18-electron rule and the 3c/4e hyperbonding saturation limit

We show that the empirical 18-electron rule of transition metal chemistry corresponds to an intrinsic saturation limit for the 3c/4e hyperbonding interactions that are a ubiquitous feature of D-block aggregation phenomena. Such a "rule" therefore requires no "p-orbital participation," "d(2) sp(3) hybridization," "valence shell expansion," or other p-type intrusions into the Aufbau orbital filling sequence. Instead, 18e stability corresponds to the natural terminus of post-Lewis 3c/4e resonance-type stabilizations that lead to successive ligand additions (and formal increments of electron count) at a transition metal center, all within the normal (s + 5d) confines of D-block valence space.