Estimate of the vibrational contribution to the entropy change associated with the spin transition in the d(4) systems [Mn (III)(pyrol)(3)tren] and [Cr-II(depe) I-2(2)]

The vibrational contribution to Delta S of the low-spin (T-3(1)) to high-spin (5 E) spin transition in two 3d(4) octahedral systems [ Mn-III(pyrol)(3)tren] and [ Cr(depe)(2)I-2] have been estimated by means of DFT calculations (B3LYP/CEP-31G) of the vibrational normal-modes frequencies. The obtained value at the transition temperature for the Mn(III) complex is Delta S-vib(44 K) = 6.3 J K-1 mol(-1), which is comparable with the proposed Jahn-Teller contribution of R ln3 = 9.1 J K-1 mol(-1) and which is approximately half of the experimentally determined 13.8 J K-1 mol(-1). The corresponding value for the Cr(II) complex is Delta S-vib(171.45 K) = 46.5 J K-1 mol(-1), as compared to the experimental value of 39.45 J K-1 mol(-1). The analysis of the vibrational normal modes reveals that for the d(4) systems under study, contrary to Fe(II) d(6) systems, not all metal-ligand stretching vibrations make a contribution. For the Mn(III) complex, the only vibration that contributes to DSvib involve the nitrogens occupying the Jahn-Teller axis, while in the case of Cr(II) the contributing vibrations involve the Cr-I bonds. Low-frequency modes due to ring vibrations, metal -ligand bending and movement of the molecule as a whole also contribute to the vibrational entropy associated with the spin transition.