Crystal and Molecular Structures of Some Metal Nitrato Complexes

Taylor, Derek Wesley (1972) Crystal and Molecular Structures of Some Metal Nitrato Complexes. PhD thesis, University of Glasgow.

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The first part of this thesis deals with some theoretical aspects of X-ray crystal-structure analysis with particular emphasis on those techniques employed in the present study. The previous structural work on metal nitrate complexes is then briefly reviewed and the results of X-ray structure determinations of ten metal-nitrato complexes with heterocyclic amines are described. The latter results are discussed in comparison with other known structures. Many physical techniques have been used in attempts to rationalise the structures of series of metal nitrate complexes. In particular, spectroscopic evidence suggested that complexes of formula A2M(NO3)2 [A= amine, M = Co(II), Ni(II), Cu(II) and Zn(II)] have structures similar to Co(Me3PO)2(NO3)2, i.e. six-coordinate with cis-bidentate nitrate groups. However, the structure analyses now reported of some bis-pyridine adducts of metal(II) nitrates have not revealed this stereochemistry. The complex, [Cu(py)2(NO3)2] 2(py) (py = pyridine), is a centrosymmetric dimer with a bridging/chelating nitrate group of a type not previously found. In contrast, Zn(py)2(NO3)2 has a monomeric, tetrahedral molecular structure with unidentate nitrate groups. It did not prove possible to isolate the anhydrous bis-pyridine complexes of cobalt(II) and nickel(II) nitrates but one of the hydrates, Ni(py)2(NO3)2(H2O)2, was examined. In this, the nickel has a fairly regular octahedral environment with oxygens of the nitrate groups involved in an extensive system of intermolecular hydrogen bonds with coordinated water molecules. The latter feature is expected to contribute to the stability of this complex. There have been conflicting reports on the stereochemistries expected for the tris-pyridine complexes, M(py)3(NO3)2 [M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)]. These have been resolved by four separate X-ray structure determinations which all reveal distorted seven-coordinate molecular geometries. Three of the complexes [M = Co(II), Cu(II) and Zn(II)] may be considered grossly isomorphous although there are significant differences in the nitrato-coordination in each case. In the series examined, the asymmetry of the nitrate groups increases in the order Cd(II) Co(II) Zn(II) Cu(II). The anomalous position of Cu(py)3(NO3)2 in this series is explicable in terms of a static Jahn-Teller effect. The detailed mode of nitrato-coordination is particularly susceptible to changes in the metal environment. The latter part of the study has been to investigate the effect on the nitrate geometry of ligands having greater steric requirements than pyridine. Cu(alpha-pic)2(NO3)2 (alpha-pic = 2-methyl pyridine) was predicted to have a structure related to that of Co(Me3PO)2(NO3)2. However, the X-ray analysis has shown that the molecular geometry is very similar to [Cu(py)2(NO3)2]2(Py) although one of the axial sites of the copper is blocked by the presence of the a-methyl groups resulting in a monomeric species in this case. In a single-crystal e.s.r. and electronic spectral study of this complex by Professor B. J. Hathaway, a second crystalline form was discovered. Subsequent structural examination has revealed that the two forms exhibit an almost unique type of polymorphism with virtually identical molecular structures and unit cells, but different crystal packing. The complex, Cd(quin)2(NO3)2 (quin = quinoline) was found to have a seven-coordinate molecular structure similar to that of Cd(py)3(NO3)2. Intermolecular hydrogen-bonding to oxygens of the nitrate groups is postulated although it is a coordinated nitrate-oxygen that is involved in contrast to the terminal oxygen utilised in Ni(py2(NO3)2(H2O). The nitrate groups are bidentate reflecting the size of the Cd(II) coordination shell in comparison with Ni(II).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Physical chemistry, Molecular chemistry, Inorganic chemistry, Analytical chemistry
Date of Award: 1972
Depositing User: Enlighten Team
Unique ID: glathesis:1972-78604
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:08
Last Modified: 30 Jan 2020 15:08

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