I,2 coordination

Quantitative comparisons of the 171 versus i 2 coordination should be made with caution, however, because Eqs. (10a) and (11) have been obtained in somewhat different approximations. The most important difference is that Eq. (10a) for the upright M -A-B coordination explicitly neglects the (repulsive) contribution from an atom B, but Eq. (11) for the side-on... 

Some of these have already been noted as 2-coordinate and linear, for example [AgfCN) ]". [Ag(NH3)2]+. [AgfS Oj)] ". Silver(I) halides dissolve in concentrated aqueous halide solutions to give complexes [AgX ]", [AgXj]. for example [AgClj] . 

Figure 13.26 (a) Structure of the anion Asu " note that the As-As distances involving the three 2-coordinate As atoms are significantly shorter than those between pairs of 3-coordinate As atoms, (b) Structure of the anion Asii i-e. [Asu-Asn] (see text). 

Cyclobutadiene complexes afford a classic example of the stabilization of a ligand by coordination lo a metal and, indeed, were predicted theoretically on this basis by H. C. Longuei-Higgins and L, E, Orgel (1956) some 3y before the first examples were synthesized, In the (hypothetical) free cyclobutadiene molecule 2 of the 4 rr-electrons would occupy t /i and there would be an unpaired electron m each of the 2 degenerate oibilals 2, Coordination to a metal provides funhei interactions and avoids this unstable configuration, See also the discussion on ferra-boranes (p. 174). 

All four monohalides of gold have been prepared but the fluoride only by mass spectrometric methods. AuCl and AuBr are formed by heating the trihalides to no more than 150°C and Aul by heating the metal and iodine. At higher temperatures they dissociate into the elements. Aul is a chain polymer which features linear 2-coordinate Au with Au-I 262 pm and the angle Au-I-Au 72°. 

Like Ag, Au also readily forms linear 2-coordinate complexes such as [AuX2] (X = Cl, Br, I) " and also the technologically important [Au(CN)2] . But it is much more susceptible to oxidation and to disproportionation into Au and Au which renders all its binary compounds, except AuCN, unstable to water. It is also more clearly a class b or soft metal with a preference for the heavier donor atoms P, As and S. Stable, linear complexes are obtained when tertiary phosphines reduce Au in ethanol. 

There are considerable numbers of the organogold compounds [3(b), 9,154], principally in the +1 and +3 oxidation states. Gold is unusual in transition metals in that, even in the +1 state, it has a marked preference for forming a-rather than zr-bonds, presumably related to the tendency of gold(I) to linear 2-coordination. 

The ions that tend to be involved in AB cements include such species as Al , Mg, Ca and Zn. These are all capable of developing a coordination number of six, and hexaquo cations are known to be formed by each of these metal ions (Huckel, 1950). The typical requirements for an ion to develop such coordination characteristics are that the ion should exist in the -I- 2 or -1-3 oxidation state, and in this state should be of small ionic radius (Greenwood Earnshaw, 1984). 

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