Number of complexes which will exhibit synergic bonding amongst, [Cr(CO)₆], [Mn(CO)₅] and [Mn₂(CO)₁₀] is ___________.

Solution

Chemistry ma’am · 3 steps

Every metal–CO bond here is synergic, so just ask whether each complex has a metal bonded to CO.

1. What synergic means. CO gives its carbon lone pair to an empty metal orbital, which forms a σ bond. The metal then pushes electrons from a filled d orbital back into the empty π^* orbital of CO, which forms a π bond. Each donation strengthens the other, and that mutual help is called synergic bonding.
2. What it needs. You need a metal in a low oxidation state with filled d orbitals to back-donate, bonded to a π-acceptor ligand like CO. In all three species the metal is in the zero oxidation state and carries CO ligands.

   Species	Metal, oxidation state	Metal–CO bonds	Synergic?
   [Cr(CO)₆]	Cr, 0 (3d⁶)	6	yes
   [Mn(CO)₅]	Mn, 0 (3d⁷)	5	yes, even though it has an unpaired electron
   [Mn₂(CO)₁₀]	Mn, 0	10, plus one Mn–Mn bond	yes

3. So. [Mn(CO)₅] is an unstable 17-electron radical that dimerises to [Mn₂(CO)₁₀], but its Mn–CO bonds are still synergic. All three qualify, so the answer is 3.

The common slip: 2. Leaving out [Mn(CO)₅] because it is an odd-electron species that does not obey the 18-electron rule gives 2. Stability does not matter here: its Mn–CO bonds are still synergic.

The idea to keep. Synergic bonding is a property of the metal–CO bond itself: σ donation from CO plus π back-donation from the metal. Any metal carbonyl has it, whether or not the complex is stable.

Answer from the NTA final answer key. Solution by PrepWiser.