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 RuC, RuH and RuRu 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 CoCo and FeFe 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.