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Unformatted text preview: he case of the trinuclear
carbonyls of Fe, Ru and Os (p. 1104) and
can be explained in a similar way.(39f The M4
tetrahedra of Co and Rh are small enough to
be accommodated in an icosahedral array of CO
ligands whereas the larger Ir4 tetrahedron forces
the adoption of the less dense cube octahedral
array of ligands.
Of the [&(C0)16] carbonyls the very darkbrown Rh compound prepared simultaneously
with and separated from [Rh4(C0)12] is the
best known. In the solid its structure consists
of an octahedral array of Rh(C0)2 units with
the remaining 4 CO’s bridging 4 faces of the
octahedron (Fig. 26.8e). A black isomorphous,
and presumably isostructural, Co analogue and
an isostructural red Ir analogue are known. A
second, black Ir isomer occurs which differs
only in that it has 4 edge-bridging rather than
face-bridging CO groups. Again rationalization
is possible on the basis of the ligand polyhedral
39 B. F. G. JOHNSON nd
977-90 (1993). Y.V. ROBERTS, olyhedron, 12,
2.. Organometallic compounds 1747 Figure 26.8 Molecular structures of some binary carbonyls of Co, Rh, and Ir. (a) Co2(CO)8 in solid state, showing
the formation of a “bent” Co-Co bond. (b) Co2(CO)s i n solution. (c) Ir4(CO)12. (d) M4(C0)12,
M = Co, Rh. (e) M6(CO)16M = Co, Rh and Ir (for its red isomer). (f) black isomer of Ir6(CO)16. model. In both structures the ligands occupy the
16 vertices of a tetracapped truncated tetrahedron.
In one case the 4 caps are the face-bridging
ligands, i n the other the edge-bridging ligands.
The two structures are related by a simple rotation
of the M6 octahedron about a c axis.(39)
Carbonyl hydrides and carbonylate anions
are obtained by reducing neutral carbonyls,
as mentioned above, and in addition to
mononuclear metal anions, anionic species of...
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