The Conceptual Dilemma of the One-Electron Picture in Describing the Uniaxial Magnetism at Linear Coordination Sites

High-spin Fe²⁺, Fe⁺ and Co²⁺ ions at linear two-coordination sites exhibit uniaxial magnetism. In the one-electron picture, the uniaxial magnetism of the Fe²⁺ ion is explained, while those of the Fe⁺ and Co²⁺ ions are not, if the d-state split pattern is 1e < 2e < 1a. The opposite is true if the d-state split pattern is 1a < 1e < 2e. We resolved this conceptual dilemma by evaluating the relative stabilities for the various L states of linear molecules (FeL₂)⁰, (FeL₂)^–, and (CoL₂)⁰ with L = C(SiH₃)₃ on the basis of first-principles broken-symmetry and configuration interaction calculations. In the first-principles picture, the total energy of an electron configuration depends not only on the energy sequence of the occupied d-states, as do the one-electron picture, but also on the electron repulsion between occupied d-states, which is neglected by the one-electron picture.