Metal Complexes -- Chapter 24 (Sections 24.3 - 24.5)
1.
Metal Complex -- consists of a set of ligands that are
bonded to a central metal
coordinate covalent bonds.
e.g.,
Cu2+ +
4 L 
ion
CuL42+ (L = NH3, H2O, etc.)
(a) Ligands are Lewis Bases and can be:
 monodentate
--
 bidentate --
two donor atoms
one donor atom
e.g., H2O, NH3, Cl-, OH-, etc.
e.g., ethylenediamine, NH2-CH2-CH2-NH2 "en"
H2C
H2C
H2N:
: NH2
=
H2N:
: NH2
M
M
 polydentate -- more than two donor atoms
e.g., EDTA -- ethylenediaminetetraacetic acid
(6 donor atoms)
O
..
:O
.. C CH2
:N
..
:O
.. C CH2
O
O
CH2
CH2
..
:
H2C C O
..
N:
..
H2C C O :
..
O
1
=
EDTA4-
by
(b) Writing Formulas of complex ions
metal ion first, then ligands
total charge = sum of metal ion + ligands
e.g.,
metal ion
ligand
complex
Cu2+
H2O
Cu(H2O)42+
Co3+
NH3
Co(NH3)63+
Fe3+
CN-
Fe(CN)63-
(c) Chelate Effect - complexes with bi- or polydentate
ligands are more stable than those with
similar monodentate ligands
e.g., Ni(en)33+ is more stable than Ni(NH3)63+
3+
N
N
N
N
N
Ni
N
(d) Nomenclature -- study rules and examples in book!
Complex
Ni(CN)42-
Name
tetracyanonickelate(II) ion
CoCl63-
hexachlorocobaltate(III) ion
Co(NH3)4Cl2+
Na3[Co(NO2)6]
tetraamminedichlorocobalt(III) ion
[Cr(en)2Cl2]2SO4
dichlorobis(ethylenediamine)chromium(III) sulfate
sodium hexanitrocobaltate(III)
2
2. Coordination Number and Structure
Coord # = number of donor atoms attached to the metal center
(a) Two-Coordinate Complexes -- linear structures
rare except for Ag+
e.g., Ag(NH3)2+ and Ag(CN)2(b) Four-Coordinate Complexes -- two structural types
 Tetrahedral structures -- common for ions with filled
d subshells, e.g., Zn2+ as in Zn(OH)42-
2-
OH
Zn
OH
HO
OH
 Square Planar structures -- common for d8 metal
ions (Ni2+, Pd2+, Pt2+) and for Cu2+
e.g.,
Cl
Cl
2-
H3N
2+
Cu
Pt
Cl
NH3
H3N
Cl
NH3
(c) Six-Coordinate Complexes -- the most common!
"always" octahedral structures, e.g.,
3+
N
N
N
Ni
Cl
N
N
H3N
H3N
N
Cr
Cl
3
NH3
Cl
3. Isomers of Coordination Complexes
Isomers --
same chemical composition (formula)
but different structures (due to either
the arrangement of atoms or 3-D shape)
(a) Linkage isomers
same ligand, with different donor atoms
e.g., In [Co(NH3)5NO2]2+, the NO2- ligand can bind
to Co through N ("nitro") or O ("nitrito").
H3N NH
3 O
H3N Co N
O
H3N NH
3
2+
2+
H 3N
H 3N
Co
H3N
nitro complex
NH3
O
N
O
NH3
nitrito complex
4
(b) Geometrical isomers
 Square Planar complexes
Cl
NH3
Cl
NH3
Pt
Pt
Cl
NH3
H3N
cis
Cl
trans
"cisplatin"
(anticancer drug)
(no biological activity)
 Octahedral complexes
Cl
H3N
H3N
Cr
Cl
NH3
Cl
H3N
H3N
NH3
NH3
NH3
Cr
Cl
cis
trans
(c) Enantiomers -- non-superimposable mirror images
also called "optical isomers"
N
N
N
Co
N
N
Cl
N
Cl
Cl
Co
Cl
5
N
N
4. Crystal Field Theory
(Bonding in Transition Metal Complexes)
Metal complexes are usually highly colored and are often
paramagnetic – such facts can be explained by a
"d-orbital splitting diagram"
dz2
dx2-y2
eg
 = crystal field
energy
splitting energy
dxy
dxz
dyz
t2g
d orbitals of the metal ion
in an octahedral field of
ligands
d orbitals of the
free metal ion
The size of  depends on
 the nature of the ligand
"spectrochemical series" --  decreases:
CN- > NO2- > en > NH3 > H2O > OH- > F- > Cl- > Br"strong field ligands"
"weak field ligands"
 the oxidation state of the metal
 is greater for M3+ than for M2+
 the row of the metal in the periodic table
for a given ligand and oxidation state of the metal,
 increases going down in a group
e.g.,  is greater in Ru(NH3)63+ than in Fe(NH3)63+
Colors of metal complexes are due to electronic transitions
between the t2g and eg energy levels
6
d orbital splitting diagrams for octahedral complexes
dz2
dz2
dxy
dx2-y2
dxz
dx2-y2
eg
large 
"low spin"
eg
small 
"high spin"
dyz
t2g
dxy
Fe(H2O)62+
dxz
dyz
t2g
Fe(CN)64-
CN- is a stronger field ligand than is H2O which leads to
a greater  value (i.e., a greater d orbital splitting)
as a result,
Fe(H2O)62+ is a "high spin" complex and is
paramagnetic (4 unpaired electrons)
while,
Fe(CN)64-
is a "low spin" complex and is
diamagnetic (no unpaired electrons)
the CN- complex with the larger  value absorbs light of higher
energy (i.e., higher frequency but shorter wavelength)
OMIT:
d orbital splitting diagrams for other geometries
(i.e., tetrahedral and square planar)
7