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

Overview
The phenomenon of aurophilicity is most commonly observed crystallographically for Au(I) compounds. The aurophilic bond has 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 in light of the fact that 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.

Another important feature of aurophilicity is the propensity of gold atoms to aggregate around nucleation sites--specifically, though not limited to, ligands that bind through phosphorus, nitrogen, and sulfur centers. While both intra- and inter-molecular aurophilic interactions exist, only intramolecular aggregation has been observed at such nucleation sites.

Applications
The similarity in strength between hydrogen bonding and aurophilic interaction has proven to be a convenient tool in the field of polymer chemistry. There has been much research into self-assembling supermolecular structures, both those that aggregate by aurophilicity alone and those that contain both aurophilic and hydrogen-bonding interactions. An important and exploitable property of aurophilic interactions relevant to their supermolecular chemistry is that while both inter- and intramolecular interactions are possible, intermolecular aurophilic linkages are comparatively weak and easily broken by solvation; most complexes that exhibit intramolecular aurophilic interactions retain such moieties in solution.



Other Phenomena
Similar metallophilic interactions exist for a few other heavy metals, such as mercury, and can also be observed between atoms of different elements. Some documented examples include Hg(II)-Au(I), Hg(II)-Pt(II), and Hg(II)-Pd(II). In accordance with theoretical calculations, which predict a local maximum for relevant relativistic effects for gold atoms, none of these other interactions are as strong as aurophilicity.