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Electrochemistry of Molybdenum Aquo Ions


Paffett, Mark Thomas (1983) Electrochemistry of Molybdenum Aquo Ions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hv5n-dz91.


The electrochemical behavior of selected molybdenum aquo ions in acidic media is examined in relation to solution structure.

The electrochemistry of Mo(VI) in non-complexing aqueous electrolytes is usually severely complicated by the oligomerization and subsequent adsorption of the reactant. This problem can be circumvented by employing dilute (≤ 10-4 M) solutions of Mo(VI) in 1 to 2 M trifluoromethanesulfonic acid. Under these conditions staircase voltammograms and pulse polarograms exhibit single, reversible waves that are consistent with the one-electron reduction of an unadsorbed, monomeric Mo(VI) species. The pH dependence of the reduction potentials suggests that two protons are consumed in the reduction of each Mo(VI). The monomeric Mo(V) reduction product undergoes spontaneous dimerization with a rate constant estimated as 103 M-1 s-1. It also reduces perchlorate and nitrate anions at a significant rate.

The Mo2(V)/Mo2(III) redox couple in acidic solution involves an overall four electron-six proton transfer connecting the two participants. This redox process is characterized by extreme electrochemical irreversibility. Reduction of the Mo2(V) aquo ion to the Mo2(III) aquo ion proceeds with αna equal to 0.73 and a proton reaction order of 1.4. A chemical step with an inverse dependence on proton concentration precedes the reoxidation of aquo Mo2(III) to aquo Mo2(V). Plausible mechanisms are given for these observations.

The trinuclear ions containing Mo(IV), Mo3O4(H2O)4+9(Mo3(IV)) and an oxalato derivative, Mo3O4(C2O4)3(H2O) 2-3, can be reversibly reduced in acidic media to trinuclear Mo(III) species. The reductions involve two sequential electron transfer steps with formal potentials that are pH dependent:

[chemical equations; see abstract in scanned thesis for details].

Two waves are evident in voltammograms and polarograms of Mo3O4(C2O4)3(H2O)2-3 but with Mo3(IV) the two formal potentials are too close together to observe separate waves. However, logarithmic analysis of the shapes of normal pulse polarograms allowed the two formal potentials to be evaluated. The reductions of both complexes are believed to be accompanied by protonation of the bridging and capping oxo-ligands. The new, trinuclear Mo3(III) species resulting from the three-electron reduction of Mo3(IV) exhibits a characteristic EPR spectrum. The mixed-valent intermediate, Mo3(III, III, IV), is diamagnetic. Possible structural changes that accompany the addition of electrons and protons to Mo3IV) are discussed.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Gray, Harry B.
Thesis Committee:
  • Anson, Fred C. (chair)
  • Grubbs, Robert H.
  • Marcus, Rudolph A.
  • Gray, Harry B.
Defense Date:12 April 1983
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Record Number:CaltechTHESIS:11012019-171533510
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapter 1. adapted for Chapter 2.
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11883
Deposited By: Mel Ray
Deposited On:02 Nov 2019 00:50
Last Modified:16 Apr 2021 22:31

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