13-4
Cyclic 6-Electron 4-Centre Bonding Units
173
13-4 Cyclic 6-Electron 4-Centre Bonding Units
A number of cyclic molecules and ions, for example
2
(SN) and
2
4
Se
, have six
π-electrons distributed amongst four pπ-atomic orbitals. The standard Lewis structures for
2
4
Se
are displayed in Fig. 13-3(a).
Figure 13-3: Standard Lewis and “increased-valence” structures for
2
4
Se
. Mirror-image
structures are not displayed for (a) and (b).
By delocalizing non- bonding π-electrons into the adjacent Se-Se π bondingorbitals, we may generate the “increased-valence” structures of (b), each of which
has an “increased-valence” bonding unit of type (7) for the six π-electrons.
However, because the atoms of (b) that correspond to the A and D atoms of
structure (7) are adjacent, their atomic orbitals can overlap well. Therefore, we
shall represent these atoms as bonded together, to form the type (c) “increasedvalence” structures. Either of the structures of (c) involves a cyclic “increasedvalence” bonding unit of type (12) or (13), each of which summarizes resonance
between four Lewis structures. These latter structures are similar to structures (8)(11), except that the “long-bond” structure (11) is replaced by a standard structure.
Cyclic 6-Electron 4-Centre Bonding Units
173
13-4 Cyclic 6-Electron 4-Centre Bonding Units
A number of cyclic molecules and ions, for example
2
(SN) and
2
4
Se
, have six
π-electrons distributed amongst four pπ-atomic orbitals. The standard Lewis structures for
2
4
Se
are displayed in Fig. 13-3(a).
Figure 13-3: Standard Lewis and “increased-valence” structures for
2
4
Se
. Mirror-image
structures are not displayed for (a) and (b).
By delocalizing non- bonding π-electrons into the adjacent Se-Se π bondingorbitals, we may generate the “increased-valence” structures of (b), each of which
has an “increased-valence” bonding unit of type (7) for the six π-electrons.
However, because the atoms of (b) that correspond to the A and D atoms of
structure (7) are adjacent, their atomic orbitals can overlap well. Therefore, we
shall represent these atoms as bonded together, to form the type (c) “increasedvalence” structures. Either of the structures of (c) involves a cyclic “increasedvalence” bonding unit of type (12) or (13), each of which summarizes resonance
between four Lewis structures. These latter structures are similar to structures (8)(11), except that the “long-bond” structure (11) is replaced by a standard structure.