Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Electron transfer. 123. Competitive redox reactions involving a Cobalt(I) intermediate

Electron transfer. 123. Competitive redox reactions involving a Cobalt(I) intermediate he hypophosphito derivative of CoIII, (NH3)5CoO2PH2 2+, decomposes in basic media, yielding Co(II) quantitatively, along with a 1:1 mixture of hypophosphite and phosphite. Earlier studies point to a cobalt (I) intermediate, which rapidly reduces a second molecule of Co(III) reactant to Co(II). In the presence of an external cobalt (III) oxidant (Co3X), the latter competes with the hypophosphito complex for Co(I), lowering the yield of free hypophosphite. From the ratio of phosphorus products (P3/P1), evaluated by proton decoupled 31P NMR, the relative reactivities (kt/kc) of the external Co(III) “traps” and the hypophosphito complex may be determined. A log-log plot comparing values of kt/kc with rates for reductions of the same series of Co(III) oxidants with Ru(NH3)6 2+ (kRu values) is badly scattered with a slope of only 0.23, well below the value of unity stipulated by the Marcus model, indicating that an outer-sphere mechanism cannot operate for all of the Co(III)-Co(I) reactions and may not operate for any, except for that with Co(NH3)5(py)3+. Among the carboxylato-substituted oxidants, there are no rate enhancements by neighbouring pyridine, -SR,-OH, -CHO, or -SO3H functions, in contrast to the accelerations previously observed for reductions by Cr(II), Eu(II) and Ti(III), which are attributed to intermediacy of chelate-stabilized precursor complexes. It is suggested that the Co(III)-Co(I) reactions, which determine the ratio of phosphorus products, are much more rapid than the ligand substitution at the Co(I) center which must precede chelate formation, and that the latter therefore does not intervene significantly in the redox processes. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Research on Chemical Intermediates Springer Journals

Electron transfer. 123. Competitive redox reactions involving a Cobalt(I) intermediate

Download PDF
9 pages

Loading next page...
 
/lp/springer-journals/electron-transfer-123-competitive-redox-reactions-involving-a-cobalt-i-E08cK3FeyD

References (23)

Publisher
Springer Journals
Copyright
Copyright
Subject
Chemistry; Catalysis; Physical Chemistry; Inorganic Chemistry
ISSN
0922-6168
eISSN
1568-5675
DOI
10.1163/156856794X00243
Publisher site
See Article on Publisher Site

Abstract

he hypophosphito derivative of CoIII, (NH3)5CoO2PH2 2+, decomposes in basic media, yielding Co(II) quantitatively, along with a 1:1 mixture of hypophosphite and phosphite. Earlier studies point to a cobalt (I) intermediate, which rapidly reduces a second molecule of Co(III) reactant to Co(II). In the presence of an external cobalt (III) oxidant (Co3X), the latter competes with the hypophosphito complex for Co(I), lowering the yield of free hypophosphite. From the ratio of phosphorus products (P3/P1), evaluated by proton decoupled 31P NMR, the relative reactivities (kt/kc) of the external Co(III) “traps” and the hypophosphito complex may be determined. A log-log plot comparing values of kt/kc with rates for reductions of the same series of Co(III) oxidants with Ru(NH3)6 2+ (kRu values) is badly scattered with a slope of only 0.23, well below the value of unity stipulated by the Marcus model, indicating that an outer-sphere mechanism cannot operate for all of the Co(III)-Co(I) reactions and may not operate for any, except for that with Co(NH3)5(py)3+. Among the carboxylato-substituted oxidants, there are no rate enhancements by neighbouring pyridine, -SR,-OH, -CHO, or -SO3H functions, in contrast to the accelerations previously observed for reductions by Cr(II), Eu(II) and Ti(III), which are attributed to intermediacy of chelate-stabilized precursor complexes. It is suggested that the Co(III)-Co(I) reactions, which determine the ratio of phosphorus products, are much more rapid than the ligand substitution at the Co(I) center which must precede chelate formation, and that the latter therefore does not intervene significantly in the redox processes.

Journal

Research on Chemical Intermediates – Springer Journals

Published: Mar 16, 2009

There are no references for this article.