Coordination number 2

    Examples of coordination number 2 are uncommon, being generally restricted to Cu(I), Ag(I), Au(I) and Hg(II), all d¹⁰ ions. Examples include [CuCl₂]^-, [Ag(NH₃)₂]⁺‏, [Au(CN)₂]^-, ) R₃P(AuCl, [Au(PR₃)₂]‏ (R = alkyl or aryl, Figure 1.1a) and Hg(CN)₂, in each of which the metal center is in a linear environment. However, in the solid state, the Cu(I) centre in K[Cu(CN)₂] is 3-coordinate by virtue of cyano-bridge formation (see Structure 21.67). Bulky amido ligands, e.g. [N(SiR₃)₂]^-, are often associated with low coordination numbers. For example, in [Fe{N(SiMePh₂)₂}₂] (∠N– Fe–N=169⁰), the sterically demanding amido groups force a 2-coordinate environment on a metal centre that usually prefers to be surrounded by a greater number of ligands.

Fig. 1.1 Examples of 2- and 3-coordinate structures (X-ray diffraction data): (a) [Au{P(cyclo-C₆H₁₁)₃}₂]⁺‏ in the chloride salt [J.A. Muir et al. (1985) Acta Crystallogr., Sect. C, vol. 41, p. 1174], (b) [AgTe₇]^3- in the salt [Et₄N][Ph₄P]₂[AgTe₇] [J.M. McConnachie et al. (1993) Inorg. Chem., vol. 32, p. 3201], and (c) [Fe{N(SiMe₃(2}₃] [M.B. Hursthouse et al. (1972) J. Chem. Soc., Dalton Trans., p. 2100]. Hydrogen atoms are omitted for clarity; colour code: Au, red; Ag, yellow; Fe, green; C, grey; P, orange; Te, dark blue; Si, pink; N, light blue.