The pyrazine-bridged diosmium(II) complex
{(μ-C4H4N2)[Os(CN)5]2}6-
has been synthesized as the hexapotassium
salt and was converted to the hexakis(tetrabutylammonium) compound
by ion exchange. Its stepwise oxidation
to the Os2
IIIOsII and
Os2
IIIOsIII states was
monitored spectroelectrochemically in acetonitrile/0.1 M
Bu4NPF6 in
the UV−vis−NIR and IR regions and by EPR. The
OsII
2 species exhibits long-wavelength
solvatochromic MLCT
bands at 14 800 and 18 480 cm-1. The
pentaanionic mixed-valent ion has a comproportionation constant
K
c of
105.8 and is distinguished by several electronic
absorptions in the IR region. The bands at 1972, 2480,
4000,
5000, 7170, and 11 900 cm-1 are similar but
appreciably lower in energy than those reported for the
analogous
complex
{(μ-C4H4N2)[Os(NH3)5]2}5+.
In addition to CN vibrational stretching features shifted on
electron transfer
there is an additional sharp band only for the 5− ion at
1582 cm-1 which is attributed to a pyrazine
ring vibration.
The intensity of this band suggests an unsymmetrical situation and
thus valence localization on the vibrational
time scale. The EPR signals at g
⊥ =
2.0563 and g
∥ = 1.761 point toward excited
states lying close to the doublet
ground state and to significant metal/cyanide interaction.
Compared to the Creutz−Taube ion
{(μ-C4H4N2)[Ru(NH3)5]2}5+ and its
osmium homologue, the 5− ion described here exhibits a lower degree
of metal−metal coupling.
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