I would like to know why cobalt cannot have a negative charge (or at least why a negative charge for cobalt isn't typical). I am not sure where I have gone wrong in my reasoning.
The electronic configuration for cobalt is: $\ce{Co}$: $[\ce{Ar}] 4s^2 3d^7$
Clearly, you could give cobalt a $+2$ charge in order to get a more stable ion as follows
$\ce{Co^{2+}}$: $[\ce{Ar}] 3d^7$
And from there, you could go further and remove $2$ more electrons from the $d$-orbital to become even more stable like this:
$\ce{Co^{4+}}$: $[\ce{Ar}] 3d^5$
If this logic is correct, then could I apply this same thinking but for adding electrons? Such that
$\ce{Co}$: $[\ce{Ar}] 4s^2 3d^7\to$ $\ce{Co^{3-}}$: $[\ce{Ar}] 4s^2 3d^{10}$
Since the $d$-orbital has been filled, the atom should have become more stable. Is it true?
Thus, am I able to argue that Cobalt has $3$ main ion charges: $+2, +4, -3$?