FURTHER-STUDIES OF THE REACTIONS INVOLVING ETHYNE AND M(2)(OBU(T))(6), WHERE M=MO AND W - POLYACETYLENE FORMATION VERSUS FORMATION OF ETHYNE ADDUCTS AND C-C COUPLED PRODUCTS

M(2)(OBu(t))(6) compounds in hydrocarbon solvents react with ethyne to give a mixture of cis- and trans-polyacetylene and ethyne- and mu-C4H 4 adducts of M(2)(OBu(t))(6) by competing pathways. The degree of ethy ne polymerization is shown to decrease with increasing concentration o f M(2)(OBu(t))(6), to increase with increasing volumes of solvent for constant concentration and to be greater for M = Mo than for M = W. Th e kinetically labile complex Mo-2(OBu(t))(6)(mu-C2H2), originally repo rted by Strutz and Schrock [Organometallics 1984, 4, 1600], has been t rapped by the addition of pyridine and 4-methylpyridine at -20 degrees C and the structure of the latter compound determined by a single cry stal X-ray study. There is a pseudo-tetrahedral Mo2C2 core, Mo-Mo = 2. 645(1) Angstrom, C-C = 1.39(1) Angstrom and Mo-C = 2.092(7) Angstrom ( ave.), supported by a single OR bridge. Each metal atom has a pseudo-t rigonal bipyramidal geometry. In the presence of an excess of ethyne, Mo-2(OBu(t))(6) yields a mixture of compounds, two of which are propos ed to be isomers of formula Mo-2(OBu(t))(6)(mu-C4H4). Addition of CO t o a hydrocarbon solution of the latter gives Mo-2(OBu(t))(6)(mu-C4H4)( CO) which is shown to be structurally related to its tungsten analogue by a single crystal X-ray determination. [nu(CO) = 1908 (M = Mo) and 1883 cm(-1) (M = W).] The isolated compounds show no activity towards ethyne polymerization which is proposed to occur by way of a carbene/a lkylidene mechanism. Consistent with this proposal is the finding that Mo-2(OBu(t))(6), which is unreactive towards norbornene, can be activ ated towards norbornene polymerization by the addition of ethyne (5 eq uiv.). Copyright (C) 1996 Elsevier Science Ltd