Gold compounds coordination number

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. 

The phosphine and arsine complexes of gold(I) have been intensively studied since the early 1970s. The possibilities of coordination numbers between 2 and 4 have been explored, though the use of bulky ligands is less essential than with the isoelectronic M(PR3)2 (M = Pd, Pt) compounds and the coordination numbers depend on both steric and electronic factors [71]. 

As an additional point, the f-factors calculated for the four sites of Aujj [25], as well as those for several smaller and larger magic-number and non-magic number cuboctahedra, for most of which no real examples of gold cluster compounds exist, exhibit a linear dependence on the coordination number of each site. Several of these are shown in Fig. 5. Thus, the coordination numbers of... 

The small difference in energy between the s, p and d states leads to the efficient formation of s/d or s/p hybridizations, which are important for explaining the pronounced tendency of gold(I) to form linear two-coordinate complexes. This tendency for two coordination is much greater than for other isoelectronic centers, such as platinum(O), silver(I), or mercury(II), which normally yield compounds with higher coordination numbers. 

Gold exhibits other interesting anomalies. For example some Au(l) compounds with an expected coordination number of two (Chapter 12) and a filled core of 5c/,a... 

The types of compounds formed by gold(I) and gold(III) often differ from those of other metals due to the constraints imposed by coordination number and electron count at the metal. Thus, for example, whereas 7r-bonded cyclopentadienyl complexes of palladium and platinum are numerous (336), and a copper(I) species of this type is known (337), cyclopentadienyl complexes of univalent (94, 96, 97) and trivalent (228) gold have invariably been found to be fluxional behavior, similar to that in dicyclopentadienylmercury, was involved (228). 

The most common oxidation states for gold are I and III. In general, Au is favored with hard ligands and Au with soft ligands. Au complexes are diamagnetic. The majority of Au compounds have a coordination number of two with linear geometry. However, compounds with trigonal planar and tetrahedral stmctmes are also known. The compounds of the type [X(Au PR3 ) ], where X = O, S, Se, Cl, Br, I, N, P, As, C, and so on, with n > 1 will be discussed in Section 15. 

In some of these mixed gold-main group compounds, the coordination numbers and stereochemistry associated with main group atoms such as carbon and nitrogen are very... 

Electron spectroscopy for chemical analysis (ESCA) has also been apphed successfully for elucidating oxidation states and coordination numbers of gold compounds, but the number of studies is stiU very limited. 

For a rough estimate of A(r ) we take the isomer shift difference between the AuF complexes and AuFj, AS= 3.4 mm/s, as representative for the one between Au(V) and Au(III). It is obvious from the above discussion that for this pair of gold compounds the difference in effective coordination number is smaller than for any other pair of the stud-... 

---