Organometallic molecules, electronic

Not included in the present review is the fascinating new chemistry which results from reaction between diazo compounds and low-valent transition-metal complexes bearing easily displaceable two-electron ligands as well as with metal-metal multiple bonds and metal hydrides whereby a variety of novel organometallic molecules could be obtained. This field has been covered, in accord with its rapid development, by successive reviews of Hermann 19 22) and Atbini23). 

Both the semiquantitative and quantitative types of calculation yield energy-level schemes from which, in principle, spectral transition energies may be calculated. Comparison of these schemes with observed electronic spectra should enable an assessment of their relative values to be made however, the spectral data on organometallic molecules are very limited and often qualitative. There are few accurate studies involving polarization data and solvent shifts from which the nature of the observed bands can be classified and since, in most cases, the theoretical schemes of bond levels yield an embarrassing number of transitions, this lack of assignment is serious. 

Polarography. In principle, it should be possible to relate the halfwave potentials Exjl for oxidation of organometallic molecules, or of the critical oxidation potential Ec, to the energy of the highest occupied orbital, since oxidation (or reduction) in many of these systems involves simple removal or addition of an electron to this orbital. 

Chemical Properties. Simple molecular-orbital theory predicts that many organometallic molecules should show electronic effects similar to conjugated systems, since the electronic structure is generally expressed in terms of molecular orbitals which involve both ring and metal orbitals. The ESR spectra (Sec. III.C) provide physical evidence for this formulation however correlation between chemical reactivities and theoretical quantities, such as charge densities and localization energies, which has been of use in aromatic systems (60) has not been attempted. Indeed, very few detailed kinetic studies of organometallic compounds have been reported with which to compare theory. We consider the different classes of compounds in turn. 

The values obtained for the ionization potentials of organometallic molecules are much lower than the ionization potentials of the ligands (Table XIV) and are much closer to the ionization potentials of the central metal atom (Table XV), indicating that ionization subsequent to electron impact involves an electron associated with the metal atom. 

Electron Paramagnetic Resonance (EPR) Spectroscopy, Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy Electron-Nuclear Double Resonance (ENDOR) Spectroscopy Nuclear Magnetic Resonance (NMR) Spectroscopy of Inorganic/Organometallic Molecules. 

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