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. (C) 2015 Wiley Periodicals, Inc.