The chemical bond zoo is relatively small (the bond being a somewhat fuzzy concept, I am not sure there is an actual count of occupants). So when two new candidates come along, it is worth taking notice. I have previously noted the Chemical Bonds at the 21st Century-2017: CB2017 Aachen conference, where both were discussed.

It is always interesting to observe conference experiments taking place. The traditional model involves travelling to a remote venue, staying in a hotel, selecting sessions to attend from a palette of parallel streams and then interweaving chatting to colleagues both old and new over coffee, lunch, dinner or excursions.

The previous post demonstrated the simple iso-electronic progression from six-coordinate carbon to five coordinate nitrogen. Here, a further progression to oxygen is investigated computationally. The systems are formally constructed from a cyclobutadienyl di-anion and firstly the HO ⁵⁺ cation, giving a tri-cationic complex. There are no examples of the resulting motif in the Cambridge structure database.

A few years back I followed a train of thought here which ended with hexacoordinate carbon, then a hypothesis rather than a demonstrated reality. That reality was recently confirmed via a crystal structure, DOI:10.5517/CCDC.CSD.CC1M71QM[cite]10.1002/anie.201608795[/cite]. Here is a similar proposal for penta-coordinate nitrogen. First, a search of the CSD (Cambridge structure database) for such nitrogen.

On February 6th I was alerted to this intriguing article[cite]10.1038/nchem.2716[/cite] by a phone call, made 55 minutes before the article embargo was due to be released. Gizmodo wanted to know if I could provide an (almost) ^† instant ^‡ quote. After a few days, this report of a stable compound of helium and sodium still seems impressive to me and I now impart a few more thoughts here.

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 CLi ₆ [cite]10.1038/355432a0[/cite] where the description "“ carbon can expand its octet of electrons to form this relatively stable molecule ” was used.