Coordination number 4

   4-Coordinate complexes are extremely common, with a tetrahedral arrangement of donor atoms being the most frequently observed. The tetrahedron is sometimes ‘flattened’, distortions being attributed to steric or crystal packing effects or, in some cases, electronic effects. Tetrahedral complexes for d³ ions are not yet known, and for d⁴ ions have only been stabilized with bulky amido ligands, e.g. [M(NPh₂)₄] and [M{N(SiMe₃)₂}₃Cl] for M = Hf or Zr. Simple tetrahedral species include:

   The solid state structures of apparently simple anions may in fact be polymeric (e.g. the presence of fluoride bridges in [CoF4]^2- and [NiF₄]^2- leads to a polymer with octahedral metal centres) or may be cation-dependent (e.g. discrete tetrahedral [MnCl₄]^2- ions are present in the Cs‏ and [Me₄N]‏⁺ salts, but a polymeric structure with Mn^_Cl^_Mn bridges is adopted by the Na‏ salt). Square planar complexes are rarer than tetrahedral, and are often associated with d⁸ configurations where electronic factors strongly favour a square planar arrangement (see Section 20.3), e.g.  [PdCl₄]^2-, [PtCl₄]^2-, [AuCl₄]^-, [AuBr₄]^-, [RhCl(PPh₃)₃] and trans-[IrCl(CO)(PPh))₂]. The classification of distorted structures such as those in [Ir(PMePh₂)₄]⁺ and [Rh)PMe₂Ph(₄]⁺ (Figure 1.1a) may be ambiguous, but in this case, the fact that each metal ion is d⁸ suggests that steric crowding causes deviation from a square planar arrangement (not from a tetrahedral one). The [Co(CN)₄]^2- ion is a rare example of a square planar d⁷ complex.

Fig. 1.1 Examples of 4- and 5-coordinate structures (X-ray diffraction data): (a) in [Rh(PMe₂Ph)₄]⁺‏, the steric demands of the ligands distort the structure from the square planar structure expected for this d 8 metal centre [J.H. Reibenspies et al. (1993) Acta Crystallogr., Sect. C, vol. 49, p. 141], (b) [Zn{N(CH₂CH₂NH₂)₃}Cl]⁺‏ in the [Ph₄B]^- salt [R.J. Sime et al. (1971) Inorg. Chem., vol. 10, p. 537], and (c) [Cu(bpy){NH(CH₂CO₂(₂}], crystallized as the hexahydrate [R.E. Marsh et al. (1995) Acta Crystallogr., Sect. B, vol. 51, p. 300]. Hydrogen atoms are omitted for clarity; colour code: Rh, dark blue; P, orange; Zn, brown; Cl, green; N, light blue; Cu, yellow; O, red; C, grey.