Download Chapter 21 Transition Metals and Coordination Chemistry

Chapter 21
Transition 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)63+ along with three Cl- ions
3Cl-
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 CNThe charge on a complex ion is the sum of the charges of the metal
ion and the ligands
Examples:
CoCl42- = Co2+ + 4ClNi(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 complexes
will 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!
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+ + 2ClA 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)
H 2O
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
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
trans
cis
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
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