____ ____Chapter 23:
The Transition Elements
and Their Coordination Compounds
Homework:
The transition elements (B
group elements) as a group are colorful, useful and fascinating. Many of their properties are due to the
filling of d-orbitals and f-orbitals, and an understanding of the electronic
configuration is essential. You may need to review the electron configuration
for these elements in your text.
Problem:
Determine the possible
charge states for Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.
General Trends
Atomic radius decreases as
you go across the periodic table toward the middle of the transition elements,
and then increase from the middle to the end of the transition series. Period 4
transition elements are significantly smaller than period 5 and period 6
transition elements. Period 5 and period 6 transition elements are virtually
the same size due to an effect called the lanthanide contraction which is
caused by a substantial increase of nuclear charge since the lanthanide f-block
elements are filled before completing the d-block. Chemical properties parallel the covalent radii similarities for
the period 5 and 6 transition elements.
The ionization energy
generally increases across the transition series. Period 4 and 5 transition elements have similar ionization
energies; however period 6 transition elements have significantly higher
ionization energies making them less reactive than period 4 and 5.
Oxidation states of the
transition elements are variable. You should know the possible charge states for these
elements. As the oxidation state of the
transition element increases the covalent nature of the bond increases.
Consider the following problem.
Problem:
Which compound has a higher
melting point, TiCl2 or TiCl4? Why?
Demo:
Chromate-dichromate acid
equilibrium.
Demo:
Complex ion formation of
several transition elements.
Complex Ions and Coordination Compounds
Complex ions are formed when
ligands bond to a central metal ion. A
ligand is a Lewis base and donates a pair of electrons to the metal. The number of ligands a metal can accept is
called the coordination number.
Problem:
What is the coordination
number for the following substances?
Na3Fe(CN)6, CaCu(NH3)4, K2Ni(H2O)4(NH3)2, Ag(NH3)2+
Many Lewis bases can act as
ligands; NH3, H2O,
halogens, CO, CN-, and OH-to name just a few. All of these ligands can donate 2 electrons
to a metal ion and are called monodentates.
Bidentate ligands can donate two pairs of electrons to the central metal
atom, and polydentate ligands can donate more than two pairs of electrons.
Bidentates: Ethylenediamine (en),
oxalate (ox)
Polydentates: ethylene diamine tetra acetate (EDTA), heme (see pages 1006-1007)
Complexes of metal ions by
polydentates are sometimes called chelates and the polydentates are called
chelating agents.
NOTE: The word dentate means
tooth and the word chelate means claw or pincer.
Complex Ion Nomenclature
1) Positive ions named first.
2) When naming the complex, ligands are named first in alphabetical
order not including Greek prefixes.
3) a) Anion ligands end in
-o.
b) Neutral
ligands are usually given the name of the molecule with the
following exceptions;
NH3 ammine, CO carbonly, and H2O aqua.
c) Greek prefixes
indicate the number of a type of ligand present in a
complex. di,
tri, tetra, penta, hexa
d) When the name of a
ligand contains a Greek prefix, an alternate
numbering system is used. bis, tris, tetrakis, pentakis, hexakis
4 The name of the central metal atoms comes last. If the complex is an anion the Latin name is
given and ends in -ate followed by the oxidation state in Roman numerals. If the complex is neutral or positive, the
English name is given followed by the oxidation state in Roman numerals.
Problem:
Name the following
substances.
Ca2Fe(CN)6
[Fe(NH3)3H2OCl2]Cl
Fe2[Ni(C2O4)2]3
[Pt(NH3)4ClBr]I2
[Cr(H2O)6]Cl3
[Cr(H2O)5Cl]Cl2•H2O
[Cr(H2O)4Cl2]Cl•2H2O
Structure and Isomerism in
Coordination Compounds
Isomerism = same molecular
formula with different structure.
Structural Isomerism
Stereoisomers = isomers
having same formula and bonded in the same order, but different in spatial
arrangement.
I.e.) 1,2 dichloroethene
Ionization isomers = isomers
having same formula, but differing in charge state of central atom because of
different coordination.
I.e.) [Co(NH3)5SO4]Br
& [Co(NH3)5Br]SO4
Hydrate isomers = isomers
having same formula, but different number of water in coordination sphere.
I.e.) [Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2•H2O,
[Cr(H2O)4Cl2]Cl•2H2O
Coordination isomers =
isomers having same formula but different atoms coordinated to anion complex and
cation complex.
I.e.)
[Cu(NH3)4][PtCl4], [CuCl4][Pt(NH3)4]
Linkage isomers = isomers
where ligands use different atoms to bond to central metal ion. I.e.) SCN or NCS,
NO2, or
ONO, (Ambidentate ligands)
Geometric isomers= isomers
where atoms are bonded to one another in the same way, but in different
relative orientation = stereoisomers.
I.e. cis-dibromodichloroplatinate
(IV) and
trans-dibromodichloroplatinate (IV)
Optical isomers = isomers
having nonsuperimposible mirror images. Only molecules having no plane of
symmetry can have optical isomers.
