[maccha]

Do complex ions form because of formal charges? For instance, in the hydroxide ion the oxygen has a negative formal charge.. is this what creates the overall negatively charged ion?

[aurophilic]

Yes, that sounds right.

[cth]

Partially yes. A negative charge on the ligand will help to bind to a positively charged metallic cation.

But some ligands are not actually charged, for example NH₃, CO, PPh₃, pyridine,... It is the lone electrons on the ligands that become shared with the metal cation when the complex form.