3.2 Analysis of the nature of electron-pair delocalization
between atomic basins
The delocalization index δ , a measure of the bond order, is
calculated between various atom-atom pairs constituting each complex.
The largest value of δ was found to be 0.896 for the O and S
atom-atom pair in the complex
Br2O···SCN– (29); this means that
approximately one electron is shared between these atomic basins. This
leads to the suggestion that there is indeed formation of a covalent
bond, and the entire complex system might be regarded as a single
molecular cation.
The A···O (X = F, Cl, Br, S, N) atom-atom pairs formed between the
O-centered monomers and other anions are also accompanied by reasonably
large δ values (Table S1). Similar δ values have been
reported for coordination bonds. For example, δ for the RuC,
RuH and RuRu bonds in the N -heterocyclic carbene triruthenium
clusters
[Ru3(μ -H)2(μ 3-MeImCH)(CO)9](Me2Im
= 1,3-dimethylimidazol-2-ylidene) were reported as 0.757–0.764, 0.474
and 0.458, respectively.81 Similarly, the δ for the
CoCo and FeFe bonds were 0.47 and 0.40 for
Co2(CO)8 and
Fe2(CO)8,
respectively.82 Clearly, the combined signature
(H b < 0 and δ>> 0.4) unequivocally indicates that the A⋯O
intermolecular interactions in 25 complexes of the entire series have
significant covalency. On the other hand, the δ values are
significantly smaller for the O···O, O···N, O···F, O···Cl and O···Br
atom-atom pairs of the complexes shown in 5-6, 11, 13-14, 22, 25, 29-30
and 33-34.