I.e.) dibromobisethylenediaminecobalt (IV). Optical isomers have the unusual property of
rotating a plane of polarized light. If
the plane of light is rotated to the right (clockwise) the substance is called
dextrorotatory (d), if the plane of light is rotated to the left
(counterclockwise) the substance is called levorotatory (l). The physical and chemical properties of the
two isomers are identical. A mixture
of equal parts d and l gives a racemic mixture which does not rotate a plane of
polarized light.
Valence Bond Theory of Complexes
Paramagnetism = substances
with unpaired electrons attracted into a magnetic field.
Diamagnetism = substance
with all electrons paired.
Ligands attach themselves by
donating electron pairs to the central metal atom. Two classes of compounds can form when ligands bond to metals;
high spin or low spin depending on the nature of the ligands. More on the
nature of the ligands will be explained in the crystal field theory. Consider the following complexes of nickel
II.
Problem:
Hexaaquairon (II) ion is
paramagnetic where hexacyanoferrate (II) is diamagnetic.
a) Explain this in terms of VBT orbital diagrams.
b) What is the hybridization for each complex?
c) Which one is high spin and which one is low spin?
Extra credit:
In some chemical text or
other chemistry resource there is a name given to the magnetic phenomenon
related to the number of unpaired electrons. I.e.) Three ____________ units are
due to three unpaired electrons. What the blank are they?
Nickel (II), palladium (II)
and platinum (II) commonly form square planar complexes commonly form square
planar complexes (dsp2).
Nickel can also form tetrahedral complexes (sp3).
Problem:
Tetracyanonickelate (II) is
a low spin complex where tetraamminenickel (II) is high spin. What is the shape of each complex?
Crystal Field Theory
Crystal field theory gives
us the answer to the question “Why does a high spin or low spin complex form
for a particular ligand-metal complex?”. The theory argues that the d-orbitals
split as a result of the electric field (crystal field) imposed on them by the
ligands. Initially all d-orbitals are
at the same energy. Consider the
d-orbitals (See page 1022) and what might happen to the energy of these
orbitals as six ligands approach along the ±x, ±y and ±z axes.
As the ligands move closed to the d-orbital set, they interact in a
head-on fashion with the dz2 and the dx2-y2
orbitals. Direct interaction raises the
energy of these orbitals. The ligands
will be directed between the lobes of the dxy, dxz, and
the dyz. When ligands are between the lobes of orbitals, the energy
is lowered for these orbitals.
_____ _____
dz2 dx2-y2
_____ _____
_____ _____ _____ D = Energy difference
_____
_____ _____
dxy dxz dyz
Orbital energy without
ligand field. Orbital energy with ligand field .
The size of the energy
difference between the lower and higher d-levels is called D (delta). This energy difference is directly related
to the ability of the ligand to apply an electric field to the d-orbitals of
the metal ion. Ligands that bond strongly to the metal impose a stronger field
than those ligands that bond weakly to the metal. A list of ligands called the spectrochemical series gives the
relative ability of ligands to bond (apply a field) to the metal ion.
The Spectrochemical Series
Weak field ligands
Strong field ligands
I- < Br- < Cl- <
F- < OH- < H2O <
NH3 < en
< NO- <
CN- < CO
High spin complexes
Low spin complexes
The magnitude of D determines the magnetic
properties (high spin or low spin) of a complex .
____ ____
dz2 dx2-y2
____ ____
dz2 dx2-y2
____ ____
____ ____ ____ D
D
____ ____
____
No field dxy dxz dyz
Weak field ____ ____ ____
Small
D dxy dxz dyz
Strong
field
Large D
Problem:
How many unpaired electrons
will Fe(CN)63-and
Fe(Cl)63- have? Use the above energy diagram.
____ ____ ____
For tetrahedral complexes
the energy diagram is: dxy dxz dyz D
____ ____
dz2 dx2-y2
--------------------------------------------------------------------------------------------
____
dx2-y2
For square planar complexes
the energy diagram is:
____
dxy
____
dz2
____ ____
dxz dyz
Problem:
Ni(CN)4-2
is diamagnetic and Ni(NH3)4+2 is paramagnetic. Describe the electron distribution in terms of crystal field
theory.
Visible Spectra of Transition-Metal Complexes
The color of a complex is
due to the absorption of frequencies of light that correspond to the difference
in energy between d-orbital energy levels. The colors not absorbed by the
complexes are the colors we see.
Problem:
Fe(CN)63-
absorbs light having wavelengths less than 620 nm. What is the color of the complex? See page 1027
Problem:
Ti(H2O)3+
absorbs light having wavelengths from 450 to 620 nm. What is the color of the complex? See page 1027
Problem:
A complex absorbs light
having wavelengths less than 500 nm and more than 560 nm. What is the color of the complex? See page 1027
Problem:
Cu(H2O)2+
absorbs light most strongly at 590 nm.
What is the value of D in kJ/mol.