The affinity in water of [Os(en)2(eta2-H-2)H2O]2+for Fe(CN)6(4-) and for the other cyano complexes featured in this report-is high, and upon mixture of the reagents [Os(en)2(eta2-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 S2O8(2-) or FeCp2+ as oxidant. The full development of the color (lambda = 660 nm, epsilon = (2.1 +/- 0.1) x 10(3) 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)H2O] (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 epsilon, with little shift in lamda(max), and which we attribute to polynuclear formation. In the presence of Fe(CN)64- in excess of 2:1, epsilon rises to 4.4 x 10(3) M-1 cm-1 and lambda(max) 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(eta2-H2)H2O]2+ reacts with Os(CN)64- and Ru(CN)64- have the following respective absorption characteristics : lambda(max) = 592 nm, epsilon = 2.2 X 10(3) M-1 cm-1; lambda(max) = 522 nm, epsilon = 1.9 x 10(3) M-1 cm-1. The trend in lambda(max) with the value of E-degrees 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, lambda(max) 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.