The unsaturated complex NiCp*W(CO)(3)Cp’ (Ni-W) adds on SO2 in high yield to form the : mu-SO2 complex Nicp*(mu-CO)(mu-SO2)W(CO)(2)Cp’ (Ni-W). This complex was characterized spectroscopically and by X-ray diffraction. The butterfly geometry of the NiWCS core of this complex was compared with two other compounds with similarly configured NiWC2 or NiWCS cores.