Metal cluster units

In all the above examples, the carbonyl group donates two electrons to the metal cluster unit. Four-electron donation by the carbonyl group has recently been observed although this appears to be a much less frequent mode of bonding in cluster systems, it has often been invoked to explain the properties of absorbed carbon monoxide on a metal surface. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

The Number of Skeletal Bonding Electrons (t -f a — 12) That Transition Metal Cluster Units Can Contribute ... 

The structural relationship between the molecular and solid-state compounds has been a hot issue in inorganic chemistry for some time (25-27). The extrusion (or excision) from preformed solid-state cluster compounds is one of the major synthetic methods of the preparation of cluster complexes (26). Use of cluster complexes as precursors to solid-state cluster compounds is the reverse reaction of excision. Both reactions utilize the structural similarity of the metal cluster units. The basic cluster units of polyhedra (deltahedra) or raft structures are triangles, and both molecular and solid-state clusters with octahedral, tetrahedral, and rhomboidal cores have been reported. Similarity of other properties such as electronic structures based on the cluster units is also important. The present review is concerned with the syntheses and structures of the cluster complexes of the group 6 metals and with their relationships to solid-state chemistry. 

Condensation of discrete triangular metal cluster units in the metal cluster plane is another way of building extended solid-state com-... 

TABLE 3.5. Skeletal Electron Contributions (r 4- x - 12)" that Transition Metal Cluster Units May Make... 

Number of Valence Shell Electrons (v) Transition Metal Cluster Unit ... 

The apparent oxidation state deduced from a molecular or empirical formula may be misleading, e.g. Lal2 is a metallic conductor and is best formulated as La (I )2(e ), and M0CI2 contains metal cluster units with metal-metal bonds and is formally [Mo6Cl8] (Cl )4. Indeed, metal-metal bond formation becomes more important for the heavier metals. 

A range of halides with M3 or Ms frameworks exist, but all have lattice structures with the metal cluster units connected by bridging halides. The structure of Nb3Cl8 is represented in 22.12, but of the nine outer Cl atoms shown, six are shared between two adjacent units, and three between three (see worked example 22.1). Alternatively, the structure can be considered in terms of an hep array of Cl atoms with three-quarters of the octahedral... 

TABLE 22.2 The Number of Skeletal Electrons Contributed by Transition Metal Cluster Units... 

Some mixed octahedral metal cluster units of the type MnM 5 Xi2 have been prepared (288). Coreduction of mixtures of the pentachlorides of tantalum and molybdenum with aluminum metal in a fused mixture of... 

Ligand migration within the metallic cluster unit. 

Ligand migration within the metal cluster unit... 

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