Nonbridging Ligand Effects In Some Electron Transfer Reactions Of Chromium Iii Complexes With Chromium Ii

Author : Robert J. Balahura
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (144 download)

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Download or read book The Preparation, Characterization, and Chromium (II) Reduction of Pentaamminecobalt (III) written by Robert J. Balahura and published by . This book was released on 1971 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The chromium(II) reduction of a series of pentaammine- cobalt(III) complexes has been studied. A classification into inner- or outer-sphere paths has been made and the position of attack of the reductant on the ligand determined. The oxygen-bonded complexes of formamide, N,N-dimethylform- amide, dimethylsulfoxide, and urea as well as the nitrogen bonded N-cyanoguanidine complex are reduced by means of an outer-sphere mechanism. On the other hand ligand transfer was shown to occur in the reduction of (NH[subscript 3])[subscript 5]CoNHCHO[superscript 2+], (NH[subscript 3])[subscript 5]CoNCNH[superscript 2+], (NH[subscript 3])[subscript 5]CoO[subscript 2]CNH[subscript 2][superscript 2+], (NH[subscript 3])[subscript 5]CoO[subscript 2]CC[subscript 6]H[subscript 4]CN[superscript 2+], and (NH[subscript 3])[subscript 5]CoNCO[superscript 2+]. Both outer- and inner-sphere paths operate in the reduction of the nitrile bonded 4-cyanophenol complex, the inner-sphere path comprising 20% of the overall reaction. For the reduction of the terephthalonitrile complex a classification into an inner- or outer-sphere path cannot be made/ and an outer-sphere electron transfer to the ligand is postulated as the rate-determining step. A large part of the work has been concerned with the characterization of the complexes and Chapter III describes the spectroscopic measurements undertaken as well as the chemical reactions of some of the complexes. The acid dissociation constant of the N isomer of formamidopentaamine- cobalt(III), cyanamidopentaamminecobalt(III), and the nitrile bonded 4-cyanophenol have been determined. The protonation equilibrium constant for carbamatopentaammine- cobalt(III) was also obtained. A kinetic study of the acid hydrolysis of isocyanatopentaamminecobalt(III) to form hexaamminecobalt(III) and carbon dioxide was carried out. The acylation of the cyanamide and 4-cyanophenol complexes was also studied. The results of the kinetic studies of the chromium(II) reductions indicate a possible correlation between the reduction rate and the half-wave potential for the reduction of the free ligand. The studies have led to the following generalizations regarding electron transfer reactions: (1) attack of reductant at a coordinated atom does not occur, (2) the atom attached to chromium(II) in the transition state must have two lone pairs of electrons, and (3) the reducibility of the complexed ligand plays an important role in the detailed mechanism for the transfer of the electron.