Page 155 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

C O O R D I N A T I O N C O M P O U N D S IR-9.1

Examples:
B B B B
B B B
A A A
B B B B B
B B

1. octahedral 2. square planar 3. tetrahedral
coordination coordination coordination
polyhedron polygon polyhedron

IR-9.1.2.6 Coordination number
For coordination compounds, the coordination number equals the number of s-bonds
between l igands and the central atom. Note that where both s- a nd p-bonding occurs

between t he ligating atom and the central atom, e.g. with ligands such as CN , C O, N 2 and
PMe 3 , t he p-bonds are not considered in determining the coordination number.

IR-9.1.2.7 Chelation

Chelation involves coordination of more than one non-contiguous s-electron pair donor
atom from a g iven ligand to the same central atom. The n umber of such ligating atoms in a
single chelating ligand is indicated by the adjectives bidentate , 2 t ridentate, tetradentate,
pentadentate, etc. ( see Table IV* for a l ist of multiplicative p refixes). The number of donor
atoms from a g iven l igand attached to the same c entral atom is called the denticity.

Examples:
1. 2.
H C 2 C H 2 H C 2 C H 2
H N NH 2 H N NH
2
2
Pt Pt CH CH NH 2
2
2
Cl Cl Cl Cl
bidentate chelation bidentate chelation
3. 4.
H C 2 C H 2 + H C 2 C H 2 2+
H N NH CH H C HN NH CH 2
2
2
Pt 2 Pt
CH H C N N CH 2
2
Cl N 2 H H
H 2 2 2
tridentate chelation tetradentate chelation
The cyclic structures f ormed when m ore t han one donor atom from the same ligand is bound
to the central atom are called chelate rings, and the process of coordination of these donor
atoms is called chelation.

* T ables numbered with a R oman numeral are collected together at the end of this book.

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