[ana2345]

My teacher presented a rule of thumb for predicting the coordination number in complex ions today, namely that the coordination number is twice the charge of the central atom. (Exceptions include Al3+.) She couldn't really explain why this is, and after searching online I haven't found a good explanation.

We analyzed why this is true for Zn2+, but this seemed to be just a coincidence. There happen to be 2 electrons removed from the 4s shell when Zn becomes a cation, and 1 s orbital + 3 p orbitals happens to be 4 = 2*2.

I'm hoping you guys can explain why this rule of thumb works!

[ardbeg]

in short, it doesnt. 

Zn2+ has a d10 electron configuration and so it can adopt 4, 5 or 6 coordinate complexes with ease as there is no difference in the ligand field.

It depends on a lot of factors, and can't really be summarized easily.