= Solution
\b[The dimer has two unpaired <electrons> and a triplet ground state, $S=1$.] Its approximately threefold-symmetric Fe$_2$(CO)$_3$ core supplies a pair of degenerate frontier <molecular orbitals>. The last two <electrons> occupy separate members with parallel <spins>, favored by exchange, rather than pairing in one. A useful qualitative picture is a <sigma bond> plus two perpendicular, singly occupied pi half-bonds: together these support a formal Fe–Fe double bond while retaining <paramagnetism>. The <spin-only magnetic moment> for $S=1$ is $\sqrt8\,\mu_B\simeq2.83\,\mu_B$; real moments can include orbital and <spin>–orbit effects.
The formal 18-<electron> count does not require every <molecular orbital> to be doubly occupied. It is therefore compatible with an open-shell dimer. The primary structural/magnetic study establishes this unusual <triplet state>; an isolated-atom high-<spin> $d^7$ diagram would not describe the coupled <metal>–ligand system.<Dinuclear, 18-electron species having a triplet ground state: isolation, characterization, and crystal structure of photogenerated (η5-C5Me5)2Fe2(μ-CO)3 by Josephine Paw Blaha and others>[1985, pp. 4561–4562, triplet electronic structure of the dinuclear 18-electron species]{ref}
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