: M–R + M → M + M–R.
In redox-transmetalation a ligand is transferred from one metal to the other through an intermolecular mechanism. During the reaction one of the metal centers is oxidized and the other is reduced. The electronegativities of the metals and ligands is what causes the reaction to go forward. If M is more electronegative than M, it is thermodynamically favorable for the R group to coordinate to the less electronegative M.