The crystal structures of Na3
[H6CoMo6O24].8H2O
and K6
[H4Co2Mo10O38].7H2O
have been determined by X-ray diffraction. The monomer,
Na3
[H6CoMo6O24].8H2O,
is triclinic, space group P-1,
a 6·451(1), b
10·866(2), c 10·922(2) Å, α
109·20(1), β 106·90(1), γ 95·43(1)°,
V 676·3(2) Å3, Z 1,
and the structure was solved to an R1 value of
0·0243 (wR2 0·0784)
for 3761 independent observed reflections. The anion exhibits the well known
Anderson structure with six octahedral MoO6 edge-sharing
units surrounding the central ‘CoO6’
octahedron, with all metals in a common plane. The dimer,
K6
[H4Co2Mo10O38].7H2O,
is monoclinic, space group P21/c, a
11·795(5), b 11·626(2), c 29·731(13) Å, β
95·33(2)°, V 4059(3)
Å3, Z 4, and the structure was solved to an
R1 value of 0·0215 (wR2
0·1040) for 6546 independent observed reflections. The anion can be
derived from the monomeric hexamolybdocobaltate(III) ion by removing one
‘MoO5’ unit (ignoring the hydrogen atoms)
from each of two monomer anions, turning one 180° around a
CoO6 octahedral diagonal, and joining them to create two
CoO6 octahedra sharing an edge. Cyclic voltammetry shows
that both anions are irreversibly reduced at low pH values
(4·0–4·5), likely as a result of chemical reactions
following the initial reduction steps. At higher pH values
(4·5–5·4), a change in speciation occurs in both cases,
most likely the result of the formation of less highly protonated species,
which also display irreversible electrochemical behaviour.