Transition Metal Chemistry and
Coordination Compounds
Chapter 23
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The Transition Metals
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3
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Oxidation States of the 1st Row Transition Metals
(most stable oxidation numbers are shown in red)
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Ionization Energies for the 1st Row Transition Metals
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Coordination Compounds
A coordination compound typically consists of a complex ion
and a counter ion.
A complex ion contains a central metal cation bonded to one
or more molecules or ions.
The molecules or ions that surround the
metal in a complex ion are called ligands.
H
H
H H H
••
Cl
••
-
C
••
O
••
••
••
N
••
A ligand has at least one unshared pair
of valence electrons.
O
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Coordination Compounds
The atom in a ligand that is bound directly to the metal atom is
the donor atom.
••
N
O
H
H
H H H
The number of donor atoms surrounding the central metal atom
in a complex ion is the coordination number.
Ligands with:
one donor atom
two donor atoms
three or more donor atoms
monodentate
bidentate
H2O, NH3, Clethylenediamine
polydentate
EDTA
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Coordination Compounds
bidentate ligand
••
H2N
CH2
CH2
••
NH2
[Co(en)3]2+
or
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polydentate ligand
(EDTA)
[PbEDTA]2-
Bidentate and polydentate ligands are called chelating agents
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Example 23.1
Specify the oxidation number of the central metal atom in each
of the following compounds:
(a) [Ru(NH3)5(H2O)]Cl2
(b) [Cr(NH3)6](NO3)3
(c) [Fe(CO)5]
(d) K4[Fe(CN)6]
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Example 23.1
Strategy
The oxidation number of the metal atom is equal to its charge.
First we examine the anion or the cation that electrically
balances the complex ion. This step gives us the net charge of
the complex ion.
Next, from the nature of the ligands (charged or neutral
species) we can deduce the net charge of the metal and hence
its oxidation number.
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Example 23.1
Solution
(a) Both NH3 and H2O are neutral species. Because each
chloride ion carries a -1 charge, and there are two Cl- ions,
the oxidation number of Ru must be +2.
(b) Each nitrate ion has a charge of -1; therefore, the cation
must be [Cr(NH3)6]3+. NH3 is neutral, so the oxidation
number of Cr is +3.
(c) Because the CO species are neutral, the oxidation number
of Fe is zero.
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Example 23.1
(d) Each potassium ion has a charge of +1; therefore, the anion
is [Fe(CN)6]4-. Next, we know that each cyanide group bears
a charge of -1, so Fe must have an oxidation number of +2.
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Naming Coordination Compounds
•
The cation is named before the anion.
•
Within a complex ion, the ligands are named first in
alphabetical order and the metal atom is named last.
•
The names of anionic ligands end with the letter o. Neutral
ligands are usually called by the name of the molecule. The
exceptions are H2O (aqua), CO (carbonyl), and NH3
(ammine).
•
When several ligands of a particular kind are present, the
Greek prefixes di-, tri-, tetra-, penta-, and hexa- are used to
indicate the number. If the ligand contains a Greek prefix,
use the prefixes bis, tris, and tetrakis to indicate the number.
•
The oxidation number of the metal is written in Roman
numerals following the name of the metal.
•
If the complex is an anion, its name ends in –ate.
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Example 23.2
Write the systematic names of the following coordination
compounds:
(a) Ni(CO)4
(b) NaAuF4
(c) K3[Fe(CN)6]
(d) [Cr(en)3]Cl3
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Example 23.2
Strategy
We follow the preceding procedure for naming coordination
compounds and refer to Tables 23.4 and 23.5 for names of
ligands and anions containing metal atoms.
Solution
(a) The CO ligands are neutral species and therefore the Ni
atom bears no net charge. The compound is called
tetracarbonylnickel(0) , or more commonly, nickel
tetracarbonyl.
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Example 23.2
(b) The sodium cation has a positive charge; therefore, the
complex anion has a negative charge (A u F4- ). Each fluoride
ion has a negative charge so the oxidation number of gold
must be +3 (to give a net negative charge). The compound
is called sodium tetrafluoroaurate(III) .
(c) The complex ion is the anion and it bears three negative
charges because each potassium ion bears a +1 charge.
Looking at [Fe(CN)6]3-, we see that the oxidation number of
Fe must be +3 because each cyanide ion bears a -1 charge
(-6 total). The compound is potassium
hexacyanoferrate(III). This compound is commonly called
potassium ferricyanide.
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Example 23.2
(d) As we noted earlier, en is the abbreviation for the ligand
ethylenediamine. Because there are three chloride ions
each with a -1 charge, the cation is [Cr(en)3]3+. The en
ligands are neutral so the oxidation number of Cr must be
+3. Because there are three en groups present and the
name of the ligand already contains di (rule 4), the
compound is called tris(ethylenediamine)chromium(III)
chloride .
