Understanding the eighteen-electron rule

It is pointed out that the preferred closed-shell electron structures, such as those in typical high-symmetry 18-electron systems, are driven both by the bonding contributions to the centre and by the kinetic-energy (nodal-structure) terms in the ligand subsystem, Ln. The latter imposes a filling order s < p < d, even for the MLn complex. Then the 18e principle can be right for the right reasons even without any np contributions at the central atom.