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Unformatted text preview: y originate from the solvent
or from cleavage of a CO ligand. The carbido C
contributes 4 electrons to the cluster bonding and
in the 90-electron species [Rh6C(C0)15]2- features trigonal prismatic coordination of Rh6 about
the central C (Fig. 26.9~).
More complex geometries are found for [RhsC(C0)19] (Fig. 26.9d)
and [CogC(C0)1& (Fig. 26.9e): these two isoelectronic clusters are not isostructural though a
slight distortion would (hypothetically) transform
one into the other. The central carbido C in the
square antiprismatic [CogC(CO>lsl2-is formally
8-coordinate, the Co-C distances being in the
range 195-220pm with a mean value of 207pm.
Even more complicated structures are found
f or the large Rh clusters containing 2 carbido C atoms: [Rh12(C~)(C0)25](Fig. 26.9f has no
symmetry elements but it is clear that the R h l z
cluster surrounds an ethanide unit C2 in which the
C-C distance is only 147 pm); the cluster also has
14 pendant terminal CO groups, 10 p -CO groups
and one p3-CO. In contrast, [ R ~ I ~ ( C ) ~ ( C O ) Z ~ ] has individual 6-coordinate (octahedral) carbido
C atoms symmetrically placed on each side of a
central Rh which itself has 12 Rh nearest neighbours in addition to the 2 C atoms. Again, the
approach to metal structures is notable and is one
of the main interests in constructing large clusters
and studying their chemical and catalytic activity.
H, P, As, S have also been encapsulated in ions
such as [ R ~ ~ ~ ( H ) ~ ( C O [Rh9P(C0)213~)Z~~~-,
[RhloAs(C0)zzI3- and [Rh17(S)z(CO)321~-. 826.3.5 Organometallic compounds 1143 Cobaltocene, [Co"(q5-C5H5)2], is a dark-purple
air-sensitive material, prepared by...
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