16-electron square-planar exception

If the 18-electron rule is the well-fed standard, the square-planar 16-electron complex is the famous regular at the diner who never orders dessert and is perfectly content. These complexes sit happily two electrons short of eighteen, and far from being unstable they are some of the most important species in catalysis, precisely because that empty seat is an open invitation for a reacting molecule to climb aboard.

The reason lies in the orbitals. When a metal has a d8 configuration — think rhodium(I), iridium(I), palladium(II), platinum(II), or nickel(II) with the right ligands — the four ligands often arrange themselves at the corners of a square in one plane rather than wrapping all the way around. In a square-planar field one particular d orbital, the one pointing up and down out of the plane (dz2-like in its mixing) together with the empty pz, ends up high and unused, so the molecule fills eight of the nine valence orbitals and stops at sixteen electrons. The geometry and the electron count go hand in hand: square planar and sixteen electrons are two faces of the same d8 situation.

This is not a failure of the rule but a different, equally legitimate magic number, and it is enormously useful. A 16-electron, coordinatively unsaturated complex has a vacant coordination site ready to bind a substrate or to undergo oxidative addition, jumping to an 18-electron, 18-then-back-to-16 cycle as catalysis turns over. Wilkinson's catalyst, the platinum drug cisplatin, and the palladium intermediates of cross-coupling all trade between 16 and 18 electrons in this way. So when you count a d8 complex and get sixteen, do not reach for an eraser; you have probably found a square-planar species doing exactly what it should.