Coordination Compounds: Definition and Nomenclature

• Coordination compounds feature a central metal atom or ion bonded to ligands.
• Ligands are molecules or ions surrounding the central metal.
• Coordination compounds exhibit diverse structures and are important in chemical and biological processes.
• Coordination compounds are also known as metal complexes.

Definition of Coordination Compounds

• Chemical formulas provide information about the type and number of atoms in the complex ion.
• Coordination compounds contain a central metal atom or ion bonded to ligands through coordinate bonds.
• The metal atom/ion functions as a Lewis acid (electron pair acceptor).
• Ligands act as Lewis bases (electron pair donors).

Formulae of Coordination Compounds

• The formula includes the metal center and ligands.
• The metal ion is written first.
• Ligands are listed alphabetically by name.
• The charge of the coordination sphere (complex ion) is indicated as a superscript outside the square brackets.
• [Co(NH3)5Cl]Cl2 includes a cobalt(III) ion, five ammonia ligands (NH3), one chloride ligand (Cl), and two chloride counterions.

Naming Conventions

• Nomenclature follows IUPAC rules for clarity and consistency.
• Naming follows specific IUPAC rules.
• The chemical formula of a coordination compound is enclosed in square brackets, [ ].
• The central metal atom or ion is listed first, followed by the ligands.
• The general naming order is naming the cation before the anion.
• Within the coordination sphere, ligands are named before the metal atom/ion.
• Ligands are listed in alphabetical order according to their names, not their charges.
• Polydentate ligands are also listed alphabetically by their names.
• The number of each ligand is indicated by prefixes: di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6).
• Use bis- (2), tris- (3), tetrakis- (4), pentakis- (5), hexakis- (6) for complicated ligands or those with prefixes.
• The charge of the complex ion is written outside the square brackets as a superscript (e.g., 2+, 3-).
• If the coordination compound is a salt, the cation is named before the anion.
• Anionic ligands end with "-o" (e.g., chloro for Cl-, cyano for CN-).
• Neutral ligands are usually called by their usual names, with exceptions.
• Water is aqua, ammonia is ammine, carbon monoxide is carbonyl, and nitrogen monoxide is nitrosyl.
• Alphabetize ligands irrespective of charge; ignore prefixes indicating ligand number.
• If the complex ion is an anion, the metal name ends with "-ate".
• Indicate the metal's oxidation state with Roman numerals in parentheses.
• [Co(NH3)5Cl]Cl2 is pentaamminechloridocobalt(III) chloride
• Examples:
  • [Ag(NH3)2]+ is Diamminesilver(I) ion
  • [CoCl2(en)2]+ is Dichlorobis(ethylenediamine)cobalt(III) ion
  • K4[Fe(CN)6] is Potassium hexacyanoferrate(II)
  • [Cu(NH3)4]SO4 is Tetraamminecopper(II) sulfate
  • Na2[Fe(CN)5NO] is Sodium pentacyanonitrosylferrate(II)
  • [Pt(NH3)2Cl2] is Diamminedichloroplatinum(II)
  • [Cr(H2O)4Cl2]Cl is Tetraaquadichlorochromium(III) chloride
  • [Co(en)3]Cl3 is Tris(ethylenediamine)cobalt(III) chloride
  • [Ni(CO)4] is Tetracarbonylnickel(0)
  • [Fe(CN)6]4- is Hexacyanoferrate(II) ion
  • [Fe(CN)6]3- is Hexacyanoferrate(III) ion
  • [CuCl4]2− is Tetrachlorocuprate(II) ion
  • [Ag(CN)2]− is Dicyanoargentate(I) ion
  • [Co(NH3)5Cl]Cl2 is Pentaamminechlorocobalt(III) chloride
  • [Pt(NH3)2Cl4] is Diamminetetrachloroplatinum(IV)
  • [Co(en)2Cl2]Cl is Dichlorobis(ethylenediamine)cobalt(III) chloride
  • [Ag(NH3)2]Cl is diamminesilver(I) chloride
  • K2[PtCl6] is potassium hexachloroplatinate(IV)

Coordination Number

• Coordination number refers to the number of ligands directly attached to the central metal atom/ion.

• Common coordination numbers are 4 and 6, but others exist.

• Influences geometry of complex.

• The coordination number depends on the size and charge of the central metal ion and the size of the ligands.

• Common Geometries:

  • Coordination Number 2: Linear
  • Coordination Number 4: Tetrahedral or Square Planar
  • Coordination Number 6: Octahedral

Common Ligands

• Classified by the number of donor atoms used to bind to the central metal

  • Unidentate ligands bind through one donor atom
  • Bidentate ligands bind through two donor atoms
  • Polydentate ligands bind through multiple donor atoms

• Examples of Unidentate Ligands:

  • Chloride (Cl-)
  • Ammonia (NH3)
  • Water (H2O)
  • Cyanide (CN-)
  • Hydroxide (OH-)
  • Carbonyl (CO)

• Examples of Bidentate Ligands:

  • Ethylenediamine (en)
  • Oxalate (C2O4^2-)

• Examples of Polydentate Ligands:

  • EDTA (ethylenediaminetetraacetate)

• Chelating Ligands:

  • Multidentate ligands are also known as chelating ligands
  • They form more stable complexes than unidentate ligands due to the chelate effect

Formula Representation

• Write the symbol of the central metal first when writing formulas from names.

