what is BACK DONATION

Ligands are Lewis bases that form complexes with metals by donating electron pairs to metal atoms or ions. In determining the formal charge on an atom, each bonding pair of electrons is divided equally between the two bonded atoms. When six pairs of electrons are donated to a metal ion, the metal ion has, in effect, gained six electrons. If the metal initially had a  3 charge, it will have a  3 formal charge after receiving six donated pairs of electrons. If the metal initially had a  2 charge, the formal charge on the metal will be  4 in a complex where the metal has a coordination number of 6.