Coordination number two

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

Other Covalent Radii. In Cu20 and Ag20 each metal atom is equidistant from two nearest oxygen atoms, the interatomic distances corresponding to the radius values 1.18 and 1.39 A for Cu1 and Agl with coordination number two. In KAg(GN)2, in which each silver atom is similarly attached to two cyanide groups1), the effective radius of Agl is 1.36 A. It has been pointed out to us by Dr. Hoard that the work of Braekken2) indicates the presence of strings —Ag—G=N—Ag—G... 

Coordination Number Two. Although rarely found in nature, complexes of Cu(I), Ag(I), Au(I) and Hg(I) adopt linear coordination geometries, such as that observed with [H3N—Ag—NH3]. 

The relativistic effect is responsible for the large 6s - 6p energy separation in gold, which is partly responsible2 for the tendency of gold(I) to form linear complexes with coordination number. two. 

Metal ions having coordination number two occur in Hg(CN)2, [Ag(CN)2]- and [Au(CN)2]-, all derivatives of d ° species in each case the structure is linear. In K[Cu(CN)2] the copper atom is actually three-coordinated, the anion being an infinite chain each copper has a carbon atom of an unshared cyanide, and a carbon atom and a nitrogen atom of shared cyanides, as nearest neighbours in approximately planar coordination. Three-coordination also occurs in the [Hg(CN)3]- ion, but there are weak interactions between two of the nitrogen atoms and mercury atoms of other anions, giving rise to a loosely bonded chain structure. 

Metal complexes have characteristic shapes, depending on the metal ion s coordination number. Two-coordinate complexes, such as [Ag(NH3)2]+, are linear. Four-coordinate complexes are either tetrahedral or square planar for example, [Zn(NH3)4]2+ is tetrahedral, and [Ni(CN)4]2 is square planar. Nearly all six-coordinate complexes are octahedral. The more common coordination geometries are illustrated in Figure 20.12. Coordination geometries were first deduced by the Swiss chemist Alfred Werner, who was awarded the 1913 Nobel Prize in chemistry for his pioneering studies. 

Derivatives of trivalent phosphorus of coordination number two are available from stable or also unisolated intermediates by elimination of small stable molecules XY according to the general... 

Today X-ray structures of pretty stable cyclic silylenes are available in these compounds silicon is bound to two nitrogen neighbours as part of a five membered ring skeleton resulting in the first stable silylene with a pure coordination number two ... 

Another bidentate ligand whose complexes have been studied in some detail is 1,8-naphthyridine (naph) which, like nitrate, has a small bite angle and is similarly capable of affording high coordination numbers two types of perchlorate complexes have been made, M(naph)6(C104)3 (M = La-Pr) and M(naph)5 (0104)3 (M = Nd-Eu). The former complexes have 12-coordinate lanthanides, confirmed by diffraction methods for the praesodymium complex. The change in stoichiometry is doubtless a manifestation of the lanthanide contraction. 

A very large number of stable copper(I) complexes exist in a variety of stoichiometries. In few of these complexes does the formal coordination number of the metal atom exceed four. Indeed, along with silver(I) and gold (I), it is one of the few oxidation states to exhibit regularly the low coordination numbers two and three. The simple amine and halo complexes isolated from aqueous solution fortuitously contain linear copper(I) ions. As a result, coordination number two is erroneously considered to be a common coordination number for this oxidation state. In fact, two-coordinate complexes are probably outnumbered by the trigonally coordinated complexes, whereas against the vast host of tetrahedral complexes the two- and three-coordinate complexes are numerically insignificant. 

Examples conforming with the S 2 mechanism can also be provided816,83l They were found, as one would expect them, for metal ions possessing a distorted coordination shell (e.g. mercury complexes with their preferred coordination number two, square planar configurations with weakly coordinated axial ligands, etc.). 

In the ideal gibbsite-like sheet (Fig. 1) only 2/3 of the octahedral sites are occupied by the same cation and the other 1/3 is vacant each oxygen atom has coordination number two and the 0-H bond is parallel to the sheet and directed towards the vacant site. 

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