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Transition metal ions and some main block elements form coordination compounds which typically consist of a coordinate complex and counter ions.  Coordination complexes are composed of ligands surrounding a central metal atom or ion.  

The coordinate complex itself consists of a transition metal atom or ion and the surrounding ligands. The coordinate complex is always enclosed in square brackets, [ ]. The coordinate complex can be an ion, cation or anion, or a neutral complex. Counter ions are needed to produce a neutral coordinate compound if the complex is an ion.

Ligands possess one or more donor atoms, atoms that donate the lone pair of electrons.  Donor atoms may possess more than one lone pair of electrons, but only one pair is involved in the bond.  An electron pair donor is referred to as a Lewis Base.  

The central metal atom or ion accepts the lone pair of electrons involved in the bond.  An electron pair receptor is referred to as a Lewis Acid.

Bonds that are formed between the ligands and the central metal atom or ion are distinctive in that the bonds of the molecule are formed with both of the electrons involved in the bond being donated by just one of the atoms involved in the bond; this is known as a coordinate covalent bond, thus giving these compounds their name.  

Example:  [Fe(OH2)6]Cl2  

Hexaaquairon (II), [Fe(OH2)6]+, is a complex ion consisting of the transition metal ion Fe+2 and six attached neutral water, OH2, ligands.  The oxygen atom of each water molecule has two lone pairs of electrons one of which the neutral ligand donates to the coordinate covalent bond. The oxygen atom is referred to as the donor atom. The donor atom is listed first in the chemical formula of the ligand in contrast with the normal pattern for the ordering of atoms in a chemical formula: cation then anion.  Also, note that when bringing together the complex ion with the counter ion, the complex ion is always enclosed in brackets [ ] while names of common ligands are enclosed in parenthesis ( ).  The counter ion in for this example is the chloride anion, Cl-.

Transition metal complexes, such as tetraamminedichlorocopper (II), [Cu(NH3)4Cl2] are neutral and do not require counter ions.

Complex Ion Nomenclature:

The chemical formula and the chemical name for Complex cations have different rules.  

The complex cation chemical formula follows traditional rules for naming ionic compounds with the cation listed first.  For example [Cu(OH2)6]2+, the cationic copper II ion is listed first and the neutral water ligand is listed second.  Complex cation chemical names are assembled by alphabetically listing the number and name of the ligands attached to the central metal atom or ion followed by the name of the central atom and its oxidation number indicated by a Roman numeral in parenthesis, for example, hexaaquacopper (II).

The chemical names and formulas for Complex anions follow the same rules as complex cations with one additional step.  The name of the central atom is changed by adding an –ate ending and then the oxidation number in parentheses.  For example: [FeF6]3- , hexafluoroferrate (III). Note here also that the Latin name for iron is used to avoid an awkward sounding name.  A list of such transition metals follows in the section on naming rules.

Ligands are ions, ionic molecules, or neutral molecules that have atoms which possess lone pairs and thus act as Lewis bases.  Ligand is derived form the Latin legare meaning to bind.  The lone electron pairs are donated to empty d orbitals of the transition metal atom or ion creating a coordinate covalent bond.  When a ligand donates only a single pair of electrons to the metal center the ligand is referred to as monodentate.  Dentate comes from the Latin dens meaning tooth. Monodentate then figuratively means

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