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Metal Clusters: These are compounds that contain metal-metal bonds. Here, those compounds are considered which contain two or more metals in a closed array. CO is the most common ligand, but other ligands can also be bound to clusters. In several metals, bonding arrangements occur for the ligand, which is not possible for monometallic compounds. Thus, various metal arrays can be seen in cluster compounds like triangular, tetrahedral, and octahedral. These three are the most common metal arrays.
Types of Metal Clusters:
- Clusters Composed of three or more Metals: These clusters get stabilized by small molecules that act as ligands and can be characterized by metal-metal bonds that can be localized or delocalized over the metal framework.
- Naked Clusters: These clusters have no ligands are electronically unsaturated, extremely reactive; their formation takes place in atomic beams.
Structure and Bonding:
(1) Types of Clusters
- π-Donor Clusters: In this, early transition metals having oxidation states +2 or +3 form bonds with π-Donor ligands such as Cl-, Br-, S2-, O2-. These clusters generally form triangular or octahedral geometries.
- π-Acceptor Clusters: In this, late transition metals having oxidation state 0 or nearly 0 form bonds with π-acceptor ligands such as CO, CNR, NO.
(2) Cluster Geometries:
- High Symmetry: It generally defines regular polyhedra as octahedra, tetrahedra, triangles.
- Positions of metals generally define triangular faces, square planar, trigonal prismatic.
- Many clusters represent fragments of ccp arrays of metal atoms like hcp, bcc.
- Forms interstitial complexes that contain H, C, S, N along with other metals. This is the case of high-nuclearity clusters.
Wade’s Rule: This rule is used to predict the shapes of boron clusters by determining the total number of skeletal electron pairs (SEP) that are available for bonding. The general method to determine the type of boron cluster is:
- First, determine the total number of valence electrons using the chemical formula- 3 electrons per B and 1 electron per H.
- Subtract 2 electrons from each B-H unit.
- To get a skeletal electron pair, divide the remaining electrons by 2.
- For close borane, a cluster has n vertices and n+1 SEP for bonding.
- For nido borane, a cluster has n-1 vertices and n+1 SEP for bonding.
- For arachnoid borane, a cluster has n-2 vertices and n+1 SEP for bonding.
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