Transition metal complexes electron-density distributions

Crystallographic studies of transition metal hydride complexes Stereochemistry of six-coordination Five-coordinate structures Stereochemistry of five-coordinate Co complexes Absolute stereochemistry of chelate complexes Stereochemistry of optically-active transition metal complexes Electron density distributions in inorganic compounds... 

Contents Introduction. - X-Ray Difraction. -Conformational Analysis. - Structure and Isomerism of Optically Active Complexes. - Electron-Density Distribution in Transition Metal Complexes. - Circular Dichroism. - References. 

Although the ligand field theory can be used to rationalize the geometry of some transition metal molecules and complex ions, the study of the shapes of transition metal molecules in terms of the electron density distribution is still the subject of research and it has not reached a sufficient stage of development to enable us to discuss it in this book. 

Fourier-transform infrared (FTIR) spectroscopy Spectroscopy based on excitation of vibrational modes of chemical bonds in a molecule. The energy of the infrared radiation absorbed is expressed in inverse centimeters (cm ), which represents a frequency unit. For transition-metal complexes, the ligands -C N and -C=0 have characteristic absorption bands at unusually high frequencies, so that they are easily distinguished from other bonds. The position of these bonds depends on the distribution of electron density between the metal and the ligand an increase of charge density at the metal results in a shift of the bands to lower frequencies. 

An X-ray atomic orbital (XAO) [77] method has also been adopted to refine electronic states directly. The method is applicable mainly to analyse the electron-density distribution in ionic solids of transition or rare earth metals, given that it is based on an atomic orbital assumption, neglecting molecular orbitals. The expansion coefficients of each atomic orbital are calculated with a perturbation theory and the coefficients of each orbital are refined to fit the observed structure factors keeping the orthonormal relationships among them. This model is somewhat similar to the valence orbital model (VOM), earlier introduced by Figgis et al. [78] to study transition metal complexes, within the Ligand field theory approach. The VOM could be applied in such complexes, within the assumption that the metal and the... 

In 1973, Iwata and Saito determined the electron-density distribution in crystals of [Co(NH3)6]fCo(CN)6l (37). This was the first determination of electron density in transition metal complexes. In the past decade, electron-density distributions in crystals of more than 20 transition metal complexes have been examined. Some selected references are tabulated in Table I. In most of the observed electron densities, aspherical distributions of 3d electron densities have been clearly detected in the vicinities of the metal nuclei. First we shall discuss the distributions of 3d electron density in the transition metal complexes. Other features, such as effective charge on transition metal atoms and charge redistribution on chemical bond formation, will be discussed in the following sections. 

Some Selected Measurements of Electron-Density Distributions in Crystals of Transition Metal Complexes... 

Stevens, E. D. Analyses of electronic structure from electron density distributions of transition metal complexes. In Electron Distributions and the Chemical Bond. (Eds., Coppens, P., and Hall, M. B.) pp. 331-349. Plenum New York, London (1982). 

The first attempt to clarify the physical basis of the Jahn-Teller theorem was due to Ruch, [3] in an introductory presentation to the 1957 annual meeting of the Bunsen-Gesellschaft in Kiel, which was organised by H. Hartmann. Ruch discussed the general connection between symmetry and chemical bonding, and also touched upon the Jahn-Teller effect in transition-metal complexes. He explained that degeneracy can always be related to the existence of a higher than twofold rotational axis and a wave function which is not totally symmetric under a rotation around this axis. Provided that the wave function is real the electron densities for such a wave function are bound to be anisotropic. The combination of an anisotropic distribution of the electron cloud and a symmetric nuclear frame leads to electrostatic distortion forces where the nuclear frame adapts itself to the anisotropic attraction force. 

The importance of the accurate structure determination of optically active transition metal compounds deserves special emphasis. If the electron-density distribution and geometrical arrangement of the atomic nuclei are well known, it is possible, at least in principle, to predict all the physical and chemical properties of the complex on the basis of quantum mechanical calculations. 

As an example of an open-shell transition-metal complex we discussed some of the pitfalls of present-day DFT and CASSCF calculations in determining accurate spin density distributions in open-shell transition-metal complexes. An accurate description of the spin density and of the electronic structure is mandatory for a subsequent qualitative analysis of the chemical bonding. This could only be accomplished by employing the DMRG algorithm to produce an accurate CASCI-type electronic wave function. 

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