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               408                                                                                Metal Cluster Chemistry









                          FIGURE 1 Structure of Fe 2 (CO) 9 .
                                                                          FIGURE 3 A square pyramidal cluster.
               X-ray single crystal diffraction, and this method remains                                 2 −
                                                                 two-electron donor) is expelled to form [Fe 3 CO 11 ] . The
               one of the most valuable tools for the characterization of
                                                                 total electron count (48) is preserved, and the triangular
               newly synthesized metal cluster compounds. In particular,
                                                                 array of metal atoms is unchanged in this process.
               the presence of short metal atom distances, determined by
                                                                   A tetrahedral array of metal atoms is generally found
               X-ray diffraction, indicate the presence of metal–metal
                                                                 for four-metal clusters having a 60 e count; an exam-
                                                                                                −
               bonds in clusters. The X-ray diffraction data also indicate
                                                                 ple is [Fe 4 CO 13 ] . When the electron count of the four-
                                                                               2−
               the identity and disposition of ligands, such as CO, which
                                                                 metal clusters is increased to 62 by a two-electron reduc-
               usually surround the metal cluster core. A simple metal
                                                                 tion, a more open butterfly geometry for the metal atoms
               carbonyl cluster compound is [Fe 2 (CO) 9 ], (Fig. 1).
                                                                 results.
                 It is possible to systematize the structures of many
                                                                   Five-metal transition cluster compounds adopt a wide
               organometallic metal cluster compounds according to the
                                                                 variety of geometries. Those having 72 cluster valence
               number of cluster valence electrons. This correlation was
                                                                 electrons often have a trigonal bipyramidal arrangement
               developed by K. Wade and D. M. P. Mingos in the England
                                                                 of metal atoms. The main group metal clusters [Sn 5 ] 2−
               and by J. Lauher in the United States. It is often referred to  2−
                                                                 and [Pb 5 ]  also adopt this geometry (Fig. 4), but unlike
               as Wades rules. In the following discussion, we will em-
                                                                 the transition metal clusters, many of these main group
               ploy the Wade–Mingos–Lauher electron counting rules to
                                                                 clusters do not have ligands attached to the metal atoms.
               describe a variety of metal clusters. For a neutral cluster,
                                                                   Examples of trigonal bipyramidal clusters are ob-
               the number of cluster valence electrons is the sum of the
                                                                 served for five metal transition metal clusters, such as
               valenceelectronsonthemetalframework,plusthenumber
                                                                 [Os 5 (CO) 16 ] and [Ni 5 (CO) 12 ] . The correlation of va-
                                                                                         2−
               of electrons donated by ligands attached to the cluster. For
                                                                 lence electron count with structure varies for many of
               a positively charged cluster, we subtract the charge num-
                                                                 the five-metal clusters. A substantial number of five-
               ber, and for a negative cluster, we add the charge number
                                                                 metal clusters, that have a 76 valence of electron count,
               to determine the total valence electron count.
                                                                 adopt a trigonal bipyramidal geometry; examples include
                                                                                             −
                                                                 [Ni 5 (CO) 12 ] 2−  and [Rh 5 (CO) 15 ] . Five-metal cluster
               II. REPRESENTATIVE TRANSITION                     compounds that contain iron group metals (iron, ruthe-
                  METAL CLUSTER COMPOUNDS                        nium, and osmium) and a 74-valence electron count of-
                                                                 ten adopt a square pyramidal geometry, as illustrated in
               Clusters with triangular metal arrays are common and  Figure  4.  Some  clusters  in  this  class  also  have  a  main
               typically clusters in this class contain 48 cluster va-  group element, such as carbon in the center of the square
                                                                                                          −
               lence electrons. Examples include Fe 3 (CO) 12 , (Fig. 2) and  base, examples are [Fe 5 C(CO) 15 ], [Ru 5 N(CO) 14 ] , and
                          −
                         2
               [Fe 3 (CO) 11 ] . A good illustration of the influence of elec-  [Os 5 S(CO) 15 ]. A variety of other metal atom arrays are
               tron count, on the chemistry of a cluster compound, is the  found for five-metal clusters.
               reduction of the neutral molecule Fe 3 (CO) 12 , which has a  The largest group of metal clusters contain six tran-
               triangular array of iron atoms. When this cluster is reduced  sition metal atoms in an octahedral geometry. The ma-
               by sodium, two electrons are transferred to Fe 3 (CO) 12 , and  jority of these octahedral clusters have an 86 valence
               to accommodate these electrons, a CO ligand (which is a  electron count. Examples of these 86-electron clusters
                          FIGURE 2 Structure of Fe 3 CO 12 .                 FIGURE 4 Structure of [Pb 5 ] 2− .
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