Intramolecular and Environmental Contributions to Electrode Half-Reaction Entropies of M(tacn)23+/2+ (M = Fe, Co, Ni, Ru; tacn = 1,4,7-Triazacyclononane) Redox Couples
详细信息 查看全文
- 作者：Jeffrey W. Turner and Franklin A. Schultz
- 刊名：Inorganic Chemistry
- 出版年：1999
- 出版时间：January 25, 1999
- 年：1999
- 卷：38
- 期：2
- 页码：358 - 364
- 全文大小：93K
- 年卷期：v.38,no.2(January 25, 1999)
- ISSN：1520-510X
文摘
Electrode half-reaction entropies, 
ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc, are measured as a function of solvent and electrolyte type and concentrationfor four M(tacn)23+/2+ (M = Fe, Ni, Co, Ru; tacn = 1,4,7-triazacyclononane) redox couples that experiencedifferent amounts of structural change in conjunction with electron transfer. Metal dependent values of ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc areobserved for these couples and are shown to arise primarily from vibrational and electronic contributions tointramolecular entropy. Vibrational terms become important when frequencies are small and change significantlywith a change in oxidation state, as occurs when an increase in number of antibonding electrons weakens metal-ligand bonds upon reduction. Entropy measurements are referenced to the Ru3+/2+ couple, which is characterizedby small inner-shell reorganization. Experimentally, mean values of ages/gifchars/Delta.gif" BORDER=0 >(ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc)M-Ru = 27 (7), 30 (6), and 69 (14)J mol-1 K-1 obtained from data in seven solvents are observed for M = Fe, Ni, and Co, respectively. The computedsums of vibrational (obtained using octahedral stretching frequencies of M(NH3)63+/2+ complexes) and electronicentropy differences equal 4, 31, and 69 J mol-1 K-1 for the same three metals relative to Ru. The unexpectedlylarge value of ages/gifchars/Delta.gif" BORDER=0 >(ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc)Fe-Ru is the result of a spin-state equilibrium in solution. Temperature-dependent magneticsusceptibility measurements yield ages/gifchars/Delta.gif" BORDER=0 >Hages/entities/deg.gif"> = 23.8 (1.0) kJ mol-1, ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif"> = 68.2 (2.8) J mol-1 K-1, and KSE = 0.25(298 K) for conversion of low- to high-spin Fe(tacn)22+ in D2O. Observation of uniform values of ages/gifchars/Delta.gif" BORDER=0 >(ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc)M-Rufor each couple in seven solvents indicates that, if inner- and outer-shell reorganizations are coupled during electrontransfer, this fact is not reflected in the solvent dependence of ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc. Ion-pair formation occurs between oxidizedcomplexes and electrolyte anions. Negative and positive contributions to ages/gifchars/Delta.gif" BORDER=0 >Sages/entities/deg.gif">rc result as ion-paired M(tacn)23+:X- is reduced to dissociated M(tacn)22+ in H2O when X- = Cl- and ClO4-, respectively.