Four-coordination idealized geometries

Phthalocyanine is a divalent ligand with a geometry appropriate for forming a four-coordinated square-planar complex and was thought ideal for generating an Au(II) species. The action of AuBr on molten 1,3-diiminoisoindoline in the absence of solvent indeed yields a neutral gold phthalocyanine (63). The EPR spectra of the complex in 1-chloronaphthalene at 77 K clearly showed the presence of Au(II), a d9 ion. The g value is 2.065, comparable to 2.042 for copper and 2.093 for silver phthalocyanine (64, 80). 

The order of preference of the polyhedra given in Table 5.6 is confirmed by the observation that the first four polyhedra have been reported in the structures reported for Lanthanide complexes. The idealized shapes of the four favoured polyhedra are shown in Fig. 5.9. It is necessary to exercise utmost care in the consideration of specific 7-coordination in view of the very small differences of spatial arrangement between the various ideal geometries. 

Fig. 6. Idealized coordination modes for metal complexes. Limiting geometries for four-coordinate (ML4, top) and five-coordinate (ML5, bottom) complexes.

For the spherically symmetric Cu d" ion, the common geometries are two-coordinate linear, three-coordinate trigonal planar, and four-coordinate tetrahedral. Some distortions from these ideal geometries are observed, particularly with chelating ligands a fairly small number of pentacoordinate Cu complexes have been isolated and characterized as well. Cu compounds are diamagnetic and colorless, except where the color results from charge-transfer bands or a counterion. Cu complexes are often fairly readily oxidized to Cu compounds the electron-transfer kinetics of several systems have been studied. ... 

For real molecules or ions, the stereochemical descriptor should be based on the nearest idealized geometry. However, some idealized geometries are closely related [e.g. square planar (SP-A), four-coordinate square pyramidal (SPY-A), see-saw (SS-4), and tetrahedral (T-4) T-shaped (TS-3), trigonal planar (TP-3), and trigonal pyramidal (TPY-3)] and care may therefore be required in making the choice. 

Ag(I) is a d10 ion and, like Au(I), can be two-, three- or four-coordinate with appropriate ligands. The idealized geometries are linear, trigonal planar and tetrahedral, but distortions from the ideal are observed in the solid state and in non-homoleptic complexes where greater silver s character is conferred on the bonds to selected elements. The chemical shift values generally increase with coordination number, other things being equal,... 

There are three ideal coordinahon geometries for four-coordinate complexes, schematically shown in Scheme 35 square planar, tetrahedral, and sawhorse (also called see-saw). The latter is idealized as a cis-divacant octahedron. Here we will provide some rules that allow us to classify and predict the stereochemistries of four-coordinate complexes from their electron configuration in a hypothetical tetrahedral complex. Those rules work well for homoleptic complexes with aU a-donor or all -donor ligands, but mixed ligand complexes may show deviations from the general trends. 

---