_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

11
Cluster compoun:ds
11.1 lntroduction
The study of compounds which contain clusters of atoms, often ofhigh symmetry,
is making significant contributions to theories of bonding. Part of the fascination
ofthis area is aesthetic, deriving from the beautiful order which these structures,
in their unanticipated variety, reveal. lmpetus for this work is enhanced by the
realization that clusters, especially of transition metal atoms, may ha ve potential
as homogeneous catalysts, where two or more sites can act in conjunction.
Moreover clusters sometimes ba ve tbe structures expected from fragments of tbe
extended close packed arrays found in bulk metals. It bas tberefore been
suggested that tbey could provide a link between bomogeneous and beterogeneous
catalyst systems. Study of cluster cbemistry could possibly lead to a deeper
understanding of the bebaviour of metal surfaces in catalysing reactions of
industrial importance such as tbe reforming of bydrocarbons or conversions of
'syntbesis gas'.
11.2 Structure and bonding
11.2 .1 Electron-precise structures
Carbon forms a wide range of catenated compounds linked togetber by strong
C-C bonds. This property is sbared to a lesser extent by otber elements of tbe
same Group (Si, Ge) and of adjacent Groups to tbe rigbt in tbe Periodic Table (P,
As, S). The fragments CR 2 , PR and S ba ve 2 available orbitals whicb contain two
electrons. Tbey can tbus link to form rings (CR 2 )", (PR)n, S" or chains with
suitable end groups. Tbe bonding in tbese 'clusters' can be described in terms of
classical two-electron two-centre bonds. Tbese ring molecules are characterized
by a total of 6n valence electrons ( e.g. 4 from C and 2 from 2R) (Table 11.1 ).
The fragment CH bas tbree available orbitals and tbree electrons for forming
skeletal C-C bonds. Polybedral three dimensional structures can arise, some of
which are illustrated in Fig. 11.1. Tbe bonding in tbese ratber strained molecules
can again be described in classical terms. In ali eacb CH group contributes five
valence electrons (four from C and one from H) so that a cluster (CH)" is
cbaracterized by Sn valence electrons. Note tbat square and pentagonal as well as
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