THERMOCHEMISTRY OF THE DI-ETA-5-CYCLOPENTADIENYL DERIVATIVES OF THE FIRST TRANSITION SERIES AND THEIR UNIPOSITIVE IONS

The heats of formation [△H0f(MCp2, c, 298)] have been determined for the di-η5-cyclopentadienyl derivatives of magnesium, vanadium, chromium, manganese, iron, cobalt and nickel, by static-bomb calorimetry. They are (in kJ mol-1) (M =) Mg, 77.0 ± 3; V, 123 ± 4; Cr, 186 ± 3; Mn, 198 ± 2; Fe, 158 ± 4; Co, 205 ± 4; and Ni, 262 ± 3. the heats of formation [△H0f(M(CO)xCp, c, 298)] of tricarbonylcyclopentadienylmanganese and dicarbonylcyclopentadienylcobalt have also been determined as -478 ± 1 and -169 ± 10 kJ mol-1, respectively. By using the measured heats of formation of the metallocenes of the first transition series and other enthalpy values from the literature in Hess's law cycles, the feasibility of the process MCp2 → MCp+2 + e (obtained from the photoelectron ionisation energies) has been analysed and shown to be the result of the interplay of exchange energy and ligand-field stabilisation energy terms in these low-spin complexes. The feasibility order, V < Cr > Mn > Fe < Co > Ni is associated with the special stability associated with half-filled and filled shelss, which, because of the large ligand-field splitting, occur at d3, d6, d8 and d10. It is shown that bond energy additivity does not occur for the above cyclopentadienylmetal carbonyls. © 1979.