2. Nomenclature

Because of the large number of complicated coordination compounds, it has been necessary to develop a systematic method for naming them. Complex species may be a cation such as [Cu(H₂O)₄]²⁺, an anion such as [Fe(CN)₆]^4-, or a neutral molecule such as [Cr(NH₃)₃Cl₃]. Many different electronically satisfied entities may act as electron-pair donor ligands. The names of some common ligands are given in the margin. It may be seen that the names of negatively-charged ligands end in o. The name of the molecule is generally used for neutral ligands. Water and ammonia are the two important exceptions. The rules listed below enable you to name a large number of common complex substances. The rules for naming chemical compounds are established by nomenclature committees of the International Union of Pure and Applied Chemistry (IUPAC).

3. Rules for naming of coordination compounds

The following rules should be taken into account while drawing the formulas of complex compounds:

1. Complex ion charge is numerically equal to algebraic sum of ion-complexing agent and ligands charges;

2. Total ion charges, located in outer sphere, are numerically equal and opposite in sign to the charge of inner coordinating sphere. For example, for K₄[Fe(CN)₆] inner sphere charge is (2+) + (6-) = 4-, outer sphere charge is 4 ∙ (1+) = 4+, complex compound charge is (4-) + (4+) = 0.

Multicharged ions of heavy metals, which have 18-electronic or transient from 8- to 18-electrinic outer configuration and have free orbitals show the ability to complex formation. Ions of some non-metals may be complexing agents. Generally the charge of complexing agent corresponds to the charge of given atom in simple compound composition. Therefore the process of complex compound may be easily presented as the composition of simple substance molecules:

4KCN + Fe(CN) ₂ = K₄[Fe(CN)₆];

NiSO₄ + 6NH₃ = [Ni(NH₃)₆]SO₄;

2NaOH + Zn(OH)₂ = Na₂[Zn(OH)₄].

The prevailing coordination numbers are 2, 4, and 6. It can be pointed out that coordination number mostly is equal to doubled or triple charge of ion-complexing agent (see Table 14).

The ability of ligands to coordination depends on atoms presented in them or groups with undivided electron pairs owing to which the link with the central atom takes place. The prevalent electron donors are the atoms of N, O, C, Cl, Br, I.

The name of the positive ion is written before the name of the negative ion.

The name of the ligands is written before the name of the metal to which it is coordinated.

The Greek prefixes mono-, di-, tri-, tetra-, penta-, hexa-, and so on are used to indicate the number of ligands when these ligands are relatively simple. The Greek prefixes bis-, tris-, and tetrakis- are used with more complicated ligands.

The names of negative ligands always end in o, as in fluoro (F^-), chloro (Cl^-), bromo (Br^-), iodo (I^-), oxo (O^2-), hydroxo (OH^-), and cyano (CN^-). A handful of neutral ligands are given common names, such as aquo (H₂O), ammine (NH₃), and carbonyl (CO).

Ligands are listed in the following order: negative ions, neutral molecules, and positive ions. Ligands with the same charge are listed in alphabetical order.

A Roman numeral in parentheses indicates the oxidation number of the metal atom after the name of the metal atom.

The names of complexes with a net negative charge end in -ate. Co(SCN)₄^2-, for example, is the Tetrathiocyanatocobaltate (II) ion. When the symbol for the metal is derived from its Latin name, -ate is added to the Latin name of the metal. Thus, negatively charged Iron complexes are ferrates and negatively charged Copper complexes are cuprates (see examples in Table 14).

Table 14. Characteristics of some complex compounds

Complexing agent	Coordination number	Formula	Name
Ag+	2	[Ag(NH3)2]Cl	Diaminesilver (I) Chloride
Cu2+	4	[Cu(NH3)4](OH)2	TetramineCopper (II) Hydroxide
Hg2+	4	K2[HgI4]	Potassium Tetraiodomercurate (II)
Zn2+	4	Na2[Zn(OH)4]	Sodium Tetra-hydroxozincate
Co2+	4	(NH4)2[Co(SCN)4]	Ammonium Tetra-thiocyanocobaltate (II)
B3+	4	Na[BF4]	Sodium Tetrafluoroborate
Co3+	6	[Co(H2O)6]Cl3	Hexaquocobalt (III) Chloride
Al3+	6	Na3[Al(OH)6]	Sodium Hexa-hydroxoaluminate

PRACTICE PROBLEMS

1. Define oxidation number, coordination number of complexing agent and charge of complex ion in the following compounds:

[Cd(NH₃)₄](OH)₂, Cu₂[Fe(CN)₆], [Cr(H₂O)₆]Br₃, K₃[FeF₆], Na₃[Cr(OH)₆], [Ni(CO)₄], [Cu(NH₃)₄]₃(PO₄)₂, K₃[Co(CN)₆].

2. Which salts are used for K₄[Fe(CN)₆] production? Equate the reactions.

3. Write given formulas as complex compounds: 2HF·SiF₂; 2KCl·PbCl₂; NiCl₂·6NH₃; CuSO₄·4NH₃; Zn(OH)₂·Ca(OH)₂; 3KNO₂·Co(NO₂)₃; 4KCN·Fe(CN)₂, CoSO₄·6H₂O.

4. Write molecular and ionic reactions between the following substances:

ZnSO₄ and K₃[Fe(CN)₆]; FeCl₃ and K₃[Fe(CN)₆]; KCl and Na₃[Co(NO₂)₆]; K₂[HgI₄] and AgNO₃; FeCl₃ and KSCN (surplus); NaCl and K[Sb(OH)₆]; CuSO₄ and NH₃; ZnCl₂ and KOH (surplus).