# How is square planar geometry possible in metal complexes?

I came across this statement and I don't understand what it meant by it:

"Square planar geometry is favoured by ligands that can form pi bonds by accepting electrons from metal atoms or ions."

How is this possible? I mean a ligand, itself, donates electrons. How is it supposed to be accepting electrons?

Maybe I'll break your world apart, but back-donation is actually pretty common. There are actually systems where it dominates, like antimony/naphthalene adducts. Back-donation is what participates to extreme stability of transition metal carbonyls. The key is, that not only transition metal has vacant orbitals, but the ligand have too, and in some cases even low-energy ones, like $CO$. Such orbitals can accept electron density from filled orbitals of the central atom, say $d_{z^2}$ in square planar complexes. Orbitals for back-donation are usually $\pi^*$ orbitals, and the interaction commonly found in phosphines, alkenes and arenes, especially if the have substituents with significant $-I$ effect