Gold belongs to group 11 of the periodic table, along with copper and silver, and has the electronic configuration [Xe]4f 145d106s1. In gold complexes, the oxidation states I, II and III are now well established and a brief overview of each is given in the following paragraphs. Au(I) and Au(III) species are present in gold drugs and gold complexes undergoing biological evaluation. While there are no Au(II) species used in medicinal chemistry, it may be that Au(II) is an intermediate between the Au(I) and Au(III) species.
Gold(I) complexes disproportionate in aqueous solution unless they are either of very low solubility, e.g. Au(I), or in presence of a strong ligand, e.g. [Au(CN)2]_, according to the equation
Since Au(I), the aurous ion, has a 5d10 electronic configuration, Au(I) complexes are diamagnetic. They usually adopt linear structures. As mentioned above, the relativistic effects operating for gold stabilize the 6s orbital, which decreases the energy gap between this and the 5d orbitals. As a result, the s, p and d orbitals have relatively small energy differences, making sp hybridization possible, and thus explaining the high tendency of Au(I) to form two-coordinate complexes, contrary to the isoelectronic Pt(0) and even Ag(1) centres.12 The two-coordinate complexes are linear, in accord with the valence-shell electron repulsion theory (VSEPR), and may be cationic, e.g. [Au(PPh3)2]+, neutral, e.g. [Ph3PAuCl] or anionic, e.g. [AuCN2]~. Being a soft metal centre, the most favoured ligands for gold are 'soft', i.e. those ligands that contain sulphur (thiolates, thioethers), selenium (selenates) and phosphorus (phosphines) donor atoms. Nitrogen (cyanide, amines) donors and alkyl (methyl, phenyl) groups are also possible but less common.
While two-coordinate linear geometries dominate Au(I) structures, higher coordination numbers are also known. Trigonal planar complexes with coordination number three such as [Au(PPh3)3]+ and [(Ph3P)2AuCl], as well as tetrahedral complexes with coordination number four such as [Au(PMe3)4]+ are known.13 Nevertheless, these complexes are rather unstable and readily lose phosphine ligands to revert to the more favoured linear geometry.14,15
Compared to the common oxidation states for gold, Au(I) and Au(III), Au(II) complexes are relatively scarce and have no known biological function. Gold(II) species may be formed as transient intermediates in redox reactions between the stable oxidation states I and III.16 The lack of stability of Au(II) arises as these species readily disproportionate to more stable Au(I) and Au(III) species.17 Complexes formed with Au(II) are often organoAu(II) species and are expected to be paramagnetic, owing to the outer 5d9 electronic configuration, unless a gold—gold bond is present.18 Gold(II) complexes are also formed with certain unsaturated sulphur-donor ligands such as the maleonitriledithiolate ion leading to square planar Au(II) centres.18
Gold(III), the auric ion, has a 5d8 electronic configuration and, with the isoelectronic Pt(II) and Rh(I) ions, is diamagnetic and adopts, almost exclusively, four-coordinate, square-planar complexes. The range of ligands able to complex Au(III) is numerous and so complexes of Au(III) can have various chemical and physical properties. For example, they can carry charges from +3 (with four neutral ligands, e.g. NH3) to —1 (with four negative ligands, e.g. Cl—). Five-coordinate Au(III) complexes with rigid bidentate nitrogen-donor ligands are square pyramidal or distorted square pyramidal, e.g. [AuCl3(2,20-biquinolyl)]19 and [AuCl3(2,9-dimethyl-1,10-phenanthroline)].20 Trigonal bipyramidal Au(III) complexes are generally formed as intermediates during ligand-substitution reactions in square-planar Au(III) complexes.20
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