Aurophilicity

Aurophilicity is the tendency of gold atoms to group together. It makes chemical bonds called aurophilic bonds. Atoms in aurophilic bonds are normally between 2.7 and 3.3 angstroms apart, similar to the distance between atoms in gold metal (2.88 Å).^[1] These bonds are stronger than van der Waals interactions but weaker than covalent or ionic bonds.

Aurophilicity is also seen in gold's chemical congeners, copper and silver,^[2] and its neighbours, platinum and mercury.^[3] Researchers have not found evidence of aurophilicity in palladium.^[4] All of these bonds are or are predicted to be much weaker than an aurophilic bond between two gold atoms.

The exact way that aurophilicity works is controversial. A common theory says aurophilicity is caused by interactions between electrons in gold's complete d orbitals. The effect is stronger in gold than with copper or silver because of relativistic effects.^[5]

Other chemists disagree with this theory, and predict that the force between the gold atoms alone should push them apart due to Pauli repulsion. The short distance between them is instead because other forces from the ligands are strnoger than this force, making a net attraction between the atoms.^[6]