One thread that runs through this blog is that of hypervalency. It was therefore nice to come across a recent review of the concept[1] which revisits the topic, and where a helpful summary is given of the evolving meanings over time of the term hypervalent. The key phrase “it soon became clear that the two principles of the 2-centre-2-electron bond and the octet rule were sometimes in conflict” succinctly summarises the issue. Two molecules that are discussed in this review caught my eye, CLi6 and SeMe6. The former is stated as “anomalous in terms of the Lewis model“, but as I have shown in an earlier post, the carbon is in fact not anomalous in a Lewis sense because of a large degree of Li-Li bonding. When this is taken into account, the Lewis model of the carbon becomes more “normal”. Here I take a look at the other cited molecule, SeMe6.
I should start by summarising what I think are two fundamental ways in which electrons can be added to the valence shell of a main group element.
One can then address the issue of hypervalency (and any octet expansion) by analysing the basis set contributions of the orbitals. These orbitals can be either canonical molecular orbitals or localised (NBO) orbitals. If a single determinant wavefunction is appropriate, then the orbitals would be doubly occupied (for closed shell species). If the molecule has multi-reference character, then of course fractional electron occupancy of these orbitals may be required (as would e.g. be the case for ozone, O3, another molecule asserted in the review as hypervalent[1]).
There are other ways of analysing the wavefunction. The one discussed at length in the review[1] is from an atomic charge map, but also mentioned is an ELF partitioning. This derives not from an orbital population but from the distribution of a function (ELF) calculated from the electron density itself. It was this latter method that was cited for SeMe6. The ELF method partitions electrons into so-called basins, which can be monosynaptic (lone pairs and ionic bonds), disynaptic (covalent bonds) and more rarely trisynaptic (3-centre bonds). Using this analysis, six disynaptic octahedrally-arranged ELF basins were located for SeMe6 (“in which the Se–C bonds are relatively non-polar, can have electron populations exceeding 8 at the central atom”[1]) and for which the total integration cames to 11.34e (FAIR Data DOI for this calculation can be see at 10.14469/hpc/3219).
One interesting property of the centroid of the ELF basins is that you can infer the polarity of the bond from its position along the bond axis. For II7, the centroids are displaced towards the central iodine, indicating it is more electronegative, and away from the terminal iodine, indicating it is the electropositive partner. I mention this since the ELF basins for SeMe6 show the centroids to be strongly displaced towards the carbon and away from the Se (0.38/0.62), indicating that this molecule is in fact polar and NOT non-polar as was asserted.
To follow-up this latter observation, I did an NBO analysis of the wavefunction for SeMe6 (FAIR Data DOI: 10.14469/hpc/3220). This reveals the following properties.
So according to this orbital-based analysis, SeMe6 is in effect a partially ionic compound with no evidence of significant Rydberg occupancies and hence no evidence of any octet expansion at Se. Thus we see two different interpretations emerging, depending on the analytical method used:
Well, SeMe6 has turned out to be rather less clear-cut than implied by the assertion “in which the Se–C bonds are relatively non-polar”. There are however possible modifications to SeMe6 that might yet make it less polar. These may be the subject of a follow-up post.
In the mid to late 1990s as the Web developed, it was becoming more obvious…
I have written a few times about the so-called "anomeric effect", which relates to stereoelectronic…
The recent release of the DataCite Data Citation corpus, which has the stated aim of…
Following on from my template exploration of the Wilkinson hydrogenation catalyst, I now repeat this…
In the late 1980s, as I recollected here the equipment needed for real time molecular…
On 24th January 1984, the Macintosh computer was released, as all the media are informing…
View Comments
Nice post Professor Rzepa.
Could QTAIM analysis shed light in this issue? For example describing the bonds as ionic or covalent? or maybe accounting charge around atomic nucleus basin?
What could QTAIM say about hypervalency in this SeMe6 example?
QTAIM was discussed extensively as a technique in the review article cited, although it was not applied to SeMe6. I will put up a QTAIM analysis shortly.
I have added a full QTAIM analysis to the FAIR data deposition https://doi.org/10.14469/hpc/3219 for anyone who wishes to go through the (extensive) set of calculated properties. Certainly it appears polar, but less so than indicated by the NBO analysis.
Here is the equivalent analysis for Se(SiH3)6 (FAIR Data 10.14469/hpc/3231 ).
The six ELF basins integrate to 11.77e. The NBO charges are Se; -0.18765, Si +0.44317, H -0.14285 which indicates reversed polarity at the Se. The natural electronic configurations are Se: [core]4S(1.64)4p(4.46)4d(0.08), Si [core]3S(1.16)3p(2.37)4S(0.01)3d(0.02)4p(0.01) and the total Rydberg population is 0.34703e. The Wiberg bond index per atom at Se is 3.0467.
All this suggests this species is hypervalent as indicated by ELF density analysis and merely hypercoordinate as suggested by NBO orbital analysis.