Coordination number 7

    High coordination numbers (≥7) are observed most frequently for ions of the early second and third row d-block metals and for the lanthanoids and actinoids, i.e. rcation must be relatively large. Figure 1.1a shows the arrangement of the donor atoms for the three idealized 7- coordinate structures; in the capped trigonal prism, the ‘cap’ is over one of the square faces of the prism. In reality, there is much distortion from these idealized structures, and this is readily apparent for the example of a capped octahedral complex shown in Figure 1.1b. The anions in [Li(OEt₂)]‏[MoMe₇]^- and [Li(OEt₂)]‏[WMe₇]^- are further examples of capped octahedral structures. A problem in the chemical literature is that the distortions may lead to ambiguity in the way in which a given structure is described. Among binary metal halides and pseudo-halides, 7-coordinate structures are exemplified by the pentagonal bipyramidal ions [V(CN)₇]^4- (d²) and [NbF₇]^3- (d¹). In the ammonium salt, [ZrF₇]^3- (d⁰) is pentagonal bipyramidal, but in the guanidinium salt, it has a monocapped trigonal prismatic structure (Figure 1.1c).

Fig. 1.1 (a) The coordination spheres defined by the donor atoms in idealized 7-coordinate structures. Examples of

7-coordinate complexes (X-ray diffraction data): (b) the capped octahedral structure of [TaCl₄(PMe₃)₃] [F.A. Cotton et al. (1984) Inorg. Chem., vol. 23, p. 4046], (c) the capped trigonal prismatic [ZrF₇]^3- in the guanidinium salt [A.V. Gerasimenko et al. (1985) Koord. Khim., vol. 11, p. 566], and (d) the pentagonal bipyramidal cation in [ScCl₂(15-crown-5)]₂[CuCl₄] with the crown ether occupying the equatorial plane [N.R. Strel’tsova et al. (1992) Zh. Neorg. Khim., vol. 37, p. 1822]. Hydrogen atoms have been omitted for clarity; colour code: Ta, silver; Cl, green; P, orange; Zr, yellow; F, green; Sc, brown; C, grey; O, red.