Electronic Spectroscopy and Electrochemistry of Alkyne Dithiocarbamate Complexes of Molybdenum(II) and Tungsten(II)

The divergent influence of ligand x acceptance and x donation on the frontier orbitals of M(CO)(RC≡CR')(dtc)₂ (dtc = ^-S₂CNR₂; M = Mo and W) and Mo(RC≡CR')₂(dtc)₂ has been probed by electronic spectral and cyclic voltammetric studies. The prevailing molecular orbital model with the dπ LUMO dependent upon alkyne π_⊥ donation is compatible with trends in both optical transition energies (E_hv) and reduction potentials (E_R°). These formal 16-electron monomers, stabilized by ligand π donation, tend to display chemically reversible reductions and irreversible oxidations, in accord with naive chemical expectations for unsaturated molecules. The observed relationship between E_hv and E_R° supports the proposed LUMO parentage while the v(CO) correlation with E_p/2 (oxidation) is consistent with the dπ HOMO origin. These results illustrate the dual π donor/π acceptor role of alkyne ligands toward orthogonal dπ orbitals.