Transition metals coordination geometries

In 1937 Jahn and Teller applied group-theoretical methods to derive a remarkable theorem nonlinear molecules in orbitally degenerate states are intrinsically unstable with respect to distortions that lower the symmetry and remove the orbital degeneracy.37 Although Jahn-Teller theory can predict neither the degree of distortion nor the final symmetry, it is widely applied in transition-metal chemistry to rationalize observed distortions from an expected high-symmetry structure.38 In this section we briefly illustrate the application of Jahn-Teller theory and describe how a localized-bond viewpoint can provide a complementary alternative picture of transition-metal coordination geometries. 

Figure 13 Common transition metal coordination geometries.

Pauling further extended the sp"dm hybridization approach to the d-block compounds.3 By varying the relative importance of p and d orbitals, Pauling was able to construct hybrid orbitals that rationalized the geometries and magnetic properties of many transition-metal coordination complexes. For example, the square-planar... 

PM3 semi-empirical calculations furnish a solid account of the geometries of transition-metal coordination compounds. Most distances are within a few hundredths of an A of their experimental values, but large errors appear for a few compounds. PM3 can be recommended (with due caution) for preliminary structure determinations of transition-metal coordination compounds. 

Carbonyl Complexes of the Transition Metals Coordination Numbers Geometries Coordination Organometalhc Chemistry Principles Diffraction Methods in Inorganic Chemistry Molecular Orbital Theory. 

This example illustrates the complexity of the excited states dynamics in this class of molecules. The early stage dynamical behaviour (in the first ps) may be tailored by the metal centre, the a-diimine group or the surrounding ligands. These chemical factors govern the character and electronic localization of the excited states, their relative position, the presence of critical geometries. Moreover the shape and relative positions of the PES may be also modified by the other experimental conditions like solvent effects. Obviously the development of new theoretical tools able to compute accurate multidimensional PES is needed to investigate the dynamics of photochemical reactions in transition metal coordination compounds. 

A large and diverse number of transition-metal coordination numbers and geometries can be used in the construction of coordination-driven assemblies, giving access to different topologies rather difficult to obtain with the classical synthetic methods. The synthetic process is under thermodynamic control when relatively labile metal centers are used (true supramolecular self-assembly), while it is under kinetic control with inert metals (unless high temperatures are used). 

PM3/TM is an extension of the PM3 method to include d orbitals for use with transition metals. Unlike the case with many other semiempirical methods, PM3/TM s parameterization is based solely on reproducing geometries from X-ray diffraction results. Results with PM3/TM can be either reasonable or not depending on the coordination of the metal center. Certain transition metals tend to prefer a specific hybridization for which it works well. 

PM3/TM is an extension of the PM3 method to transition metals. Unlike the parameterization of PM3 for organics, PM3/TM has been parameterized only to reproduce geometries. This does, of course, require a reasonable description of energies, but the other criteria used for PM3 parameterization, such as dipole moments, are not included in the PM3/TM parameterization. PM3/TM tends to exhibit a dichotomy. It will compute reasonable geometries for some compounds and completely unreasonable geometries for other compounds. It seems to favor one coordination number or hybridization for some metals. 

---