 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. 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.