Hypervalency is defined as a molecule that contains one or more main group elements formally bearing more than eight electrons in their valence shell. One example of a molecule so characterised was CLi6[1] where the description "“carbon can expand its octet of electrons to form this relatively stable molecule“ was used. Yet, in this latter case, the octet expansion is in fact an illusion, as indeed are many examples that are cited. The octet shell remains resolutely un-expanded. Here I will explore the tiny molecule CH3F2- where two extra electrons have been added to fluoromethane.
Two such electrons added to e.g. such a methane derivative can be in principle accommodated in two ways:
Here are some ωB97XD/Def2-TZVPPD/scrf=water calculations. All these species are molecules with all-real vibrations, being stable toward dissociation to e.g. CH3– + H– or CH3– + F–. A transition state for this latter dissocation with IRC[2] can be characterised. In all cases the energy of the highest occupied MO or NBO is -ve, meaning that the electrons are bound, at least in part due to the solvent field applied.
Molecule | Wiberg CH order | Wiberg CF order | Natural Populations | E HONBO, au | dataDOI |
---|---|---|---|---|---|
CH42- | 0.773 | – |
C:[core]2S(1.98)2p(3.82)3S( 0.15)4d( 0.01) H:1S( 1.00) |
-0.144 |
[3] |
CH3F2- | 0.980 | 1.213 |
C:[core]2S(1.05)2p( 3.20)3S(1.26)4p( 0.01)4d( 0.01) H:1S( 0.84)2S( 0.01)2p( 0.02) F:[core]2S(1.88)2p( 5.61)3S( 0.30)3p( 0.04)3d( 0.01)4p( 0.01) |
-0.068 | [4] |
CH2F22- | 0.871 | 0.897 |
C:[core]2S(1.60)2p( 2.64)3S(0.39)3p( 0.01)4d( 0.01) H:1S(1.19)2S( 0.06) F:[core]2S(1.86)2p( 5.52)3S( 0.01)3p( 0.01)4p( 0.01) |
-0.281 | [5] |
CF42- | – | 0.801 |
C:[core]2S(1.94)2p( 1.96)3S( 0.19)3p( 0.04)5d( 0.01) F:[core]2S(1.89)2p( 5.54)3p( 0.01)3d( 0.02)
|
-0.148 |
[6] |
So of these systems, CH3F2- can be reasonably called hypervalent, whilst the others have much less such character. It does appear that there is a fine balance between placing extra electrons into Rydberg orbitals to expand the "octet" and hence valencies, and placing them in anti-bonding orbitals where the individual valencies are actually reduced. It seems that substituting methane with just one fluorine encourages population of the Rydberg orbitals, but that more fluorines encourage instead population of the antibonding orbitals. What is remarkable is that CH3F2- actually has a (small) barrier to dissociation. The challenge now is to try to design a system which has a significant Rydberg population, a low antibonding population AND is stable to dissociation; this will require some inspiration. So do not hold your breaths!
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Another way of analysing what the electrons are up to is the ELF (electron localization function). That for CH3F2- is shown below. Note the existence of six "Rydberg" basins, integrating to 1.44e, and located some distance away from the atoms. These are the electrons contributing to the expanded octet. The "unexpanded" shell surrounding the carbon integrates to 7.23e, a "normal" octet.