What is "aurophilicity"

In chemistry, aurophilicity refers to the tendency of gold complexes to aggregate via formation of weak gold-gold bonds.

The main evidence for aurophilicity is from the crystallographic analysis of Au(I) complexes. The aurophilic bond is assigned a length of about 3.0 Å and a strength of about 7–12 kcal/mol, which is comparable to the strength of a hydrogen bond. The aurophilic interaction is thought to result from electron correlation of the closed-shell components, which is unusual because closed-shell atoms generally have negligible interaction with one another at distances on the scale of the Au-Au bond. This is somewhat similar to van der Waals interactions, but is unusually strong due to relativistic effects. Observations and theory show that, on average, 28% of the binding energy in aurophilic interaction can be attributed to relativistic expansion of the gold d orbitals.

An example of aurophilicity is the propensity of gold centres to aggregate. While both intra- and inter-molecular aurophilic interactions have been observed, only intramolecular aggregation has been observed at such nucleation sites.