11/29/2010

1

Chapter 21Transition Metals and

Coordination Chemistry

Some History

In the 19th century, chemists started to prepare colored compounds containing transition metals and other substances like ammonia,

chloride, water and cyanide

They were very interesting because their formulae gave little clue as to how they were bonded together

Example:

Co(NH3)6Cl3

Sophus Mads Jørgensen (1837-1914) Chain Theory

Metal ammonia complexes contain chains of ammonia molecules connected to metal with chlorides at the end of each chain

Alfred Werner (1866-1919)Coordination Theory

Octahedral arrangement of ammonia molecules around metal forming the complex ion Co(NH3)6

3+ along with three Cl- ions

3Cl-

11/29/2010

2

Complex Ions

A complex ion is a charged species consisting of a metal ion (most commonly a transition metal) surrounded by a number of molecules

or ions called ligands

A ligand is a Lewis base having a lone pair of electrons that can be donated to an empty orbital on the metal ion (acting as a Lewis acid)

forming a coordinate covalent bond

The most common ligands are H2O, NH3, Cl- and CN-

The charge on a complex ion is the sum of the charges of the metal ion and the ligands

Examples:CoCl42- = Co2+ + 4Cl-

Ni(NH3)62+ = Ni2+ + 6NH3

The ligands do not all have to be the same

Example: Cu(NH3)4(H2O)22+

Coordination Number

The number of ligands attached to the metal ion is called the coordination number

The most common coordination numbers are 6, 4 and 2

Examples: Co(H2O)62+, CoCl42-, Ag(NH3)2

+

Complex Ion Shapes

Coordination number 6 complexeswill always be octahedral

Coordination number 2 complexes will always be linear

Coordination number 4 complex can be tetrahedral or square planar

Octahedral Complexes

Tetrahedral Complexes

Square Planar Complexes

Linear Complexes

[H3N: Ag :NH3]+

[Ag(NH3)2]+

VSEPR Theory and Complex Ions

Valence Shell Electron Pair Repulsion (VSEPR) theory works for six and two coordinate complex ions but cannot be used to predict whether a four coordinate complex will be tetrahedral or square

planar!

11/29/2010

3

Coordination Compounds

A coordination compound consists of a complex ion along with one or more counterions which are anions or cations required to produce a

compound with no net charge

Square brackets are placed around the complex ion to indicate the composition of the complex ion

Counterions are shown outside the brackets

Example:

[Co(NH3)5Cl]Cl2 = Co(NH3)5Cl2+ + 2Cl-

A coordination compound behaves like any other ionic compound when dissolved in water forming independent cations and anions:

[Co(NH3)5Cl]Cl2(s) Co(NH3)5Cl2+(aq) + 2Cl-(aq)2H O

Colors of Coordination Compounds

Many coordination compounds are very colorful since the transition metal ions absorb light in the visible spectrum

Ligand Types

Ligands that can form one bond to a metal ion are called

monodenate or unidentate(“single toothed”)

Ligands with more than one atom with a lone pair that can bond to a metal are said to chelating ligands or chelates (“clawed”)

A ligand that can form two bonds to a metal ion is called a

bidentate ligand

Ligands that can form more than two bonds are called polydentate

Etheylenediaminetetraacetate (EDTA)

A hexadenatate ligand!

Common Ligands Naming Coordination Compounds

11/29/2010

4

Isomerism in Coordination Compounds

Occurs when two or more complex ions contain exactly the same types and numbers of atoms (hence same formula) but the

arrangement of ligands differ leading to different physical and/or chemical properties

Linkage Isomerism

The composition of the complex is the same but one or more of the ligands attaches with a different atom

Example:

[Co(NH3)4(NO2)Cl]Cl

[Co(NH3)4(ONO)Cl]Cl

Geometrical Isomerism

Occurs when ligands occupy different spatial positions around the central atom

cis trans

cistrans

11/29/2010

5

Review of Electronic Configurations of Transition Metal Atoms

Iron?1s22s22p63s23p64s23d6

or[Ar] 4s23d6

Chromium?Expect? 1s22s22p63s23p64s23d4 or [Ar]4s23d4

Find: 1s22s22p63s23p64s13d5 or [Ar]4s13d5

Half-filled 3d level is very

stable!

Gadolinium?1s22s22p63s23p64s23d104p65s24d105p66s25d14f7

or [Xe]6s25d14f7

11/29/2010

6

Electronic Configurations of Transition Metal Ions

In neutral transition elements, the s- and d-orbitals have very similar energies

However, in transition metal ions the energy of the d-orbitals is significantly lower than that of the s-orbitals so that the electrons remaining after the ion forms occupy the lower energy d-orbitals.

Example:

Fe3+ is [Ar]3d5 not [Ar]4s23d3

Bonding in Coordination Compounds

Can be considered a Lewis acid-base reaction where lone pairs on the ligands are shared with orbitals on the metal ion