Formation, structure and reactivity of boryloxycarbyne complexes of group 6 metals

Reaction of the diborane(4) B-2(NMe2)(2)I-2 with two equivalents of K[(eta(5)-C5H5)M(CO)(3)] (M = Cr, Mo, W) yielded the dinuclear boryloxycarbyne complexes [{(eta(5)-C5H5)(OC)(2)Mequivalent toCO}(2)- B-2(NMe2)(2)} (4a, M = Mo; b, M = W; c, M = Cr), which were fully characterised in solution by multinuclear NMR methods. The Mo and W complexes 4a, b proved to be kinetically favoured products of this reaction and underwent quantitative rearrangement in solution to afford the complexes [{(eta(5)-C5H5)-(OC)(2)Mequivalent toCO}B(NMe2)B(NMe2)- {M(CO)(3)(eta(5)-C5H5)}] (5a, M=Mo; b, M = W); 5a was characterised by X-ray crystallography in the solid state. Corresponding reactions of B-2(NMe2)(2)I-2 with only one equivalent of K[(eta(5)-C5H5)-M(CO)(3)} (M = Mo, W) initially afforded 1:1 mixtures of the boryloxycarbyne complexes 4a,b and unconsumed B-2(NMe2)(2)I-2. This mixture, however, yielded finally the diborane(4)yl complexes [(eta(5)-C5H5)(OC)(3)M{B(NMe2)- B(NMe2)I}] (6a, M = Mo; b, M = W) by [(eta(5)-C5H5)(OC)(3)M] transfer and rearrangement. Density functional calculations were carried out for 4 c and 5a, b.