121Sb NMR and SCF-MS-Xα Studies of Quadrupole Interaction and The Electronic Structure of Mixed-Valence Compound, Cs2SbCl6

Cs2SbCl6 is known as a typical mixed-valence compound. It crystallizes into a tetragonal space group I41/amd and contains two different complex anions, Sb(III)Cl3-6 and Sb(V)Cl-6 . The dark blue color of this compound has been considered to originate from a charge transfer between the above two anions. In order to study the electronic structure of these complex anions and the existence of charge transfer between them we measured the 121Sb NMR spectrum and carried out molecular orbital calculations on the electronic states of these anions. The 121Sb NMR spectrum consists of two peaks at 0 and 30 kHz which can be assigned to the central transition of 121Sb in Sb(V)Cl-6 and Sb(III)Cl3-6 , respectively. The line shape analyses of the spectra led to nuclear quadrupole coupling constants of nearly zero for Sb(V)Cl-6 and 4.9 ± 0.5 MHz for Sb(III)Cl3-6 at room temperature. The quadrupole coupling constant of 121Sb(III) decreases steadily on heating. The calculations of the electronic ground state energies of both anions were calculated by the MS-Xα molecular orbital method. The calculated charge-transfer band from the A1g state of Sb(III)Cl3-6 to the A1g state of Sb(V)Cl-6 appears at 610 nm and can account for the experimental electronic spectrum, the calculated quadrupole coupling constant in Sb(III)Cl3-6 however is far larger than the experimental one. The contribution of the charge-transferred state to the ground state is negligible and so the temperature dependence of the quadrupole coupling constant of 121Sb(III) is attributed to an anisotropic thermal expansion of the compound.