Cyclic 6-Electron 4-Centre Bonding Units

A number of cyclic molecules and ions, for example (SN) and, have six 7i-electrons distributed amongst four px-atomic orbitals. The standard Lewis struc- 

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

The Se-Se bond lengths of 2.28 A for are shorter than the estimate of 2.34 A for an Se-Se single bond, and resonance between either the four standard Lewis structures of type (a) or the two increased-valence structures of type (c) accounts for this observation. However, the increased-valence representation does this in a more economical maimer. 

The molecnlar orbitals for the jr-electrons are given by Eqn. (13-1), with g = L = 1. For systems such as with symmetry, the molecular orbitals /2 and /j are degenerate, as are the excited configurations 

---

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 ... 

A cyclic 6-electron 5-centre bonding unit obtains for the Tc-electrons of pyrrole (Figure 2-7), for which the standard Lewis stmctmes are... 

The 71-electrons of pyrrole form a cyclic 6-electron 5-centre bonding unit, in which the Y and D atoms are adjacent in the generalized Lewis structures (26)-(32). The Y-D bond of structure (32) then becomes a normal electron-pair bond, as it is in structure (43). Another type of cyclic increased-valence structure can also be constructed, namely structure (44) (as in the pyrrole structures (45) and (46)). Increased-valence structure (44) should be more stable than (29), but we shall not pursue this matter here. To obtain stmctures (45) and (46), it is necessary to write down the non-octet structures (47) and (48), and then to delocalize the non-bonding 7t-electrons into the adjacent bonding orbitals. 

A single-crystal X-ray diffraction study revealed that the two ferrocene units are in anti conformation. Cyclic voltammetric results are in accord with a strong interaction between the iron centres. Electronic interactions through bonds are mediated by the bridging dimethylsilyl groups and this is discussed in more detail in Section n.E.3. 

Investigation of the nature and activity of catalytic species in copper(i) chloride-promoted dimerisation of acetylene indicates that the mechanism either involves insertion of a C2H unit into a copper-chlorine bond, or the reaction of CgH with two copper atoms to give a 3-centre, 2-electron-bonded species. Cyclic oligomerisation of diphenylacetylene in the presence of AlPha proceeds through ir-complexes as intermediates benzyne intermediates are suggested, on the basis of observed products, in oligomerisation of but-2-yne and of oct-4-yne in the presence of n-butyl- or /7-tolyl-chromium compounds. 

---