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Complexation and Coordination:

Nomenclature of Complex Ions and Coordination Compounds

Complex ions are ions which are made up of a single central atom or ion, usually a transition metal ion, to which other atoms, molecules, or ions called ligands can be attached. The naming of complex cations and complex anions is similar, except that anions are always made to end in -ate. Coordination compounds, like simpler inorganic compounds, are named with the cation preceding the anion regardless of which (if either) one of them is a complex ion.

The rules for naming complex ions or compounds are as follows:

  1. To name any complex ion, list first the ligands, then the central atom.
  2. The ligand names are made to end in -O if they are negative (chloro, bromo, nitrito, etc.), and unless they are negative the ligand names must not end in -O. Examples of ligand names are chloro, hydroxo, cyano, aqua (for H2O), ammine (for NH3), and thiosulfato (for S2O32-).
  3. Anions end in -ate and, when a Latin symbol is used for the element, the element takes the Latin name in anions but not in cations. For example, Cu(NH3)42+ is called the tetraamminecopper(II) ion but Cu(CN)64- is called the hexacyanocuprate(II) ion. Likewise Al(NH3)63+ is called the hexaamminealuminum(III) ion but Al(OH)4- is called the tetrahydroxoaluminate(III) ion.
  4. The number of each kind of ligand is specified by the usual Greek prefix. The ligands are named in alphabetic order. For example, Cu(Cl2Br2I2)4- is the dibromodichlorodiiodocuprate(II) ion while Cu(Cl2Br2INH3)4- is the amminedibromodichloroiodocuprate(I) ion.
  5. The oxidation state of the central metal atom to which the ligands are attached must be indicated, unless it is an element which has only one known oxidation state such as sodium. The oxidation state is not always obvious; the compound GaCl2 was named gallium dichloride until it was discovered to be the dimeric Ga2Cl4. Its renaming as digallium tetrachloride was not sufficient, since its actual structure is Ga+[GaCl4]- and so its name really should be gallium(I) tetrachlorogallate(III) in order to indicate the oxidation states.

Example. We will name the following salts:
  1. [Co(NH3)4Cl2]Cl: dichlorotetramminecobalt(III) chloride.
  2. [Co(NH3)5Cl]Cl2: monochloropentamminecobalt(III) chloride.
  3. K3[Co(NO2)6], known as Fischer's Salt: potassium hexanitritocobaltate(III).
  4. [Pt(NH3)3Cl]2[PtCl4], known as Magnus' Pink Salt: di(monochlorotriammineplatinum(II)) tetrachloroplatinate(II).
  5. [Pt(NH3)4][PtCl4], known as Magnus' Green Salt: tetrammineplatinum(II) tetrachloroplatinate(II).
  6. Reinecke's Salt, NH4[Cr(NH3)2(SCN)4]: ammonium tetrathiocyanatodiamminechromate(III).
  7. Drechsel's Chloride, [Pt(NH3)6]Cl4: hexammineplatinum(IV) chloride.
  8. Cossa's First Salt, K[Pt(NH3)Cl3]: potassium trichloroammineplatinate(II).
  9. Cossa's Second Salt, K[Pt(NH3)Cl5]: potassium pentachloroammineplatinate(IV)

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Copyright 1995 James A. Plambeck (Jim.Plambeck@ualberta.ca). Updated November 17, 1996 jp.