Structural Studies of Nitrato Cobalt Complexes, Ylides and Two Related Organic Molecules by X-Ray Analysis

McElhatton, James (1976) Structural Studies of Nitrato Cobalt Complexes, Ylides and Two Related Organic Molecules by X-Ray Analysis. PhD thesis, University of Glasgow.

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In this thesis the techniques of crystal structure analysis are applied to the study of bonding and molecular conformations in three classes of compounds. The contents are divided into four parts; in PART I historical and theoretical aspects of structure determination are surveyed, with particular emphasis being placed on those techniques which were used in the present studies. PART II is concerned with the study of five nitrato-cobalt complexes. In an introduction, aspects of the co-ordinated nitrato group are surveyed, with particular emphasis being placed on the rationalization of the distortions observed in co-ordinated nitrato groups relative to the symmetrical structure of the free nitrate ion. This is followed by a description of the analyses of three novel cobalt complexes of formula [COA2(NO3)Cl]2, [A = pyridine, acetonitrile and tetrahydrofuran]. The three complexes are found to be centrosymmetric dimers, with asymmetrically bridging chlorine atoms. The monomeric units are described as octahedra and as sharing a meridional edge. The octahedra exhibit grossly distorted geometries, the distortions being accountable in terms of the severe constraint imposed by the short 'bite' of the nitrato group on two cis octahedral sites. Many physical techniques have been used in attempts to rationalize the structures of series of nitrato-metal complexes. In particular, spectroscopic evidence suggested that complexes of formula MA2(NO3)2 [A = amine, M = Co(II), Ni(II), Cu(II) and Zn(II)] have structures which are similar to that observed for Co(Me3PO)2(NO3)2, ie six co-ordinate with cis-bidentate nitrato groups. However no previous analyses had revealed this stereochemistry. Thus it was of interest to investigate the structures of Coa2(NO3)2 [A = pyridine, acetonitrile], the results of which are reported in the final section of PART II. Both analyses have revealed monomeric molecular structures having quasi C2 symmetry, and with geometries in accord with the above mentioned prediction. PART III is devoted to a study of ylides. In an introduction, some aspects of the chemistry of ylides containing second-row 'onium species are surveyed, with particular reference to their marked stability relative to their first-row analogues. In view of the paucity of data on S(VI) 'onium ylides, the crystal and molecular structures of three dimethylsulphoxonium ylides were undertaken. The results are presented in the following three sections, and compared with their sulphur(IV) analogues and other related systems. The final section of PART III is devoted to a discussion of aspects of these analyses that are of overall relevance to the study of such systems. The literature is surveyed in tabular form, and the competition of 'onium and stabilizing groups to delocalize the negative charge on the anionic atom is discussed. Finally the conformations of ylide structures are discussed in terms of the steric requirements for electron delocalization and the minimization of non-bonding interactions. Accounts of the structure analyses of two xanthene-9-spiro-4'-piperidine derivatives are contained in PART IV. These are the first of a series of related compounds which it is proposed to study in this laboratory. The observed geometries of the two molecules are accountable in terms of the theory of isovalent hybridization and also in terms of intramolecular non-bonded interactions.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Physical chemistry, Inorganic chemistry, Organic chemistry, Molecular chemistry
Date of Award: 1976
Depositing User: Enlighten Team
Unique ID: glathesis:1976-78721
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:59
Last Modified: 30 Jan 2020 14:59

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