= Solution
Neither formula requires a bridging <carbonyl> to repair an <electron> deficiency: the terminal-CO donor–acceptor structures in (d) and (e) already give an electron-counting rationale for the metal–metal interaction. \b[A predominantly terminal-carbonyl model is a reasonable starting proposal for A; bridging or semibridging CO is more geometrically plausible for the shorter Fe–Fe contact in B.] Larger metal separation makes a <carbonyl> simultaneously engaging both Mo atoms less favourable, while <carbonyl> redistribution or tilting can be easier around a compact Fe pair.
This is a qualified structural prediction, not a determination from molecular formula. A bridging CO can change the relative hapticity and local <electron> allocation, and the two metal centres are inequivalent; a large frequency shift by itself need not prove a symmetric bridge. Infrared <carbonyl> frequencies combined with diffraction would test terminal versus bridging or semibridging assignments. The given low-temperature preparation can trap a coordination pattern before complete <carbonyl> redistribution.
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