Electron 5-Centre Bonding Units C3O2, Succinimide, and Pyrrole

13-6 6-Electron 5-Centre Bonding Units C3O2, Succinimide, and Pyrrole 

The phenomenon of 6-electron 5-centre bonding is conveniently introduced by consideration of the Tc-electron distribution for linear C3O2. The standard Lewis stractures (22)-(25) (together with the mirror image structures for structures (22)-(24)) reveal that this molecule has two sets of six -electrons (k and n or and TCy), each of which is distributed amongst five overlapping atomie orbitals. The orbitals are displayed in Fig. 13-7 for one set of eleetrons. 

Chapter 13 Increased-Valence Structures for V-Centre Bonding Units 

For each set of -electrons, the standard Lewis structures are of the general types (26)-(28), with two electron-pair bonds and a lone-pair of electrons. The C-C and C-0 bond lengths of 1.28 A and 1.16 A may be compared with 1.34 A 

Molecules such as succinimide and pyrrole also provide a 6-electron 5-centre bonding unit for the Tc-electrons. For succinimide with the geometry reported in the Lewis stracture (36), the increased-valence stractnre (37) accounts immediately for the observed lengthenings and shortening of the C-0 and C-N bonds relative to the standard donble and single-bond lengths of 1.21 A and 1.47 A. 

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Electron 5-Centre Bonding Units C3O2, Succinimide, and Pyrrole t----------A B C-----------... 

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