COMPLEX IONS

Compounds in which metal ion is surrounded by a group

of anions or neutral molecules.

Metal Ions. . . do not exist in isolation. are accompanied by anions whose

negative charge balances the positive charge of metal ion, producing neutral compounds.

have empty valence orbitals therefore act as electron pair acceptors.

Metal Ions. . . act as Lewis acids.

Anions or molecules with unshared pairs of electrons can act as Lewis bases and bind to metal center.

ability to form complexes increases positive charge increases and size decreases. Weakest complexes formed by alkali metals. 2+ and 3+ ions of transition elements show

greatest tendency to form complexes. Cr 3+ forms more stable complexes than does Al 3+

Ligands. . . Molecules or ions surrounding the metal

ion. Are known as complexing agents. Normally either anions or polar molecules.

Must have at least one unshared pair of valence electrons. Therefore act as electron pair donors/Lewis

bases.

Coordination Sphere. . .

The central metal and the ligands.

[Cu(NH3)4]SO4

Placed inside brackets to set it off from other parts of the compound.

Complex Ions. . . Metal ions in water, the ligand is

water. In aqueous solution, ligands such as SCN-

and CN- replace H2O in the coordination sphere.

Different properties than metal or ligands. Color. Oxidation state stabilized. Reduction

potentials change.

Charge. . . Is the sum of the charges on the

central metal and surrounding ligands.

[Cu(NH3)4]SO4SO4 = 2- (charge of the polyatomic ion)

NH3 = 0 (molecule therefore no charge)

Cu = 2+ (based on what is needed to balance the charges)

Practice determining charges 1. What is the oxidation number of the

central metal in [Co(NH3)5Cl](NO3)2? 2. What is the charge of the complex

formed by a platinum (V) metal ion surrounded by three ammonia molecules and three bromide ions? Write the formula for this complex ion.

3. Given that a complex ion contains a chromium (III) bound to four water molecules and two chloride ions, write its formula.

Nomenclature. . . [Co(NH3)5Cl]Cl2

In naming salts, the name of the cation is given before the name of the anion.

[Co(NH3)5Cl]2+ Cl-

Ligands are named before the metal. [Co(NH3)5Cl]2+

Ligands named in alphabetical order. Do not consider charge or prefix. In writing formula, metal is written first.

Anionic ligands end in the letter o. Others bear the name of the molecule.

Ligand Ligand NameAzide,N3

- azido

Bromide, Br- bromoChloride,Cl- chloroCyanide, CN- cyanoHydroxide, OH- hydroxoCarbonate,CO3

2- carbonato

Oxalate, C2O4 2- oxalato

Neutral Molecules. . .

Ligand Ligand NameAmmonia, NH3 ammine

Ethylenediamine, en ethylenediamineWater,H2O aqua

CH3NH2 methylamine

CO carbonylNO nitrosyl

A Greek prefix is used to indicate the number and kind of each ligand when more than one is present.

[Co(NH3)5Cl]Cl2Pentaammine

If the name of the ligand contains a prefix, then the name of the ligand is enlosed in parentheses. Alternate prefixes are used bis-, tris-, tetrakis, petakis-, and hexakis for the name enclosed in the parentheses.

[Co(en)3]Cl3

Tris(ethylenediamine)cobalt (III)

Monodentate LigandUnidentate Ligand Form one bond to the metal ion.

H2O NH3

NO2-

SCN-

OH-

X- (halides)

Bidentate Ligand Form two bonds to the metal

ion. . . Oxalate Ethylenediamine

Use bis-, tris-, tetrakis-, etc.

If the complex is an anion, its name ends in –ate: K4[Fe(CN)6] The suffix –ate is added to the latin stem.

[Fe(CN)6] 4- hexacyanoferrate (II) ion.

K4[Fe(CN)6] Iron Ferrate Copper cuprate Lead plumbate Silver argenate Gold aurate Tin stannate

The oxidation number of the metal is given in parentheses in Roman numerals following the name of the metal.

hexacyanoferrate (II)Tris(ethylenediamine)cobalt (III)

Practice. . .[Ni(C5H5N)6]Br2

[Co(NH3)4(H2O)CN]Cl2

Na2[MoOCl4]

Na[Al(OH)4]

Hexapyridinenickel(II)bromide

Tetraammineaquacyanocobalt(III)chloride

Sodium tetrachlorooxomolybdate(IV)

Sodium tetrahydroxoaluminate