We have seen in the series of posts on the topic of hypervalency how the first row main group elements such as Be, B, C and N can sustain apparent hypercoordination and arguably hypervalency. The latter is defined not so much by expanding the total valence shell of electrons surrounding the hypervalent atom beyond eight, but in having more than four well defined bonds to it, as quantified by AIM and ELF analysis. The previous post made the suggestion of how a compound involving hypervalent boron could also sustain a genuine bond to the rare gas helium. It is surely time to seek evidence that this type of bonding can be sustained in reality. Fortunately, a crystal structure of a reasonably analogous compound IS available (DOI: 10.1016/0022-328X(94)05089-T).
YOCVIV: Crystal structure of hexacoordinate boron. Click for 3D
AIM analysis for YOCVIV
The AIM analysis shows five bond critical points in the B-C regions and one in the B-Br region. with ρ(r) values of 0.121 and 0.146 respectively. The corresponding ∇2ρ values were -0.07 and -0.22. These BCPs are matched by equally well defined disynaptic basins in the ELF analysis with electron populations of respectively 0.67 and 2.1 electrons. This compares with ρ(r) values of 0.157 and 0.069, and ELF integrations of 1.22 and 2.0 calculated for the structurally similar proposed B-He compound.
ELF analysis for YOCVIV. Click for 3D
The analogy is sufficiently similar to suggest that (in this case boron) hypervalency for such first row main group elements can be reflected in real systems.
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