Conversion of a Dithiocarbamate Ligand to a Thiocarboxamido Ligand To Form W(S)(PhC≡CPh)(S₂CNEt₂)(SCNEt₂)

The W(IV) bis(dithiocarbamate) complexes W(S)(PhC≡CPh)(S₂CNR2)₂ (R = CH₃, C₂H₅) lose one sulfur atom when treated with an equivalent of triethylphosphine at -78 °C. The products W(S)(PhC≡ CPh)(SCNR₂)(S₂CNR₂) contain a thiocarboxamido ligand in place of one dithiocarbamate. The molecular structure of the ethyl derivative was determined by a single-crystal X-ray diffraction study (monoclinic, P2_t/a; a = 16.927 (6) Å, b = 9.951 (3) A, c = 17.669 (6) Å, β= 118.49 (3)°, and Z = 4; R = 0.078 and = 0.064 for 1962 reflections with I > 3σ(I)). The carbenoid carbon of the thiocarboxamido ligand lies cis to the alkyne ligand in the plane defined by the metal-alkyne triangle. The orientation of the η ^2-SCNEt2fragment allows the unique p orbital of the three-coordinate carbon to overlap with the lone filled dπ orbital of the roughly octahedral d² tungsten(IV) ion. A dynamic process equilibrates the two dithiocarbamate alkyl substituents on the NMR time scale with an activation barrier of 15.9 kcal/mol. We suggest that this process is promoted by dechelation of the η²-thiocarboxamido sulfur to generate a fluxional five-co-ordinate intermediate.