Picosecond nonlinear absorption dynamics of neutral metal-dithiolenes belonging to the general class [M(R,R' timdt)2] (M = Ni, Pd, Pt; R,R' timdt = monoreduced form of disubstituted imidazolidine-2,4,5-trithione) has been investigated at lambda = 1064 nm using the pump-probe technique. Both central metal ion M and substituents R,R' have been found to strongly influence the temporal evolution of the absorption saturation. In particular, it has been observed that the intersystem crossing rate always increases by a factor similar to1.5 when changing the central metal from Pd to Pt. The mechanisms leading to the measured dynamics have been explained using a multilevel rate equation analysis, based on the results of relativistic time-dependent density functional calculations. (C) 2003 American Institute of Physics.