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

Structure and bonding
plT t
1u
Fig. 11.3 Radial molecular orbitals in an eicosahedral cluster. (After Hali and Mingos
(1984).)
Thus the clasa boranes (Greek dwf36a, a bird cage), require a total of 4n + 2
valence electrons (sometimes termed the 'polyhedral electron count', p.e.c.) to fiii
ali the bonding m.o.s in the cluster.
It has been recognized that less symmetrical borane structures can be derived
formally from B"H/- by removal of one or more BH vertices (Fig. 11.4), or by
capping one triangular face. Carboranes (strictly carbaboranes) formally arise by
isoelectronic replacements e.g. BH 2 or BH- by CH. The p.e.c.s listed in Table 11.2
follow from the result that an open cluster has the same number ofbonding m.o.s
as the closed figure from which it is derived. The same applies to a mono-capped
deltahedral structure.
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