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Coordination Compounds: Nomenclature & Properties

Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

This lesson will discuss the different rules for naming coordination compounds along with examples. It will also talk about geometry, isomerism and magnetic properties of some mononuclear coordination complexes.

Nomenclature of Coordination Compounds

There are some specific rules for naming coordination compounds. Following are the main rules for naming coordination compounds according to the IUPAC recommendations of 1990.

1. In the written formula of a coordination compound, the symbol of the central atom is written first.

2. This is followed by the ionic ligands and then by the neutral ligands. Within each category, the formula of the ligands is listed alphabetically, by the first letter (or next).

3. Ligands containing carbon and hydrogen only are grouped under C. For organic ligands containing heteroatoms, the position is determined by the alphabetical sequence of the heteroatom; for example, N of C5 H5 N. If two ligands are present with the same defining atom, one with fewer such atoms precedes that with more. When the number of defining atoms are the same, subsequent symbols are used to determine the sequence. Thus, C5 H5 N precedes NH3 and C2 H8 N2 precedes C10 H8 N2.

4. The entire coordination entity is enclosed under square brackets.

5. Common abbreviations for ligands may be used. They should be lower case and enclosed in parentheses. Below are a few illustrative examples:

[Ru (NH3)5 (N2)] Cl2 pentaamminenitrogenruthenium(II)chloride

[PtBrCl(NO2) NH3] - Amminebromochloronitrito-N-platinum(II) ion - Here, nitro is an ambidentate ligand, meaning nitro can bind to the metal either through N or O, so we need to specify the dentation through nitrogen and since it is an ion, we write the term 'ion' at the end.

[Al(OH)(H2 O)5]2+ Pentaaquahydroxoaluminium(III)

When naming mononuclear coordination compounds, a specific set of rules apply, outlined below.

1. The central atom is named at the end, in accordance with rule 4 stated below.

2. This is preceded by the names of ligands in alphabetical order. Numerical prefixes (like di, tri, etc.) indicating the number of ligands are ignored. Thus, position of dichloro is determined by the alphabet 'C' while diphenylphosphine is determined by 'd' since this is an overall name of the ligand.

3. The prefixes di-, tri-, etc. are replaced by bis-, tris-, etc. (derived from ordinals) in complex expressions, and in cases of probable ambiguity, e.g., bis(methylamine), to exclude dimethylamine.

4. All anionic coordination entities end with -ate; no distinguishing terminology is used for cationic or neutral coordination entities.

5. The oxidation number of the central atom in roman numeral or arabic zero is indicated in parentheses after the name of the central atom. Positive signs are not used before the oxidation numbers. No space is left between this number and the name of the central atom.

Examples:

K3[Fe(CN)6] Potassium hexacyanoferrate(III) - Here, cyanide being an anionic ligand, the suffix-o is added, 6 cyanide ligands brings the term 'hexa' and since this is an A(B) type complex, the metal gets the suffix -ate.

[CoCl(H2 O)2 (NH3) 3]Cl2 triamminediaquachlorocobalt(III)chloride - Here, ligands are alphabetically named, three ammonia ligands brings the term 'tri', and two water ligands get the term 'di'. Since this is not an A(B) type complex, the metal only gets the oxidation number within parantheses, with its name as it is.

cis-[PtCl2 (Ph3 P) 2] cis-dichlorobis(triphenylphosphine)platinum(II)

K[PtCl3 (C2 H4)] Potassium trichloro(n2-ethene)platinate(II)

The symbol 'n' with a numerical superscript (hapticity) indicates the number of ligating atoms in the ligand which bind to the metal atom.

Geometry and Isomerism

Coordination compounds with the same empirical formula but different structures (either in atom connectivity or in the orientation of atoms in space) are called isomers.


Example of Isomers
picture of isomers


The number of isomers of a complex depends on the geometry, the coordination number of the metal, and the nature of the ligands. In coordination compounds, a four-coordinate metal may have either tetrahedral or square planar geometry and a six-coordinate metal will have octahedral geometry. The stereochemistry (three dimensional arrangement of atoms) of coordination compounds is more varied since metals involve a much larger number of orbitals (s, p and d) whose energy and spatial distribution are widely different. The isomerism in coordination compounds may be classified in two general types, described below.

1. Constitutional isomerism is where two or more compounds differ in bond connectivities. This is also called structural isomerism. Constitutional isomerism may be further subdivided as follows:

a) Ionization isomerism - Coordination compounds having different ions in and outside coordination sphere., e.g.:

[PtCl2 (NH3) 4] Br2 → [PtCl2 (NH3) 4]2+ + 2Br-

[PtBr2 (NH3) 4]Cl2 → [PtBr2 (NH3) 4]2+ + 2Cl-

b) Hydrate (Solvate) isomerism - This is a special type of ionization isomerism where water molecules are shifted outside the coordination sphere, e.g. [Cr(H2 O)6]Cl3 (violet), [CrCl(H2 O) 5] Cl2. H2 O (blue green), [Cr(H2 O) 4 Cl2]Cl. 2H2 O (green). Please note they yield a different number of chloride ions on ionization.

c) Coordination isomerism - Ligands are interchanged between cationic and anionic parts of a compound, e.g. [Co(NH3) 6][Cr(CN)6] and [Cr(NH3) 6][Co(CN)6]

d) Linkage isomerism - Some ligands may coordinate through either of two atoms. Such ligands are called ambidentate ligands. For example, the Nitro group may coordinate through either N or O in coordination compounds, and phenyl thiocyanate may coordinate to the metal either through N or S.


Linkage Isomerism
linkage isomerism


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