The (1)A(1) reversible arrow T-5(2) spin transition has been investigated in the solid solutions of FexM1-x(pyrazine)[Pt(CN)(4)] (M = Ni or Co, 0 <= x <= 1) having a three-dimensional polynuclear structure. Both Ni and Co dilutions tend to decrease the hysteresis width and smooth the transition curves. The enthalpy (entropy) change associated with the spin transition was found to decrease from 26 kJ mol(-1) (84 J K-1 mol(-1)) for x = I to 12 W mol(-1) (47 J K-1 mol(-1)) for 47% Co dilution and to 15 kJ mol(-1) (54 J K-1 mol(-1)) for 59% Ni dilution. Raman spectroscopy revealed a mixed one- and two-mode behavior in the solid solutions. For the first time, a correlation between vibrational frequencies exhibiting one-mode behavior and the entropy change, which drives the spin crossover, is established.