A copper-oxygen bond that appears to defy the law of repulsion between like charges has been discovered by chemists in the UK. 

Jane Nelson at the University of Loughborough and colleagues discovered the unusual copper-oxygen bond while they were investigating bonding between two copper ions inside molecular cages called cryptands.

When the researchers reduced the size of the cage, they found that it contained only a single copper cation. The second copper cation had been replaced by an H₃0⁺ ligand. The researchers say this ligand was bound to the copper by an extremely short copper-oxygen bond - even though the copper ion and the ligand are both positively charged. 

Nelson said the explanation for the unexpectedly short bond lies in the structure of the cryptand. The cage has three nitrogen atoms that are perfectly placed to bond to each hydrogen of the H₃0⁺ ligand. These hydrogen bonds are highly polarised towards the nitrogen, effectively giving the oxygen atom a negative charge.

AP de Silva, a supramolecular chemist at Queen's University, Belfast, said, 'by encasing a copper-oxygen bond within a small cryptand, [Nelson and co-workers] have taken a fine step forward in our understanding of the interplay between hydrogen bonding and metal coordination in restricted spaces. The opportunity to arrange other metal-oxygen bonds in such environments will now open up, with valuable implications for bioinorganic chemistry.'

Nelson said the cryptands could also find uses in synthetic chemistry. 'Scientists can now begin to investigate the potential of cryptate hosts to act as sites of chemical transformation.'

James Mitchell Crow