Coordination Compounds and Coordination Sphere

Coordination Compounds

• Coordination number of Fe and Co in [Fe(C2O4)3] and [Co(en)3] is 6 because C2O42– and en (ethane-1,2-diamine) are didentate ligands.

Coordination Sphere

• The central atom/ion and the ligands attached to it are enclosed in square brackets and collectively termed as the coordination sphere.
• The ionisable groups are written outside the bracket and are called counter ions.
• Example: in K4[Fe(CN)6], the coordination sphere is [Fe(CN)6]4– and the counter ion is K+.

Coordination Polyhedron

• The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom.
• The most common coordination polyhedra are octahedral, square planar, and tetrahedral.
• Examples: [Co(NH3)6] is octahedral, [Ni(CO)4] is tetrahedral, and [PtCl4] is square planar.

Mononuclear Coordination Entities

• Mononuclear coordination entities contain a single central metal atom.
• Rules for writing formulas:
  • The central atom is listed first.
  • The ligands are then listed in alphabetical order.
  • Polydentate ligands are also listed alphabetically.
  • The formula for the entire coordination entity is enclosed in square brackets.
  • Ligand abbreviations are enclosed in parentheses.
  • No space between the ligands and the metal within a coordination sphere.
  • When the formula of a charged coordination entity is to be written without the counter ion, the charge is indicated outside the square brackets as a right superscript.

Naming of Mononuclear Coordination Compounds

• The names of coordination compounds are derived by following the principles of additive nomenclature.
• The groups that surround the central atom must be identified in the name.
• A di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand.
• The number of such ligating groups is called the denticity of the ligand.
• Chelate complexes tend to be more stable than similar complexes containing unidentate ligands.
• Ligand which has two different donor atoms and either of the two ligates in the complex is called ambidentate ligand.

Coordination Number

• The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.
• Examples: in the complex ions, [PtCl6] and [Ni(NH3)4], the coordination number of Pt and Ni are 6 and 4 respectively.