Geometry of metal complexes

Fig. 8.3. Established geometries of metal complexes with EDTA-type ligands.

The intriguing question is how the seven-coordinate geometry of metal complexes engenders its remarkable catalytic activity, exceeding that of the native mitochondrial MnSOD enzymes (Fig. 2), knowing that the coordination sphere of active metal centers in the native SOD enz5mes is of different geometry. 

Figure 8.2 Coordination geometries of metal complex catalysts (redrawn from Collman et al., 1987).

The pure Hartree-Fock approximation works fairly well for predicting the geometries of metal complexes. Vibrational frequencies are overestimated while formation energies are underestimated. These errors are rather systematic so that, for example, it is standard practice to scale calculated frequencies some empirical factor (Pople et al. 1993) to predict experimental values. The problem is that the single-Slater-determinant approximation to the wavefunction is too restrictive it doesn t allow the electrons with opposite spin to avoid each other as much as they would like. In accordance with the variational principle, the Hartree-Fock total energy is always too... 

The structure theory of inorganic chemistry may be said to have been bom only fifty years ago, when Werner, Nobel Laureate in Chemistry in 1913, found that the chemical composition and properties of complex inorganic substances could be explained by assuming that metal atoms often coordinate about themselves a number of atoms different from their valence, usually four atoms at the comers either of a tetrahedron or of a square coplanar with the central atom, or six atoms at the comers of an octahedron. His ideas about the geometry of inorganic complexes were completely verified twenty years later, through the application of the technique of x-ray diffraction. 

The molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

A simple example of aromatic CT complexation to transition metals is illustrated in the Cr(CO)3- -CefL complex shown in Fig. 5.49. In the geometry of this complex, each of the three polar aoc acceptor orbitals of Cr(CO)3 is oriented to point the d-type metal hybrid lobe toward one of the benzene C=C bonds, somewhat analogous to the nitrosyl positioning shown in Fig. 5.48(b). 

The chemistry of metal complexes featuring alkyne and alkynyl (acetylide) ligands has been an area of immense interest for decades. Even the simplest examples of these, the mononuclear metal acetylide complexes L MC=CR, are now so numerous and the extent of their reaction chemistry is so diverse as to defy efforts at a comprehensive review. " The utility of these complexes is well documented. Some metal alkynyl complexes have been used as intermediates in preparative organic chemistry and together with derived polymeric materials, many have useful physical properties including liquid crystallinity and nonlinear optical behaviour. The structural properties of the M—C=C moiety have been used in the construction of remarkable supramolecular architectures based upon squares, boxes, and other geometries. ... 

Although several phenyl derivatives of the lanthanides and actinides have been characterized, only one re-arene complex of the / transition metals is known to date. This is the uranium(III) benzene complex, U(AlCl4)s CeHe 153), prepared by the combination of uranium tetrachloride, aluminum trichloride and aluminum powder in refluxing benzene, the Fischer-Hafner method [154). The molecular geometry of the complex is shown in Fig. 18. 

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