7-7 C-Nitroso Dimers and S4N4
101
lengths. A similar arrangement of positive formal charges occurs in the standard
Lewis structures for a number of other molecular systems
6, 12a, 28-30 and we shall
discuss two examples here, namely the C-nitroso dimers (RNO) 2 (R = alkyl or
aryl) and S 4 N 4 .
The standard Lewis structures for these molecules, structures (32) and (33),
carry positive formal charges on the adjacent nitrogen and sulphur atoms,
respectively. If we assume that the greater electronegativity of the N
+ and S
+
relative to the O
– and N
– leads to appreciable delocalization of the O
– and N
– lonepair electrons into the antibonding N-N * and S-S
*
 orbitals that are vacant in
structures (32) and (33), we can explain the observed lengthenings of the N-N and
S-S bonds relative to double and single bond lengths. For a number of C-nitroso
dimers, the N-N bond-lengths range in value between 1.30 Å and 1.32 Å
31
; the
length of an N-N double bond (as in
3
3
CH N NCH

) is 1.24 Å. For 4 4
S N , the SS bond-lengths of 2.58 Å
32 may be compared with the S-S single-bond length of
2.06 Å.
Other systems with adjacent positive charges in their standard Lewis structures
include
2
Ru(II) N Ru(II)


and 2 2
S O . Two of their standard Lewis structures
are (34) and (35),
and the bonding for these systems is discussed in Sections 18-2 and 11-7.