The temperature-induced charge transfer between the Mn and Fe sites in RbMn[Fe(CN)(6)] was analyzed by density functional theory calculations. Our analysis indicates that the extent of electron correlation (equivalently, the pairing energy or the on-site repulsion) is much greater for the Mnn+ ion than for the Fen+ ion (n = 2, 3). This surprising and counterintuitive finding is a consequence of the pi-back-donation effect of the CN ligands.