5.4 Inorganic Polymers and Clusters

Inorganic polymers and clusters are fascinating compounds that break away from traditional organic structures. These materials, including silicones and boron hydrides, showcase unique bonding and properties that make them valuable in various applications.

From flexible silicones to electron-deficient boranes, these compounds demonstrate the diverse chemistry of main group elements. Understanding their structures and bonding helps us predict and manipulate their properties, opening doors to new materials and technologies.

Silicon and Phosphorus Polymers

Silicone-Based Polymers

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• Silicones consist of Si-O-Si backbone with organic side groups attached to silicon atoms
• Polysiloxanes represent the most common type of silicones with general formula $[R_2SiO]_n$
• Properties of silicones include flexibility, thermal stability, and water repellency
• Applications of silicones range from lubricants to medical implants
• Synthesis of silicones involves hydrolysis of chlorosilanes followed by condensation polymerization

Phosphorus-Containing Polymers

• Polysilanes feature Si-Si bonds in the main chain with organic substituents
• Synthesis of polysilanes uses Wurtz coupling of dichlorosilanes with sodium metal
• Polyphosphazenes contain alternating phosphorus and nitrogen atoms in the backbone
• General formula of polyphosphazenes $[NPR_2]_n$ where R represents organic or inorganic substituents
• Applications of polyphosphazenes include flame retardants and biomedical materials

Polymerization Mechanisms

• Polycatenation involves formation of chains through single covalent bonds between atoms of the same element
• Silicon and phosphorus undergo polycatenation to form extended structures
• Polymerization of silicon and phosphorus compounds occurs through various mechanisms
  • Condensation polymerization for silicones
  • Addition polymerization for some phosphazenes
  • Ring-opening polymerization for cyclic phosphazenes

Boron and Boron-Containing Compounds

Boron Hydrides and Their Structure

• Boron hydrides (boranes) consist of boron-hydrogen compounds with general formula $B_nH_m$
• Diborane (B₂H₆) serves as the simplest stable borane with a unique bridged structure
• Boranes form various structures including
  • Closo-boranes (closed polyhedra)
  • Nido-boranes (nest-like structures)
  • Arachno-boranes (web-like structures)
• Bonding in boranes involves 3-center-2-electron bonds due to electron deficiency

Carboranes and Electron-Deficient Compounds

• Carboranes incorporate carbon atoms into borane frameworks
• General formula of carboranes $C_2B_nH_{n+2}$ with icosahedral structures common
• Electron-deficient compounds contain fewer valence electrons than predicted by octet rule
• Boron compounds often exhibit electron deficiency due to boron's trivalent nature
• Multicenter bonding compensates for electron deficiency in these compounds

Wade's Rules and Structural Predictions

• Wade's rules predict structures of borane and carborane clusters
• Skeletal electron pair theory forms the basis of Wade's rules
• Closo structures have n+1 skeletal electron pairs for $B_nH_n^{2-}$ or $C_2B_{n-2}H_n$
• Nido structures possess n+2 skeletal electron pairs
• Arachno structures contain n+3 skeletal electron pairs
• Application of Wade's rules helps determine cluster geometry and electron count

Metal and Cage Compounds

Metal Clusters and Their Properties

• Metal clusters consist of three or more metal atoms held together by metal-metal bonds
• Bonding in metal clusters involves delocalized electrons similar to metallic bonding
• Nuclearity refers to the number of metal atoms in a cluster
• Low-nuclearity clusters (3-12 metal atoms) exhibit molecular-like properties
• High-nuclearity clusters (13+ metal atoms) show bulk metal characteristics
• Applications of metal clusters include catalysis and materials science

Zintl Ions and Their Structures

• Zintl ions represent polyatomic anions formed by post-transition metals or metalloids
• General formula of Zintl ions $[E_n]^{m-}$ where E represents the main group element
• Structures of Zintl ions range from chains to cages (Sn₅²⁻, Pb₅²⁻)
• Zintl phases combine electropositive metals with Zintl ions
• Applications of Zintl compounds include thermoelectric materials and battery technologies

Cage Compounds and Polyhedral Structures

• Cage compounds feature atoms arranged in polyhedral structures
• Fullerenes represent carbon-based cage compounds (C₆₀, C₇₀)
• Clathrates form cage-like structures that can encapsulate guest molecules
• Polyoxometalates consist of metal-oxygen cage structures with various applications
• Synthesis of cage compounds involves self-assembly processes or template-directed methods