Transient absorption and emission spectroscopic studies on a series of diimineplatinum(II) dichalcogenolenes, LPtL', reveal charge-separated dichalcogenolene -> diimine charge-transfer (LL'CT) excited-state lifetimes that display a remarkable and nonperiodic dependence on the heteroatoms of the dichalcogenolene ligand. Namely, there is no linear relationship between the observed lifetimes and the principle quantum number of the E donors. The results are explained in terms of heteroatom-dependent singlet-triplet (S-T) energy gaps and anisotropic covalency contributions to the M-E (E = O, S, Se) bonding scheme that control rates of intersystem crossing. For the dioxolene complex, 1-O,O', E(T-2) > E(S-1) and rapid nonradiative decay occurs from S-1 to S-0. However, E(T-2) <= E(S-1) for the heavy-atom congeners, and this provides a mechanism for rapid intersystem crossing. Subsequent internal conversion to T-1 in 3-S,S produces a long-lived, emissive triplet. The two LPtL' complexes with mixed chalcogen donors and 5-Se,Se show lifetimes intermediate between those of 1-O,O' and 3-S,S.