Coordination number 5

   The limiting structures for 5-coordination are the trigonal bipyramid and square-based pyramid. In practice, many structures lie between these two extremes, and we have already emphasized that the energy differences between trigonal bipyramidal and square-based pyramidal structures are often small. Among simple 5-coordinate complexes are trigonal bipyramidal [CdCl₅]^3-, [HgCl₅]^3- and [CuCl₅]^3- (d¹⁰) and a series of square-based pyramidal oxo- or nitrido-complexes in which the oxo or nitrido ligand occupies the axial site:

(19.6)                                                         (19.7)

  The formulae of some complexes may misleadingly suggest ‘5-coordinate’ metal centres: e.g. Cs₃CoCl₅ is actually Cs₃[CoCl₄]Cl. 5-Coordinate structures are found for many compounds with polydentate amine, phosphine or arsine ligands. Of particular interest among these are complexes containing tripodal ligands (19.3) in which the central atom is a donor atom; this makes the ligand ideally suited to occupy one axial and the three equatorial sites of a trigonal bipyramidal complex as in [CoBr{N(CH₂CH₂NMe₂)₃}]‏, [Rh(SH){P(CH₂CH₂PPh₂)₃}] and [Zn{N(CH₂CH₂NH₂)₃}Cl]⁺‏ (Figure 19.5b). On the other hand, the conformational constraints of the ligands may result in a preference for a square-based pyramidal complex in the solid state, e.g. [Cu(bpy){NH(CH₂CO₂)₂}].6H₂O (Figure 19.5c).