Branching 6-Electron 4-Centre Bonding Units

The bonding for S l has been described in Section 11-7 each Sjlj component has a cyclic 6-electron 4-centre bonding unit. For such bonding units, the concomitant bonding has also been designated as a 2-electron 4-centre bond  

If an atom A is sp hybridized to form three coplanar a-bonds, it is often possible to find numerous molecules that have Lewis valence-bond arrangements of type (17) of Fig. 13-4 for six electrons that occupy four overlapping atomic orbitals. This bonding unit pertains for the 7c-electrons of (NHj)2CO, for example. For this molecule, the standard Lewis structures are stractures (l)-(3) of Fig. 13-5. 

Chapter 13 Increased-Valence Structures foriV-Centre Bonding Units (+) 

Each of the increased-valence structures of Fig. 13-4 involves an increased-valence bonding unit of type (17) for the six rc-electrons. It summarizes resonance between the standard Lewis structure (18) and the two long-bond Lewis stractures (19) and (20). (Because structure (21) involves three electrons located in the A-atom atomic orbital, this structure cannot be included in the Lewis structure resonance scheme.) 

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