13-5
Branching 6-Electron 4-Centre Bonding Units
175
the length of 3.03 Å for these bonds is  1 Å longer than a “normal” S-S single
bond
16 .
The bonding for
2
2 4
S I
 has been described in Section 11-7; each 2 2
S I
 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
16, 17 .
13-5 Branching 6-Electron 4-Centre Bonding Units
If an atom A is
2
sp hybridized to form three coplanar σ-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 π-electrons of
2 2
(NH ) CO , for example. For
this molecule, the standard Lewis structures are structures (1)-(3) of Fig. 13-5.
Figure 13-4: Standard Lewis and “increased-valence” structures for
2
3
CO
 and
3
NO
 .