We have examined cobalt based valence tautomer molecules such as Co(SQ)(2)(phen) using density functional theory (DFT) and variational configuration interaction (VCI) approaches based upon a model Hamiltonian. Our DFT results extend earlier work by finding a reduced total energy gap (order 0.6 eV) between high-temperature and low-temperature states when we fully relax the coordinates (relative to experimental ones). Furthermore, we demonstrate that a charge transfer picture based upon formal valence arguments succeeds qualitatively while failing quantitatively due to strong covalency between the Co 3d orbitals and ligand p orbitals. With the VCI approach, we argue that the high-temperature, high spin phase is strongly mixed valent, with about 30% admixture of Co (III) into the predominantly Co(II) ground state. We confirm this mixed valence through a fit to the XANES spectra. Moreover, the strong electron correlations of the mixed valent phase provide an energy lowering of about 0.2-0.3 eV of the high-temperature phase relative to the low-temperature one. Finally, we use the domain model to account for the extraordinarily large entropy and enthalpy values associated with the transition.