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···Br3–illustrated 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