Three-center, four-electron hyperbonding

The intact molecular species on the left appears to have too many electrons to be viewed as a conventional Lewis strucmre, whereas neither dissociated form I, II on the right is objectionable. As first recognized by Coulson, the alternative formulations on the right of (8.23) can be sensibly regarded as the intermolecular resonance structures that contribute (near-)equal weightings Wi, Wu and approximate halfbonds to the resonance hybrid on the left, namely. 

As expressed in the prescient phrase of Nobelist Gerhard Herzberg, the resonance is the binding. 

Because the resonance mixing in (8.23) involves three atomic centers competing for two electron pairs, it is also described as three-center, four-electron (3c/4e) hypervalency. The phenomenon can also be identified as hyperbonding and denoted by a distinctive stroke-symbol (L -A— L ) and m-bond notation that suggests its unique electronic character, namely. 

As shown by Pimentel, Bundle, and Coulson (cf. V B, p. 278ff), hyperbonded 3c/4e species can also be rationalized in molecular orbital terms, but the resonance-hybrid formulation (8.23-8.28) seems to capture the essence of m-bonding most elegantly and succinctly. 

In exceptional cases, a still weirder resonance form of 3c/4e long-bonding (denoted with a caret or connecting brad symbol) may become dominant, as depicted schematically in (8.29)  

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