Metal Complexes -- Chapter 24
Metal Complex -- consists of a set of ligands that are bonded to
a central metal ion by coordinate covalent
bonds.
e.g. Cu2+ + 4 L --> [CuL4]2+ ( L = NH3, H2O, etc)
Ligands are Lewis Bases and can be:
– monodentate -- one donor atom
e.g. H2O, NH3, Cl–, OH–, etc.
– bidentate -- two donor atoms
e.g. ethylenediamine, NH2–CH2–CH2–NH2 “en”
H2C
CH 2
=
H2N
NH 2
NH 2
H2N
M
M
Polydentate ligands
– polydentate -- more than two donor atoms
e.g. EDTA -- ethylenediaminetetraacetic acid
(6 donor atoms)
O
O
O
C
H2
C
H2C
N
O
C
O
CH 2
CH 2 C
CH 2
O
= EDTA4–
N
H2C
C
O
O
Chelate Effect
Chelate effect -- complexes with bi- or polydentate ligands are
more stable than those with similar monodentate ligands
e.g. [Ni(en)3]3+ is more stable than [Ni(NH3)6]3+
3+
NH 2
NH 2
H2N
Ni
H2N
N
H2
NH 2
Know Table 24.2!
Writing Formulas of Complex Ions
Metal ion first, then ligands. Charge outside brackets.
total charge = sum of metal ion + ligands
e.g.
metal ion
Cu2+
Co3+
Fe3+
ligand
H2O
NH3
CN–
complex
[Cu(H2O)]2+
[Co(NH3)6]3+
[Fe(CN)6]3–
Nomenclature
• 1. Name the ligands, put in alphabetical order.
• 2. Add prefixes for ligands that appear more than once.
• 3. Name the metal
– Cationic complex; metal name
– Anionic complex; metal prefix + ate
• 4. Add oxidation state in roman numerals
• 5. As separate word, write “ion” if not neutral, or list
counterions (positive first, negative second).
Examples:
•
Complex
Name
[Ni(CN)4]2–
tetracyanonickelate(II) ion
[CoCl6]3–
hexachlorocobaltate(III) ion
[CoCl2(NH3)4]+
tetraaminedichlorocobalt(III) ion
Na3[Co(NO2)6]
sodium hexanitrocobaltate(III)
[CrCl2(en)2]2SO4
dichlorobis(ethylenediamine)chromium(III) sulfate
Coordination Number and Structure
Coordination # -- 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)4]2–
2
OH
Zn
OH
HO
OH
More Coordination Number and Structure
(b) Four-Coordinate Complexes -- two structural types
square planar structures -- common for d8 metal ions (Ni2+, Pd2+,
Pt2+) and for Cu2+
e.g.
2
2+
NH3
H3N
Cl
Cl
Cu
Pt
NH3
H3N
Cl
Cl
(c) Six-Coordinate Complexes -- the most common!
“always” octahedral structures, e.g.
3+
NH 2
Cl
NH 2
H2N
Ni
H2N
N
H2
NH 3
NH 3
Cr
NH 3
NH 2
Cl
Cl
Sample Problem
1. Give the name or formula, as required. Draw the structure
of each complex.
a) [AgI2]–
b) [Co(en)3]3+
c) cis-dichlorobis(ethylenediamine)cobalt(III) chloride
d) sodium tetracyanonickelate(II)
Structural Isomers of Coordination Complexes
• Linkage isomers
pentaamminenitrocobalt(II) ion
pentaamminenitritocobalt(II) ion
• Coordination isomers
pentaamminechlorocobalt(II) bromide
pentaamminebromocobalt(II) chloride
Isomerism; Overview
10
Stereoisomers of Coordination Complexes
(a) Geometrical Isomers
Cl
Cl
NH 3
Cr
NH 3
NH 3
NH 3
Cl
NH 3
Cr
NH 3
NH 3
NH 3
cis
Cl
trans
fac
mer
(b) Enantiomers
Three common ways to get enantiomers:
--chiral ligand
--tetrahedral complex with 4 different groups
--octahedral complexes with chelating ligands (and no mirror plane)
Sample Questions
Draw a clear, 3-dimensional structure of the geometrical
isomer of Co(en)(NH3)2Cl2 that is optically active. (define any
abbreviations)
Excess silver nitrate is added to a solution containing 0.0522
mol of [Co(NH3)4Cl2]Cl. How many g of AgCl (FW = 143.3)
will precipitate?
Sample Questions
Draw a clear, 3-dimensional structure of the geometrical
isomer of Co(en)(NH3)2Cl2 that is optically active. (define any
abbreviations)
NH 2
NH 2
NH 2
Co
Cl
NH 2
NH 3
NH 3
Cl
NH 3
Cr
Cl
NH 3
Cl
Excess silver nitrate is added to a solution containing 0.0522
mol of [Co(NH3)4Cl2]Cl. How many g of AgCl (FW = 143.3)
will precipitate?
7.48 g
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
energy
eg
dx2-y2
D = crystal field 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
Shape and Directionality of the d Orbitals
eg
D
t2g
Crystal Field Strength
The size of D depends on…
• The nature of the ligand (ligand-metal bond strength!)
“spectrochemical series” -- D decreases:
CN– > NO2– > en > NH3 > H2O > OH– > F– > Cl– > Br–
“strong field ligands”
“weak field ligands”
• The oxidation state of the metal (charge!)
D is greater for M3+ than for M2+
• The row of the metal in the periodic table (size!)
for a given ligand and oxidation state of the metal, D
increases going down in a group
e.g. D is greater in Ru(NH3)63+ than in Fe(NH3)63+
Colors of metal complexes are due to electronic transition
between the t2g and eg energy levels
d Orbital Splitting Diagrams for Octahedral
Complexes
dx2
dx2
dx2–y2
eg
dx2–y2
eg
large D
“low spin”
small D
“high spin”
dxy
dxz
dyz
Fe(H2O)62+
t2g
dxy
dxz
dyz
Fe(CN)64–
t2g
High Spin vs. Low Spin
CN– is a stronger field ligand than is H2O which leads to a
greater D 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 D 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)
Sample Questions
A complex [CoA6]3+ is red. The complex [CoB6]3+ is green.
a) Which ligand, A or B, produces the larger crystal field
splitting D?
b) If the two ligands are ammonia and water, which is A and
which is B?
c) Draw the d orbital diagram of either complex, assuming it
is “high spin.” How many unpaired electrons will it have?
Give the order of increase of D in the following sets:
a) Cr(NH3)63+, CrCl63–, Cr(CN)63–
b) Co(H2O)62+, Co(H2O)63+, Rh(H2O)63+