Describe the bonding in metal carbonyls. Name the shapes of [Ni(CO)₄] and [Cr(CO)₆].

1. 1 · Open with the definition. Metal carbonyls are complexes with CO ligands; homoleptic examples are [Ni(CO)₄], [Fe(CO)₅] and [Cr(CO)₆].
2. 2 · The σ bond. The lone pair on carbon is donated into an empty metal orbital.
3. 3 · The π back bond. A filled metal d orbital donates into the empty antibonding π* orbital of CO.
4. 4 · Draw it. Show an arrow from the C lone pair to the metal (σ) and a curved arrow from a metal d lobe to the CO π* lobe (π).
5. 5 · Synergic effect and shapes. Each bond strengthens the other (synergic). [Ni(CO)₄] is tetrahedral; [Cr(CO)₆] is octahedral.
6. 6 · Close in one line. So CO, a weak donor, still binds strongly to metals in low oxidation states such as 0.

Scoring tip: Name both bonds and use the word 'synergic'. Marking schemes usually split the marks this way.