Coordination Compounds

• Transition metals form complex compounds where metal atoms are bound to anions or neutral molecules by sharing electrons, known as coordination compounds
• Coordination compounds are an important area of modern inorganic chemistry, providing insights into biological systems
• Examples of coordination compounds include chlorophyll (magnesium), haemoglobin (iron), and vitamin B12 (cobalt)
• Coordination compounds are used in metallurgical processes, industrial catalysts, analytical reagents, electroplating, textile dyeing, and medicinal chemistry
• Alfred Werner was the first to formulate ideas about coordination compounds, introducing primary and secondary valences for metal ions
• Werner's theory of coordination compounds includes postulates about primary and secondary valences, spatial arrangements, and coordination polyhedra
• Coordination compounds exhibit isomerism and common geometrical shapes like octahedral, tetrahedral, and square planar
• Double salts dissociate into simple ions in water, while complex ions like [Fe(CN)6]4– do not dissociate
• Coordination entities consist of a central metal atom/ion surrounded by ligands
• Central atom/ion in a coordination entity is the atom/ion to which ions/groups are bound in a specific arrangement
• Ligands are ions or molecules bound to the central atom/ion in a coordination entity
• In coordination chemistry, ligands bound to the central atom/ion in the coordination entity are called ligands
• Ligands can be simple ions like Cl

  –, small molecules like H2O or NH3, larger molecules like H2NCH2CH2NH2 or N(CH2CH2NH2)3, or even macromolecules like proteins
• When a ligand is bound to a metal ion through a single donor atom, it is unidentate (e.g., Cl

  –, H2O, NH3)
• If a ligand can bind through two donor atoms, it is didentate (e.g., H2NCH2CH2NH2, C2O4

  2–)
• When a ligand has several donor atoms in a single ligand, it is polydentate (e.g., N(CH2CH2NH2)3)
• Ethylenediaminetetraacetate ion (EDTA

  4–) is an important hexadentate ligand
• A chelate ligand is a di- or polydentate ligand that uses two or more donor atoms simultaneously to bind a single metal ion
• The coordination number of a metal ion in a complex is the number of ligand donor atoms to which the metal is directly bonded
• The coordination sphere includes the central atom/ion and the ligands attached to it, enclosed in square brackets
• The oxidation number of the central atom in a complex is the charge it would carry if all ligands are removed along with shared electron pairs
• Homoleptic complexes have a metal bound to only one kind of donor groups, while heteroleptic complexes have a metal bound to more than one kind of donor groups
• Nomenclature in coordination chemistry is based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC)
• The formula of a compound in coordination chemistry provides basic information about its constitution in a concise manner
• Mononuclear coordination entities contain a single central metal atom
• When writing formulas for coordination entities:

  • List the central atom first
  • List ligands in alphabetical order
  • Enclose the formula in square brackets
  • No space between ligands and the metal within a coordination sphere
• When naming coordination compounds:

  • Name the cation first
  • List ligands alphabetically before the central atom/ion
  • Use prefixes to indicate the number of individual ligands
  • Indicate the oxidation state of the metal with a Roman numeral in parenthesis
• Examples of nomenclature for coordination compounds:

  • [Cr(NH3)3(H2O)3]Cl3 is named as triamminetriaquachromium(III) chloride
  • [Co(H2NCH2CH2NH2)3]2(SO4)3 is named as tris(ethane-1,2–diamine)cobalt(III) sulphate
• Formulas for coordination compounds:

  • (a) [Co(NH3)4(H2O)Cl]Cl2
  • (b) K2[Zn(OH)4]
  • (c) K3[Al(C2O4)3]
  • (d) [CoCl2(en)2]+
  • (e) [Ni(CO)4]
• IUPAC names of coordination compounds:

  • (a) [Pt(NH3)2Cl(NO2)] is named as diamminechloridonitrito-N-platinum(II)
  • (b) K3[Cr(C2O4)3] is named as potassium trioxalatochromate(III)
  • (c) [CoCl2(en)2]Cl is named as dichloridobis(ethane-1,2-diamine)cobalt(III) chloride
  • (d) [Co(NH3)5(CO3)]Cl is named as pentaamminecarbonatocobalt(III) chloride
  • (e) Hg[Co(SCN)4] is named as mercury (I) tetrathiocyanato-S-cobaltate(III)
• Isomerism in coordination compounds:

  • Two principal types of isomerism are known among coordination compounds: stereoisomerism and structural isomerism
  • Stereoisomers have the same chemical formula and chemical bonds but have different spatial arrangements
  • Structural isomers have different bonds
• Hydrate isomerism occurs when water is involved as a solvent in coordination compounds
• Solvate isomers differ by whether a solvent molecule is directly bonded to the metal ion or present as free solvent molecules in the crystal lattice
• An example of solvate isomerism is the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green)
• Werner was the first to describe the bonding features in coordination compounds
• Approaches to explain bonding in coordination compounds include Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT), and Molecular Orbital Theory (MOT)
• According to Valence Bond Theory (VBT) and Crystal Field Theory (CFT), metal atoms or ions under the influence of ligands use their orbitals for hybridization to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar, etc.
• In coordination compounds, the magnetic moment can be used to predict the geometry of a complex based on the valence bond theory
• Magnetic properties of coordination compounds can be measured by magnetic susceptibility experiments to obtain information about the number of unpaired electrons and structures adopted by metal complexes
• Valence Bond Theory (VBT) explains the formation, structures, and magnetic behavior of coordination compounds but has limitations such as not giving a quantitative interpretation of magnetic data or explaining the color exhibited by coordination compounds