A crowded neighbourhood

US researchers have broken the record for a compound with the highest coordination number - the number of neighbours an atom is capable of binding to. The compound, a thorium aminodiboranate, Th(H₃BNMe₂BH₃)_4,   has a central thorium atom bound to 15 separate hydrogens, beating the previous record, held by several other compounds, by one.

To achieve a high coordination number a large atom needs to be surrounded by small ones, which are presented in such a way not to crowd each other out. ’Both electronic and steric - crowding - factors can limit the coordination number,’ says Gregory Girolami, of the University of Illinois at Urbana-Champaign, one of the team’s leaders. ’In the thorium complex electronic factors are not limiting and the coordination number is almost completely dictated by the crowding.’

The compound’s high coordination number relies on thorium being extremely large and hydrogen atoms extremely small. ’What we have done is figure out how to make a compound in which the thorium forms bonds only with hydrogen atoms,’ says Girolami. There are only three other thorium compounds that can manage this, but the coordination number of one is 12 and for the other two remains unknown. ’We also used a coordinating group that is compact and chelating, [so] binds to the thorium atom by means of four hydrogen atoms, and these features enhance the stability,’ Girolami explains. 

While the new compound  has no immediate practical applications, high coordination numbers are useful, says Girolami, for keeping metal complexes ’in splendid isolation’, which in turn enhances other properties such as vapour pressure, which is useful in applications such as metal deposition from the gas phase. 

Keith Izod, an expert in coordination chemistry at Newcastle University in the UK, says, ’Although the determination of coordination number can be somewhat subjective, the reported 15-coordinate thorium compound certainly pushes the limit of maximum coordination number observed so far.’ 

’It is a very pretty structure that is perhaps more of a chemical curiosity than anything else - although there is nothing wrong with that!  Overall it is a very nice piece of work.’ 

Simon Hadlington 

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