For heavy elements, electrons in the core are travelling at a significant fraction of the speed of light. The resulting increase in mass of the electrons affects orbital energies, particularly those of the 6s orbital. This maximally affects the chemistry of Au, and elements in its vicinity. The structure of Cd in its complexes with crown ethers shows extreme plasticity of the coordination sphere, with short bonds to axially coordinated ligands that form more covalent Cd-L bonds, and very long bonds to the oxygen donors of a crown such as 18-crown-6 that occupy the equatorial coordination sites. The solution chemistry of the Cd(II) is greatly altered by this distortion, induced by binding to the crown, with higher binding constants for axially bound ligands such as bromide than is found for the free metal ion. DFT calculations are used to predict formation constants of a wide variety of metal ions in aqueous solution. It is shown that for Au(I) log K1 with ammonia would be 9.1 with relativistic effects included in the calculations, but that this drops down to only 2.0 if relativistic effects are excluded. The DFT calculations suggest that 'soft' behavior in the HSAB sense is largely caused by relativistic effects.
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