where V i is the number of valence electrons for atom i, {ψ k } is the set of molecular orbitals generated with extended Hückel theory, E(ψ k ) is the energy of orbital k and E C = 10 eV is the cutoff energy under which the molecular orbitals are filled with double occupancy. If this results in a positive ligand charge combined with a low lying LUMO (E(ψ LU M O < -9 eV), we iteratively add two electrons until this is no longer the case. Similarly, if this results in a ligand charge of -2 or lower combined with a high lying HOMO (E(ψ HOM O ) > -10.2 eV), we iteratively remove two electrons until either of the two criteria are no longer true.