Molecular clusters, in the broad sense that the term is commonly
understood, today comprise an enormous class of species extending into
virtually every important area of chemistry: "naked" metal clusters,
transition metal carbonyl clusters, hydrocarbon cages such as cubane (C
H ) and dodecahedrane (C H ), 8 8 20 20 organometallic cluster
complexes, enzymes containing Fe S or MoFe S 4 4 3 4 cores, high
polymers based on carborane units, and, of course, the many kinds of
polyhedral borane species. So large is the area spanned by these diverse
classes that any attempt to deal with them comprehensively in one volume
would, to say the least, be ambitious-and also premature. We are
presently at a stage where intriguing relationships between the various
cluster families are becoming apparent (particularly in terms of bonding
descriptions), and despite large dif- ferences in their chemistry an
underlying unity is gradually developing in the field. For example,
structural changes occurring in Fe S cores as electrons are 4 4 pumped
in and out, in some measure resemble those observed in boranes and
carboranes. The cleavage of alkynes via incorporation into carborane
cages and subsequent cage rearrangement, a sequence familiar to boron
chemists, is a thermodynamically favored process which may be related to
the behavior of unsaturated hydrocarbons on metal surfaces; analogies of
this sort have drawn attention from theorists and experimentalists.