)-(4e)-C=C(Ph)Me}(CO)(4)(eta-C5H5)(2)] affords the diastereoisomers [Mo-2{mu-eta(2):eta(2)-CH2=C=C(Ph)Me}(CO)(4)(eta-C5H5)(2)] and [Mo-2{mu-eta(2):eta(2)-CH2=C=C(Ph)Me}(CO)(4)(eta- C5H5)(2)]. The EHMO calculations implicate dipolar intermediates in these reactions formed by nucleophilic attack under frontier-orbital control on the lowest unoccupied molecular orbital of the vinylidene ligand located on the alpha-carbon.