Halogen bonds 4: The strongest (?) halogen bond.

Continuing my hunt, here is a candidate for a strong(est?) halogen bond, this time between Se and I.[cite]10.1021/ic50038a006[/cite]. The features of interest include:

1. The six-membered ring is in the chair conformation.

2. The (relatively enormous) I…I substituent is axial!

3. It is attached to the Se rather than the O.

4. The Se…I distance is 2.75Å, some 1.13Å shorter than the combined atom ver der Waals radii (1.90 + 1.98 = 3.88)

5. The Wiberg bond index is 0.42 for the Se-I bond and 0.63 for the I-I bond (at the crystal geometry). It is tending towards a symmetrical disposition of the central iodine (a feat also achieved by the iodine in the NI₃ complex).

6. The NBO E(2) perturbation involving donation from the Se lone pair into the I-I antibond is 77 kcal/mol, almost twice the value of the one involving DABCO…I-I and way above the values found for the related hydrogen bond.

7. The B3LYP+D3/Def2-TZVPP+PP(I) optimised structure expands the Se-I bond distance to 3.04 and contracts the I-I distance to 2.81Å indicating (as with DABCO…I-I) that there may be compression of this bond induced by the lattice.

8. The NCI surface shows a classical "strong" interaction between Se and I (blue), but significant additional features arising from the axial hydrogens that might account for the axial orientation of the Se…I-I group.

   Click for 3D

9. For good measure, here is the complete crystal structure search, defining any non-bonded contact between any element of group six and group seven that is <0.5Å shorter than the van del Waals contact. Our candidate is on the left hand edge of the plot.