Gold complexes bonding

The remarkable stability of the gold complexes is due to significant metal-metal bonding. However, their isolation and structural study are remarkable and greatly contributed to our knowledge of higher-coordinate carbocations. 

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. 

A related dinuclear species 77, recently described, constitutes the first dinuclear gold(I) complex with heterobridged phosphor-1,1 -dithiolato moieties and bis(ylide) bridging ligands [ 102]. It is obtained by reaction between [ AuS2PPh2] and the diylide gold complex 74 (R=Me). No intermolecular Au-Au interaction is observed in 77 but the oxidative addition of chlorine to the product leads to a new complex 78 in which a single bond is formed between the two Au(II) centers (Scheme 26). 

The cyclic carbene complex shown in equation 3.4 was studied by X-ray diffraction [66], it shows a linear complex (angle C—Au—C 178.6(4)°) and the gold aryl bond distance is 1.993(10) A which is in accordance with such bonds in other known pentafluorophenyl complexes. The gold carbene carbon distance is 1.961(9) A, the dihedral angle between the planes formed by the two organic ligands is 5.35° and the shortest intermolecular Au—Au distance is 3.95 A. 

In summary, [Au(alkynyl)(CNR)j complexes also show mesomorphic behavior but, in contrast to the halide-isocyanide derivatives, they are thermally less stable and usually decompose before or at the clearing temperatures. This thermal instability is thought to be associated with the gold-(alkynyl) bond and makes these compounds... 

The first method has been used with haloaryl,1639-1643 urt/m-nitrophenyl,1644 mesityl,255 or ylidegold(I)1645-1648 complexes, which are sufficiently stable to undergo the oxidative addition without cleavage of the gold-carbon bond, an example can be seen in Equation (11) ... 

Phosphonium ylides, which can be written in the two familiar canonical forms, are available with a wide variety of substituents both at the phosphorus and at the carbon atoms (Scheme 30). In gold complexes, without any exceptions, they function as two-electron donors, as proposed by the dipolar form to give discrete Au-C cr-bonds (771, monohapto). No side-on, 7r-coordination (t 2), as might be expected out of the ylene form, has been observed to date. 

In other words, the gold complex is of formal MX type. Thus, the NBO description of the bonding goes considerably beyond the hapticity labels in (4.110) and suggests that only a severely distorted and reorganized form of the allyl anion exists in either of these complexes. 

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