18‐electron rule and the 3c/4e hyperbonding saturation limit. Issue 2 (14th November 2015) - Landis, C. R.
Abstract : 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. © 2015 Wiley Periodicals, Inc. Abstract : Electronic iconography of the 18e rule, illustrating how a normal‐valent (12e) transition metal MX3 species (using s and d orbitals at M to accommodate three MX bonds, blue dots, and three lone pairs, red dots) achieves the maximally stabilizing e M = 18 "count" (all dots) by adding three L donor ligands (green dots) that participate in all possible 3c/4e hyperbonding interactions ( n L →σ*MX ) with MX bonds.