Vibrational spectral studies and crystal analysis of coordination complexes (1972)
AuthorsJanson, V.show all
There are many physical or physico-chemical methods available for studying the various properties of chemical compounds. These methods can be conveniently classified into classes which are determined by the purpose of the investigation. Those used to elucidate molecular structure fall into two classes. The first, which includes x-ray, neutron, and electron diffraction yields detailed information about the whole structure of the molecule. These techniques introduce a metrical element into the understanding of the complex, revealing the lengths of the chemical bonds, the angles between them, and other structural details such as the various forms of isomerism in the topological and conformational arrangements of polydentate ligands about the metal centre. The second gives fragmentary information concerning individual bonds of a particular group of atoms in the molecule. This class includes optical rotatory dispersion, circular dichrosism, the measurement of electric and magnetic moments, and various kinds of spectroscopy of the region ranging from microwave to ultraviolet. The last two groups should yield information mostly in the area of the electronic structure of the molecule. The last group covers vibrational spectroscopy which originates in the vibrations of the nuclei constituting a molecule. Vibrational spectra are observed both as infrared and Raman spectra, and the frequencies of the vibrational transitions are determined by the masses of the constituent atoms, the molecular geometry and the interatomic forces. The intensities of infrared and Raman spectra are related to the changes in dipole moment and polarisability, respectively. Attempts have been made to analyse the vibrational spectra quantitatively. Though there has been some success, work on many aspects (such as intensity in relation to bonding) is sparse. Nevertheless, the understanding of the molecular and the electronic structure of some molecules can be much extended by employing a combination of methods. This thesis consists of two main sections. The first describes the vibrational spectral studies of diarsine complexes. Most of the spectral work is centred around the low infrared frequency region. Raman spectra of two complexes are reported and characterised by their metal-ligand sensitive absorptions. The frequencies of these assignments are used to calculate the frequencies of the infrared active metal-ligand vibrations. The frequencies assigned to the metal-ligand vibration of diarsine complexes are analysed in terms of the change in electronic configuration of the transition element. It is hoped that the inferences obtained from the vibrational may elucidate the bonding in these complexes. These results are checked against those derived from the data given in x-ray and electron spin resonance (e.s.r.) spectral studies. The second main section gives a description of the crystal and the molecular structure of cobalt triethylenetetramine glycinato dichloride, 𝚫-𝛃2'-(RS)-(Co(trien)(gly)) Cl2.H2O. It belongs to a series of cobalt(III) polyamino compounds containing asymmetric nitrogen centres. These polyamino complexes can display various forms of isomerism in their topological and confrontational arrangements. The reasons for the study of this particular complex are many, and are given in the Introduction to Crystal Studies. However, the primary reason is that precise molecular geometry is of great importance as a check on the predicted molecular geometry derived from energy minimisation techniques.