Question

What is the oxidation number of the metal in `Ni(CO)_4`?

Answer 1

Well, we should review the definition of `"oxidation state"`, `"oxidation number..........."`

Explanation:

The oxidation state of an atom in a compound is the charge left on the atom when all the bonding pairs of electrons are removed, with the charge, the electron, assigned to the more electronegative atom. This goes back to our earliest definitions of ionic and covalent bonding, where ionic bonds result from the transfer of electrons between atoms to give ions, and covalent bonds result from the sharing of electrons to form bonds...........

And when we do this for `Ni(CO)_4`, we break the `Ni-C` bonds to get `Ni^0` and `4xxstackrel(-):C-=O^(+)`, and we are left with a neutral nickel atom, and a neutral carbon monoxide ligand (we normally write the Lewis structure of `CO` with charge separation, even tho it is a neutral, gaseous molecule). And thus the metal centre has a FORMAL oxidation state of `0`.

Satisfied?