During the first half of this century, the principles associated with
Werner-type coordination complexes, namely, those containing a central
metal cation surrounded by a group of neutral donor molecules or anions
(e.g., H20, NH,, CI-, and CN-), were delineated. However, the last 15 years
(1) have seen the arrival of a new wave of coordination compounds in
which a group of ligands surround two or more metal atoms that are di￾rectly bonded to each other. While perhaps even the majority of these
compounds owe their existence to the use of nonaqueous solvents and
anaerobic conditions. there are many that are readily synthesized in, and
quite stable to, water and our otherwise noninnocent atmosphere. At this
time, virtually all the transition elements are known to form homo- or het￾erodinuclear compounds with metal-to-metal bonds that may be of integral
(1, 2, 3, or 4) or fractional (i, 11, 21, or 34) order. There are also large
classes of cluster compounds, ranging from polynuclear metal carbonyls
and other organometallics to polynuclear metal halides, oxides, and chal￾cogenides, that contain delocalized metal-metal bonds. Initially, most of
the interest in these compounds centered on elucidation of their structure,
bonding and spectroscopic properties. More recently, there has been a
growing interest in the reactivity patterns associated with these compounds
(2). This chapter presents a survey of the types of reactions that have been
established as of this writing. The survey is not intended to be comprehen￾sive, but rather to illustrate the trends that are emerging in this new field.
With time, perspectives, emphasis, and fashionable opinions may change,
but there is no doubt that the development of chemistry surrounding metal￾to-metal bonds will be one of the most significant aspects of the post￾Wernerian age of coordination chemistry.