The syntheses and single crystal X-ray structural analysis of five novel hetero- and homometallic μ₃-oxo trinuclear cluster with the formula [Fe^III₂M^II(μ₃-O)(μ- O₂CCH₃)₆(4-Rpy)₃]·;x(4-Rpy) ·y(CH₃CN) where R = Ph for 1(Fe₂Mn), 2(Fe ₂Fe), 3(Fe₂Co), 4(Fe₂Ni) and R = CF₃ for 5(Fe₂Co), are reported. The persistence of the structure for compounds 2-5 in dichloromethane solution in the temperature range 190-320 K is demonstrated by ¹H and ¹⁹F NMR spectroscopy. Even at the lowest temperature, the electron exchange in the homometallic mixed-valence compound 2(Fe₂Fe) is in the fast regime at the NMR time scale. Variable temperature and pressure NMR line broadening allowed quantifying the fast coordinated/free 4-Rpy exchanges at the two labile metal centers in these clusters: 2: Fe^III(k²⁹⁸/10³ s^-1 = 166; ΔH^‡ = 60.32 kJ mol^-1; ΔS ^‡ = + 34.8 J K^-1 mol^-1; ΔV ^‡ = + 12.5 cm³ mol^-1); 3: Fe (11.9; 58.92; +30.7; +10.6) and Co (2.8; 68.24; +49.8; +13.9); 4: Fe(12.2; 67.91; +61.0; -) and Ni (0.37; 78.62; +67.8; +12.3); 5: Fe (46; 58.21; +39.3; +14.2) and Co (4.7; 55.37; +11.2; +10.9). A limiting D mechanism is assigned to these exchange reactions. This assignment is based on a first-order rate law, the detection of intermediates, the positive and large entropies and volumes of activation. The order of reactivity k^Co > k^Ni is expected for a D mechanism at these metal centers: their low exchange rates are due to their strong binding with the 4-Rpy donor. Surrounded by oxygen donors the d⁵ iron(III) usually reacts associatively; however, here due to low affinity of this ion for nitrogen the mechanism is D and the rate of exchange is very fast, even faster than on the divalent ions. There is no significant effect of the divalent ion in cluster 2, 3, and 5 on the exchange rates of 4-Phpy at the iron center, which seems to indicate that the specific electronic interactions between the three ions making the clusters do not influence the Fe^III-N bond strength.