• Write the formulas of the ligands next, enclosing the entire complex in square brackets.

• Determine the charge of the complex ion by considering the charge of the metal ion and the charges of the ligands.

• Balance the charge with counter-ions as needed to form a neutral compound.

• Central metal atom listed first.

• Ligands follow, listed alphabetically.

• Entire coordination entity in square brackets [ ].

• Counter-ions written outside brackets to balance charge.

• Examples:

  • [Co(NH3)5Cl]Cl2: Pentaamminechloridocobalt(III) chloride
  • K4[Fe(CN)6]: Potassium hexacyanoferrate(II)

Charge of Complex Ion

• The overall charge of the complex ion is the sum of the charges of the metal ion and the ligands.
• Knowing the charge on the ligands and the overall charge allows determination of the oxidation state of the metal.

Isomers

• Isomers have the same chemical formula but different properties.

• Coordination compounds exhibit structural and stereoisomerism.

• Coordination compounds can exhibit different types of isomerism, including structural isomerism and stereoisomerism.

• Structural isomers have different connectivity between the metal and the ligands.

• Stereoisomers have the same connectivity but different spatial arrangements.

  • Structural Isomers:

    • Ionization isomers: Differ in ions coordinated to the metal vs. counterions, e.g., [Co(en)2Cl2]NO2 and [Co(en)2(NO2)Cl]Cl
    • Hydrate isomers: Differ in placement of water molecules, e.g., CrCl3.6H2O exists as [Cr(H2O)6]Cl3 (violet), [Cr(H2O)5Cl]Cl2.H2O (light green), and [Cr(H2O)4Cl2]Cl.2H2O (dark green)
    • Linkage isomers: Ambidentate ligand coordinates through different donor atoms, e.g., [Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl
    • Coordination isomers occur when both the cation and anion are complex ions, and the ligands are exchanged between the two metal centers, e.g., [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].

  • Stereoisomers:

    • Geometric isomers (cis-trans isomers): Different spatial arrangements of ligands
    • Optical isomers (enantiomers): Non-superimposable mirror images

Stereoisomers

• Stereoisomers have the same formula and atomic connectivity, but differ in spatial arrangement.
• Coordination compounds exhibit geometrical and optical isomerism.

Geometrical Isomerism

• Arises when ligands arrange differently around the metal center.
• Common in square planar and octahedral complexes.
• Cis isomers have identical ligands on the same side; trans isomers have them on opposite sides in square planar complexes.
• In octahedral complexes, cis isomers have identical ligands adjacent; trans isomers have them on opposite sides.
• [Pt(NH3)2Cl2] exhibits cis and trans isomers.

Optical Isomerism

• Also known as enantiomers.
• Optical isomers are non-superimposable mirror images.
• Occurs when the coordination compound is chiral (lacks a plane of symmetry).
• Chiral structures can rotate plane-polarized light.
• Labeled as d (dextrorotatory) or l (levorotatory) based on the direction of rotation.
• [Co(en)3]3+ exhibits d and l isomers.

Structural Isomerism

• Structural isomers have the same chemical formula but differ in atomic connectivity.
• Types include ionization, hydrate, linkage, and coordination isomerism.

Ionization Isomerism

• Involves the exchange of ligands inside and outside the coordination sphere.
• Results in different ions.
• [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are ionization isomers.

Hydrate Isomerism

• Isomers differ in the number of water molecules inside the coordination sphere versus free water molecules.
• [Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2·H2O, and [Cr(H2O)4Cl2]Cl·2H2O are hydrate isomers.

Linkage Isomerism

• Occurs when an ambidentate ligand coordinates through different atoms.
• Common ambidentate ligands: NO2− (nitro or nitrito) and SCN− (thiocyanato or isothiocyanato).
• [Co(NH3)5(NO2)]Cl2 and [Co(NH3)5(ONO)]Cl2 are linkage isomers.

Drawing Stereoisomers

• Requires careful consideration of the spatial arrangement of ligands.
• For square planar, draw the metal center and four ligands, then switch ligand positions for the geometrical isomer.
• For octahedral complexes, use wedges and dashed lines to indicate ligands above and below the plane, then switch positions for the geometrical isomer.
• Create a mirror image of the original complex and verify that it is non-superimposable for optical isomers.

Applications

• Coordination compounds have diverse applications in areas such as catalysis, medicine, and analytical chemistry.
• Examples: cisplatin as an anticancer drug, EDTA in chelation therapy, and metal complexes in industrial catalysis.