synthesis, characterization, and low-temperature crystal structures of a family of group 4 complexes containing different numbers of imidodithiodiphosphinate ligands and different metal oxidation states is presented. Their catalytic activity in the polymerization of ethylene and propylene was studied. For ethylene, when these complexes are activated with methylalumoxane (MAO), they are active catalysts in the polymerization producing extremely high molecular weight, high-density polyethylene with very narrow polydispersities. However, for propylene, the activity of these complexes is low, producing only atactic polypropylene. The bis(imidodithiodiphosphinate) titanium(bis(amido)) complex 6e has a singular reactivity, which upon heating forms the corresponding imido complex with the concomitant elimination of Et3N and the formation of the homoleptic complex tris(imidodithiodiphosphinate) titanium(III) (6h).