The values of ρ b at the bcps between OX2 and anions were found to be between 0.008 and 0.253 a.u. (Table S1 of the Supplementary Information). Except at the bcp of Br2O···SCN (29), the Laplacian of the charge density, ∇2ρ b, is large and positive for all complexes, with values varying between 0.020 and 0.416 a.u. Since ∇2ρ b> 0 and ∇2ρ b< 0 have been regarded as a signature of closed- and open-shell (ionic) interactions,78 the ∇2ρ b = –0.258 a.u. at the bcp of Br2O···SCN signifies the presence of an open-shell (covalent) interaction.73,74
The O···A (A = F, Cl, Br, C, N) contacts in the complexes of O2F2 with the anions A (A = F, Cl, Br, CN) (1-5, Fig. 1) exhibit similar closed-shell bonding features (∇2ρ b > 0), yet the angles of approach of the electrophile varies between 162.0 and 170.4o (Table 1). The directionality of the interaction is comparable with that obtained for the OFCN complexes of the anions A (A = F, Cl, Br) (15-17), with the A···O–C (A = F, Cl, Br) angles ranging between 168.9 and 175.7o. It is comparatively weaker for the complexes of OF2, OCl2 and OBr2with the same anions (7-9, 18-20 & 26-28), with the A···O–X angles commensurate with the range typically found for Type-II interactions (150–180o).17
The complexes F2O···Br3, Cl2O···Br3 and Br2O···Br3illustrated as 14, 25 and 34 in Fig. 1, respectively, display the presence of a bifurcated topology of bonding, in which the O atom in X2O acts as an electrophile for the Br3 anion. Because of the involvement of a secondary interaction in each of these three complexes, the Br···O–X angles are significantly non-linear. Similar topologies of bonding are also found for the complexes shown as 6 and 33 ((CN)2O···NO3 and Br2O···OCN, respectively). In 6, the bond path topologies are developed between the (O)Cπatom of the (CN)2O molecule and the two O atoms of the NO3 anion. In 33, such topologies are seen between the O (and Br) atom(s) of the OBr2molecule and the O atom of the OCN anion. Since the σ-hole on the O atom in OBr2 is weakly positive (VS,max = +1.6 kcal mol-1, Fig. 2), its attractive interaction with the entirely negative O atom in OCN results in the formation of the O···O Type-II chalcogen bond (∠O···O–Br = 166.8o). The other interaction in the same complex is secondary, and is classified as Type-I bonding (∠Br···O–C = 139.2o). The combination of both the Type-I and -II interactions provides geometrical stability to the entire ion-molecule complex. Details of the intermolecular distance and directional nature of various A···O interactions in other complexes are summarized in Table 1.
Table 1. The uncorrected and BSSE-corrected binding energies and selected geometrical properties of the 34 O-bonded complexes, obtained with [MP2/aug-cc-pVTZ].a