In which one of the following metal carbonyls, CO forms a bridge between metal atoms?

🧠 Bridging vs Terminal CO

A bridging CO (μ-CO) connects two metal centres simultaneously: $\mathrm{M\!-\!C(\!=\!O)\!-\!M}$ with both M–C bonds.

Walk the four candidates:

| Carbonyl | Metals | Bridging COs | Notes | |---|---|---|---| | $\mathrm{Co_2(CO)_8}$ | 2 Co | 2 ✓ | Solid-state structure: 2 μ-CO + 6 terminal + Co–Co | | $\mathrm{Mn_2(CO)_{10}}$ | 2 Mn | 0 | All 10 CO terminal; only Mn–Mn metal bond | | $\mathrm{Os_3(CO)_{12}}$ | 3 Os | 0 | All 12 CO terminal (4 per Os); 3 Os–Os bonds | | $\mathrm{Ru_3(CO)_{12}}$ | 3 Ru | 0 | All 12 CO terminal; 3 Ru–Ru bonds |

Only $\mathrm{Co_2(CO)_8}$ shows bridging CO.

🗺️ Why Mn₂(CO)₁₀ Has No Bridges

In Mn₂(CO)₁₀, each Mn satisfies 18-electron rule with 5 terminal CO + 1 Mn–Mn bond: $7 + 5(2) + 1 = 18 \checkmark$

Adding bridging COs would over-count electrons — so the symmetry-allowed terminal-only structure is preferred.

In Co₂(CO)₈, the alternative all-terminal structure exists in solution but the solid-state form has the bridging arrangement to satisfy crystal packing + electron count.

⚡ Bridging vs Terminal as a Function of M–M Distance

Bridging CO is favoured when M–M distance allows: in Co–Co (2.49 Å), the metals are close enough for 2 bridging COs to fit. In Mn–Mn (longer, ~2.93 Å) or Os–Os/Ru–Ru triangles, the geometry doesn't favour bridging.

⚠️ Solution vs Solid Structures

Many di- and trinuclear carbonyls have multiple isomeric forms differing by bridge count. Always specify "solid-state structure" when comparing — the solution form may differ.

$\boxed{\text{Answer: (1) } [\mathrm{Co_2(CO)_8}]}$