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Example 23.3
Write the formulas for the following compounds:
(a) pentaamminechlorocobalt(III) chloride
(b) dichlorobis(ethylenediamine)platinum(IV) nitrate
(c) sodium hexanitrocobaltate(III)
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Example 23.3
Strategy
We follow the preceding procedure and refer to Tables 23.4
and 23.5 for names of ligands and anions containing metal
atoms.
Solution
(a) The complex cation contains five NH3 groups, a Cl- ion, and
a Co ion having a +3 oxidation number. The net charge of
the cation must be +2, [Co(NH3)5Cl]2+. Two chloride anions
are needed to balance the positive charges. Therefore, the
formula of the compound is [Co(NH3)5Cl]Cl2.
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Example 23.3
(b) There are two chloride ions (-1 each), two en groups
(neutral), and a Pt ion with an oxidation number of +4. The
net charge on the cation must be +2, [Pt(en)2Cl2]2+. Two
nitrate ions are needed to balance the +2 charge of the
complex cation. Therefore, the formula of the compound is
[Pt(en)2Cl2](NO3)2 .
(c) The complex anion contains six nitro groups (-1 each) and a
cobalt ion with an oxidation number of +3. The net charge
on the complex anion must be -3, [Co(NO2)6]3-. Three
sodium cations are needed to balance the -3 charge of the
complex anion. Therefore, the formula of the compound is
Na3[Co(NO2)6] .
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Structure of Coordination Compounds
Coordination number
2
Structure
Linear
4
Tetrahedral or Square planar
6
Octahedral
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Structure of Coordination Compounds
Stereoisomers are compounds that are made up of the same
types and numbers of atoms bonded together in the same
sequence but with different spatial arrangements.
Geometric isomers are stereoisomers that cannot be
interconverted without breaking a chemical bond.
cis-[Pt(NH3)2Cl2]
trans-[Pt(NH3)2Cl2]
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Structure of Coordination Compounds
trans
cis
cis-[Co(NH3)4Cl2]+
Are these
additional
geometric
isomers of
[Co(NH3)4Cl2]+?
trans-[Co(NH3)4Cl2]+
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Structure of Coordination Compounds
Optical isomers are nonsuperimposable mirror images.
cis-[Co(en)2Cl2]+
trans-[Co(en)2Cl2]+
optical isomers
not optical isomers
chiral
achiral
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Structure of Coordination Compounds
Chiral molecules are optically active.
Rotate the plane of polarized light.
Polarimeter
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Bonding in Coordination Compounds
d-orbital orientation relative to x, y, and z axes
All equal in energy in the absence of ligands!
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Bonding in Coordination Compounds
Isolated
transition metal
atom
Bonded
transition metal
atom
Crystal field splitting (D) is the energy difference between
two sets of d orbitals in a metal atom when ligands are present
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Bonding in Coordination Compounds
DE = hn
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Bonding in Coordination Compounds
Spectrochemical Series
I- < Br- < Cl- < OH- < F- < H2O < NH3 < en < CN- < CO
Weak field ligands
Small D
Strong field ligands
Large D 35
Colors of Some Transition Metal Ions
Ti3+
Cr3+
Mn2+
Fe3+
Co2+
Ni2+
Cu2+
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Bonding in Coordination Compounds
weak ligand field
strong ligand field
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Orbital Diagrams for High
Spin and Low Spin
Octahedral Complexes
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Example 23.4
Predict the number of unpaired spins in the [Cr(en)3]2+ ion.
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Example 23.4
Strategy
The magnetic properties of a complex ion depend on the
strength of the ligands.
Strong-field ligands, which cause a high degree of splitting
among the d orbital energy levels, result in low-spin complexes.
Weak-field ligands, which cause a small degree of splitting
among the d orbital energy levels, result in high-spin
complexes.
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Example 23.4
Solution
The electron configuration of Cr2+ is [Ar]3d4.
Because en is a strong-field ligand, we expect [Cr(en)3]2+ to be
a low-spin complex.
According to Figure 23.22, all four electrons will be placed in
the lower-energy d orbitals (dxy, dyz, and dxz) and there will be a
total of two unpaired spins.
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Crystal Field splitting in a Tetrahedral Complex
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Crystal Field splitting in a Square Planar Complex
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Reactions of Coordination Compounds
Exchange or substitution reactions
Stability
Unstable in acid solution
Kinetic lability: tendency to react
Reaction takes several days to complete
Complex is inert.
Labile complexes react rapidly.
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Chemistry In Action: Coordination Compounds in Living Systems
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Chemistry In Action: Cisplatin – The Anticancer Drug
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