Page 299 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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308 Nanosized Inorganic Clusters

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FIGURE 8 Se skeleton in [Ag 114 Se 34 (Se Bu) 46 (P Bu 3 ) 14 ] FIGURE 9 Molybdenum-oxide-based building blocks.
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(above) and [Ag 172 Se 40 (Se Bu) 92 (dppp) 4 ] (below). Se atoms are
depicted by the horizontally hatched spheres and the Se atoms
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of the Se Bu groups as diagonally hatched spheres.
The most important strategy to promote growth of
polyoxometalate clusters requires the generation of
sufficiently negatively charged fragments/intermediates
formed during the aggregation process. This can be at-
tained not only by substituting some lower valence metal
centers for ones of higher oxidation state or by substitut-
ing less positively charged for higher positively charged
groups (e.g., by exchanging [Mo(NO)] 3+ for [MoO] ),
4+
but mainly by the presence of an appropriate reducing
agent or even different types of these. The growth pro-
cesses are promoted by the overall solubility of the grow-
ing cluster in solution-preventing aggregation, which can
be achieved by avoiding nucleophilicity of the peripheral
oxygen atoms that is too high. The presence of numerous
FIGURE 10 Schematic representation of the 132 molybdenum
H 2 O ligands causes high solubility in protic media and ter-
atom framework of the Keplerate cluster highlighting its spheri-
minal double-bonded oxygen atoms of the O=Mo groups cal nature. Two pentagonal {(Mo)Mo 5 } groups linked by an {Mo -
V
V
prevent aggregation. Mo } bridge are emphasized.