Mixed Olefin-Alkyne Complexes of Molybdenum(II) and Tungsten(II) Dithiocarbamates

Mixed olefin—alkyne complexes of the type M(η²-olefin)(η²-alkyne)(S₂CNR₂)₂ (M = Mo and W) have been prepared from M(CO)(η²-alkyne)(S₂CNR₂)₂ and electron poor olefins (maleic anhydride, tetracyanoethylene, and trans-dicyanoethylene). The range of acceptable alkyne ligands spans alkyl, aryl, and terminal alkynes. The motivation for this work was to incorporate a single-faced r-acid ligand into the octahedral position cis to the alkyne ligand of the M(η²-alkyne)(S₂CNR₂)₂ fragment. The resulting family of mixed olefin—alkyne complexes has been characterized by 1H NMR, ¹³C NMR, infrared, and electronic spectroscopies; cyclic voltammograms have been recorded for a number of the compounds reported here. The structure of W(η²-maleicanhydride)(7₇²-PhC₂H)(S₂CNMe₂)₂ has been determined: a = 22.744 (5) A, b = 12.589 (3) A, c = 21.440 (8) A, = 121.52 (2)°, Z = 8, d_calcd = 1.69 g cm^-3, and space group C2/c. Several of these olefin—alkyne complexes undergo nucleophilic attack by the phosphorus lone pair of phosphites and phosphines at the terminal carbon of the coordinated alkyne to form 77²-vinyl products. The η²-vinyl ligand present in the M(η²-PhCCHPR₃) moiety which results can also be described as a cyclic alkylidene or as a metallacyclopropene. These names are in accord with the considerable carbenoid character of the more tightly bound a-carbon as reflected in a low-field ¹³C chemical shift of 220–230 ppm.