Techniques of studying Coordination Compounds And NMR


Techniques of studying Coordination Compounds

There are many technique to study the coordination compounds;
Vibrational spectroscopy
Resonance Raman spectroscopy
Spectroscopic methods unique to optically active molecules
Nuclear spectroscopies
Electron paramagnetic (spin) resonance spectroscopy (EPR, ESR)
Photoelectron spectroscopy (PES)
Evidence for covalency in transition metal complexes
Molar conductivities
Cyclic voltammetry
X-ray crystallography


Nuclear Spectroscopies:

In this section three methods will be outlined which have been used to study the properties of nuclei in coordination compounds.
1. Nuclear magnetic resonance (NMR)
2. Nuclear quadrupole resonance (NQR)
3. Mossbauer spectroscopy


Working of NMR:
 None of the techniques. Is of universal application and they all suffer from the disadvantage that the connection between the spectra obtained and the molecular bonding is seldom simple so that in this application they are generally best  used to compare two compounds rather than discuss either  in isolation. However, the spectra can be outstandingly useful in obtaining details of molecular geometry and reactions. 


Nuclear Magnetic Resonance (NMR):

Introduction:

The use of NMR in kinetic studies in particular the way that it is used to study intramolecular gymnastics (fluxionality). At this point, it has to be emphasized that spectra obtained at room  temperature may be misleadingly simple because of dynamic exchange processes in which two (or more) nuclei interconvert their chemical environments so rapidly that they give an averaged spectrum. NMR has developed to the point at which it is the second most important technique in chemistry for determining molecular structure (the most important being X-ray diffraction). 
The measurements such as the variety indicated above are available, they require a considerable amount of time and expertise in order to arrive at the optimum experimental pattern for a given measurement a fair amount of trial and error may be involved. Not surprisingly, the vast majority of NMR measurements remain of the relatively simple variety. Fortunately, too, much of the sophisticated work is aimed at taking complicated spectra and making them simple in the more general case, where such simplifications cannot be achieved by use of experimental ingenuity, use of computer programs to match observed and calculated spectra is normal. It is usually possible to be confident that such a match is unique and so the relevant molecular quantities number of nuclei of a particular type, chemical shifts and coupling constants are uniquely determined. Finally, mention should be made of the fact that it is becoming .increasingly possible to study solids by NMR. 


Example of Tungsten and Fluorine:

Complexes of WF6, [WF6L] where L is a ligand, exist and in which the tungsten atom might be seven coordinate. A low-resolution 19F spectrum shows three lines of relative intensity, which does not appear to be consistent with a seven-coordinate structures. The most likely structure is [WF5L]+ F-, in which the tungsten atom is octahedrally coordinated by six ligands, five of which are fluorines. The four coplanar fluorines give rise to the largest peak and the axial fluorines to one of the others. The fluoride anion gives the final peak.  The fine structure of the peaks confirms this assignment as does the conductivity of the compounds in liquid sulfur dioxide and, finally, X-ray crystallography. This example, as presented, depends on an interpretation of the number of observed peaks. In practice, empirical relationships between structure, chemical shifts and coupling constants would also be used in such structure determinations.


The opening out of spectra due to Paramagnetism is exploited by the use of so-called shift reagents.  These are paramagnetic compounds, soluble in organic solvents, which have a through-space effect on the species in solution.  By careful choice of shift reagent, chemical shift differences in the solute molecules are enormously increased, thus simplifying spectra, without any undue broadening of the peaks. The detailed mechanism of the effect is not fully known but it is likely that a small proportion of the solute molecules form a transient, weak, complex with the shift reagent, the effect perhaps affecting all solute molecules by a process akin to.  For 1H spectra, a lanthanide complex of the ligand Me3 CCOCHCOCMe3 with an ion such as EIII or PII is frequently used. There has long been known a particularly good illustration of the origin of one contribution to such correlations in coordination compounds. This example concerns 59Co resonances, for which it has been predicted that the chemical shift by octahedral cobalt(III) complexes should  inversely proportional to the energy separation between the 1A1g ground state and the lowest excited 1T1g, term, a separation which may, of course, be measured from the d--> d  spectra. A plot of chemical shift against the wavelength of this transition (the wavelength is proportional to the inverse of the energy separation) for some octahedral cobalt(III)  complexes.  The amount of the (small) mixing of the 1T1g term into the 1A1g term by the magnetic field is inversely dependent on their energy separation, thus explaining the observation.


Techniques of studying Coordination Compounds




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