Coordination Chemistry

Introduction

  • Central ion: center of coordination compound; ion that all surrounding atoms are bound to   * Generally a transition element ion
  • Ligand: Lewis bases bounded to/surrounding the central ion
  • Coordinate covalent bond: one in which both shared electrons come from the same atom. 
  • Coordination number: the number of electron pairs gained by the central ion
  • Brackets are used to set off the complex ion from the remainder of a given compound. 
  • Coordination sphere: the part of the compound that is in the bracket

Chelates

  • The coordination number does not have to equal the number of ligands.
  • Chelates: ligands that can donate 2 or more electron pairs. 
  • Bidentate: a chelate that can donate 2 electron pairs

Coordination Numbers

  • The coordination number of a metal depends upon the size of the metal and the size of the ligands.

Nomenclature in Coordination Chemistry

  • In naming complex ions or molecules, the ligands are named before the metal. 
  • Ligands are listed in alphabetical order, regardless of their charges. 
  • The names of anionic ligands end in the letter o, but electrically neutral ligands ordinarily bear the name of the molecules.
  • In naming complexes that are salts, the name of the cation is given before the name of the anion
  • Greek prefixes (di-, tri-, tetra-, etc.) are used to indicate the number of each kind of ligand when more than one is present.    * If the ligand contains a Greek prefix or is polydentate, the prefixes bis-, tris-, tetrakis-, etc. are used and the ligand name is placed in parentheses.
  • If the complex is an anion its name ends in “ate.”   * The name often corresponds to the symbol, not the English name. 
  • The oxidation number of the metal appears as a Roman numeral following the name of the metal.

The Structure of Complexes

  • Many transition metal complexes adopt octahedral geometries, with six donor atoms forming bond angles of 90° about the central atom with adjacent ligands.
  • Note that only ligands within the coordination sphere affect the geometry around the metal center.

Isomers

  • Isomers: the same molecular formula but a different arrangement of atoms.
  • There are two main subgroupings: structural isomers and stereoisomers.
  • Linkage isomers: at type of structural isomer where the ligand is bound to the metal by a different atom.
  • Coordination sphere isomers: structural isomers that differ in what ligands are bound to the metal and which fall outside the coordination sphere.
  • Geometric isomers: stereoisomers in which the ligands have a different spatial relationship.
  • Optical isomers, or enantiomers: stereoisomers that are non-superimposable mirror images of one another.

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