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single toothed.  Ligands that donate two, three, four, five, or six lone pairs of electrons are collectively referred to as polydentate, and bidentate, tridentate, tetradentate, pentadentate, and sexadentate respectively. Polydentate ligands typically form five- and six-membered rings and are referred to as chelating agents and the complex is referred to as chelate. Chelate comes from the Greek chela meaning claw.

The number of coordinate covalent bonds formed typically ranges from two to eight depending on the charge, size, and electron configuration of the transition metal atom or ion. The coordination number may also be as low as 1 and as high as 12.  The number of ligands attached to the central metal atom or ion, the number of coordinate covalent bonds, is the coordination number, (CN); with two, four and six the most common coordination numbers.  The coordination sphere is the number of coordinate covalent bonds plus the number of central metal atoms or ions, often the coordination sphere is one more than the coordination number.

Simplified IUPAC Rules for Naming Coordination Compounds:

1.

Cations are always named before the anions.

2.

Ligands are named before the metal atom or ion.

3.

Ligand names are modified with an -o added to the root name of an anion.  For neutral ligands the name of the molecule is used, with the exception of OH2, NH3, CO and NO.  Names of common neutral ligands are listed in Table 1, and common anionic ligands are listed in Table 2.

Table 1 – Neutral Molecules

aqua

OH2

ammine

NH3

benzene

C6H6

carbonyl

CO

nitrosyl

NO

methylamine

CH3NH2

dimethylamine

(CH3)2NH

trimethylamine

N(CH3)3

ethylenediamine or en

H2NCH2CH2NH2

bidentate

diethylenetriamine or dien

NH(CH2CH2NH2)2

tridentate

triethylenetetraamine or trien

N(CH2CH2NH2)3

tetradentate

ethylenediaminetetraacetato (EDTA)

(COOH)2N(CH2)N(COOH)2

hexadentate

1,10-phenanthroline or phen

C12H8N2

bidentate

pyrdidine  or py

C5H5N

2,2-bipyridine or bipy

(C5H4N)2

bidentate

2,2,6′,2′′terpyridine or terpy

(C5H4N)3

tridentate

bis(diphenylphosphine) ethylene or dppe

(C6H5)2P(CH2)2 P(C6H5)2

bidentate

bis(dimethylphosphino) ethylene or dmpe

(CH3)2P(CH2)2 P(CH3)2

bidentate

Table 2 – Anions

fluoro

F-

nitrito

ONO-

chloro

Cl-

nitrato

NO3-

bromo

Br-

oxalate  (ox)

C2O42-

bidentate

iodo

I-

oxo

O2-

carbonato

CO32-

sulfato

SO42-

cyano

CN-

thiocyanato

SCN-

hydrido

H-

azido

N3-

hydroxo

OH-

thiosulfato

S2O32-

nitro

NO2-

acetylacetonate (acac)

CH(COCH3)2-

bidentate

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