Oxidation state of Ni in Ni(CO)4 and possible molecular shapes

[Big-Daddy]

Homework Statement

In Ni(CO)₄ each carbon monoxide acts as a ligand coordinating to nickel. Suggest two possible shapes for Ni(CO)₄; what is the oxidation state of Ni?

The Attempt at a Solution

The oxidation state of Ni seems to be likely to be 0, because carbon monoxide itself is neutral. But is it always safe to say that the oxidation state of the atoms in a ligand do not change when the coordination bond is formed?

As for the possible shapes, they most likely mean one tetrahedral structure and one square planar structure. But my doubt there is this: with the carbon monoxide ligand, it is ever possible to get the O providing the lone electron pair and thus you could have linkage isomers where O donates instead of C? (After all, even though it is formally positive in the carbon monoxide molecule, O still has a lone pair of electrons.)

 

[TeethWhitener]

The oxidation state of Ni seems to be likely to be 0, because carbon monoxide itself is neutral.

This is correct, and the correct way to think about the problem: add up all the charges on the ligands, and enforce neutrality with the metal's oxidation state.

But is it always safe to say that the oxidation state of the atoms in a ligand do not change when the coordination bond is formed?

I don't quite understand this. If the ligand is reducing or oxidizing, then the oxidation number of the metal may change. But if you're simply referring to something along the lines of: if I have two Cl^- ligands, will the metal be anything but 2+ charged if the overall species is neutral? then the answer is no.

As for the possible shapes, they most likely mean one tetrahedral structure and one square planar structure. But my doubt there is this: with the carbon monoxide ligand, it is ever possible to get the O providing the lone electron pair and thus you could have linkage isomers where O donates instead of C? (After all, even though it is formally positive in the carbon monoxide molecule, O still has a lone pair of electrons.)

Probably these are the two they're looking for. In fact, nickel carbonyl is tetrahedral. Also, CO always bonds to transition metals through the C. There are a number of reasons for this, most importantly pi backbonding (also known as pi acidity), where the sigma metal-carbon bond is augmented by backdonation of electrons from a filled metal d orbital into the empty CO $\pi^*$ antibond.