[Big-Daddy]

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 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 the ligand do not change when the coordination bond is formed?

As for the shapes, since they've asked for 2 I've got a choice - either draw two possible linkage isomers (4 to choose from) or draw one square planar and one tetrahedral structure. Which one is more accurate? I can't rationalize why it should be impossible for either the C or O to provide the lone pair and act as the coordinator; but I also can't tell off the top of my head whether it should be square planar or tetrahedral (in general I haven't learnt how to tell whether a complex should be tetrahedral or square planar; I'm not sure if there are guidelines but if so links would be helpful). So, what should I do?