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| Title: | Mixed-valence molecules based on monohydridobis(ethylenediamine)osmium(IV) and metal cyano complexes |
| Authors: | Li, Z.-W.a, Yeh, A.ab, Taube, H.a |
| Contributors: | Department of Chemistry, Tunghai University |
| Date: | 1994 |
| Issue Date: | 2013-05-14T09:01:43Z (UTC)
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| Abstract: | The affinity in water of [Os(en)2(η2-H2)H2O]2+ for Fe(CN)64- - and for the other cyano complexes featured in this report - is high, and upon mixture of the reagents [Os(en)2(η2-H2)Fe(CN)6] 2- (Os(II)-Fe(II)) forms rapidly. On exposure of the colorless solution to air, it rapidly turns blue, as is the case also with S2O82- or FeCp2+ as oxidant. The full development of the color (λmax = 660 nm, ε = (2.1 ± 0.1) × 103 M-1 cm-1) requires 2 equiv of oxidant/mol of Os(II)-Fe(II). Spectrophotometric and other evidence leads to the conclusion that the product is [OsIV(en)2-(H)(FeII(CN)6)H 2O] (Os(IV)-Fe(II)). It is formed also from [Os(en)2(H)(H2O)2]3+, as already reported,3 and Fe(CN)64-. By the latter method of preparation, it is difficult to separate the formation of Os(IV)-Fe(II) from a following reaction, which leads to a doubling of ε, with little shift in λmax, and which we attribute to polynuclear formation. In the presence of Fe(CN)64- in excess of 2:1, ε rises to 4.4 × 103 M-1 cm-1 and λmax shifts to 682 nm. Both observations lend weight to the conclusion3 that the metal center in the residue [OsIV(en)2H]3+ can assume a coordination number of 7. The products of the 2e- oxidation of the complexes formed when [Os(en)2(η2-H2)-H2O] 2+ reacts with Os(CN)64- and Ru(CN)64- have the following respective absorption characteristics: λmax = 592 nm, ε = 2.2 × 103 M-1 cm-1; λmax = 522 nm, ε = 1.9 × 103 M-1 cm-1. The trend in λmax with the value of E° for the three M(CN)63-/4- couples and the characteristics of the absorption band indicate that the mixed-valence molecules are localized and that the absorption bands are associated with M(CN)64-→Os(IV) electron transfer. For Mo(CN)84- as nucleophile, λmax appears at 702 nm. The lower energy of the transition compared to that for Os(IV)-Fe(II) is ascribed to a lower Franck-Condon barrier to electron transfer for the octacyano complex. Association of Fe(CN)63- with the Os(IV) center also gives rise to charge transfer absorption, with characteristics much like those observed for Os(IV)-Fe(II), but which must now involve Os(IV) to Fe(CN)63- charge transfer. Acid shifts the charge-transfer band for Os(IV)-Fe(II) to higher energy, as does base (pH > 10). We infer that in acid a proton is added to Fe(CN)64-, raising the Fe(III)/Fe(II) redox potential; in base, coordinated water is deprotonated, thus lowering the Os(IV)/Os(III) redox potential, both processes increasing the energy barrier to electron transfer. ? 1994 American Chemical Society. |
| Relation: | Inorganic Chemistry
Volume 33, Issue 13, 1994, Pages 2874-2881 |
| Appears in Collections: | [化學系所] 期刊論文
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