Nomenclature of Coordination Compounds

Writing the formulas of Mononuclear Coordination Entities :

The following rules are followed while writing the formulas :

(i) The central atom is placed first.

(ii) The ligands are then placed in alphabetical order. The placement of a ligand in the list does not depend on its charge.

(iii) Polydentate ligands are also placed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.

(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligands abbreviations are also enclosed in parentheses.

(v) There should be no space between the ligands and the metal within a coordination sphere.

(vi) When the formula of a charged coordination entity is to be written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(H2O)6]3+, [Fe(CN)6]3– etc.

(vii) The charge of the cation(s) is balanced by the charge of the anion(s).

Writing the name of Mononuclear Coordination Compounds :

The following rules are followed when naming coordination compounds :

(i) Like simple salts the cation is named first in both positively and negatively charged coordination entities.

Examples :

[Ag(NH3)2]Cl,diamminesilver(I)chloride.K3[Fe(CN)6],potassium hexacyanidoferrate(III).

(ii)  The ligands are named in an alphabetical order (according to the name of ligand, not the prefix) before the name of the central atom/ion.

Examples :

[Pt(NH3)BrCl(CH3NH2)], amminebromidochloridomethylamineplatinum(II).

[Co(H2O)2(ox)2], diaquabis(oxalato)cobaltate(III).

(iii)  Names of the anionic ligands end in –o and those of neutral ligands are the same except aqua for H2O, ammine for NH3, carbonyl for CO, thiocarbonyl for CS and nitrosyl for NO. But names of cationic ligands end in–ium. The neutral an cationic are placed within enclosing marks ( ) .

Some more important examples of neutral and cationic ligands are :

tetraphosphorus   —    P4

dioxygen              —    O2

octasulphur          —    S8

(iv) Prefixes mono, di, tri, etc., are used to indicate the number of the one kind of ligands in the coordination entity. When the names of the ligands include a numerical prefix are complicated or whenever the use of normal prefixes creates some confusion, it is set off in parentheses and the second set of prefixes is used.

Example ;

[CoCl2(NH2CH2CH2NH2)2]+, dichloridobis(ethane-1,2-diamine)cobalt(III).

[NiCl2(PPh3)2], dichloridobis(triphenylphosphine)nickel(II).

(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in the parentheses after the name of metal.

(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix - ate. For example, Co in a complex anion, [Co(SCN)4]2– is called cobaltate. For some metals, the Latin names are used in the complex anions.

Examples ;     

[Co(NH3)4Cl2]+, pentaamminechloridocobalt(III).

(NH4)2 [Co(SCN)4], ammonium tetrathiocyanato-S-cobaltate(II).

(vii) The neutral complex molecule is named similar to that of the complex cation.

Example ;      

[CrCl3(py)3], trichloridotris(pyridine)chromium(III).

(viii) The prefixes cis- and trans- designate adjacent and opposite geometric locations. For examples,

[Pt(NH3)2Cl2], cis- and trans-diamminedichloridoplatinum(II),

[CoCl2(NH3)4]+, cis- and trans-tetraamminedichloridocobalt(III).

Effective Atomic Number Rule given by Sidgwick :

Effective Atomic Number (EAN) = No. of electron present on the metal atom/ion + No. of electrons  donated by ligands to it.

OR

Effective Atomic Number (EAN) = Atomic no. of central metal – Oxidation state of central metal + No. of electrons donated by ligands.

The complexes in which the EAN of the central atom equals the atomic number of the next noble gas, are found to be extra stable.

Note : The EAN rule is generally found to be not valid in case of most of the complexes but in case of metal carbonyls this rule is found to be valid in all cases except one or two exceptions.