Download Chapter 23 Transition Metals and Coordination Chemistry

Lecture Presentation
Chapter 23
Transition Metals
and Coordination
Chemistry
Yonsei University
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23.1 The Transition Metals
Minerals
• Most metals, including transition metals, are found in solid
inorganic compounds known as minerals.
• Minerals are named by common, not chemical, names.
Transition
Metals
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Metallurgy
Metallurgy: the science and technology of extracting metals from natural
sources and preparing them for practical use.
• mining (getting the ore out of the ground)
• concentrating (preparing it for further treatment)
– Differences in the chemical and physical properties of the mineral of
interest and the undesired material, called gangue, are used to
separate these components.
– Example: Iron can be separated from gangue in finely ground
magnetite by using a magnet to attract the iron.
• reduction (to obtain the free metal in the 0 oxidation
state)
• purifying or refining (to obtain the pure metal)
• mixing with other metals (to form an alloy)
– Alloys are metallic materials composed of two or more elements.
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Atomic Radii
•
•
•
As one goes from left to right
across a row, we see a decrease,
then an increase in the radius of
transition metals.
In general, increasing effective
nuclear charge tends
to make atoms smaller.
On the other hand,
the strongest (and, therefore,
shortest) metallic bonds are
found in the center of the transition
metals.
The lanthanide contraction
balances the increase in size we
anticipate between Hf (4B) and Zr.
Transition
Metals
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Traits of Transition Metals
• Because most
transition metals have
only partially occupied
d subshells, the
metals and/or their
compounds often
– Have more than one
oxidation state,
commonly +1, +2 or +3
– Are pigmented
Transition
Metals
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Traits of Transition Metals
• Because most
transition metals have
only partially occupied
d subshells, the
metals and/or their
compounds often
– Have more than one
oxidation state,
– Are pigmented,
– Have magnetic
properties.
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Transition
Metals
Paramagnetism
• Paramagnetism, as you
recall from Chapter 9,
results from an atom
having unpaired electrons.
• Such atoms, then, show
attraction to a magnet
placed close by.
Transition
Metals
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Ferromagnetism
• In ferromagnetic substances,
the unpaired spins influence
each other to align in the
same direction, thereby
exhibiting strong attractions
to an external magnetic field.
• Such species are permanent
magnets.
– SmCo5, Nd5Fe14B
Transition
Metals
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Antiferromagnetism
• Antiferromagnetic
substances have
unpaired spins on
adjacent atoms that
align in opposing
directions.
• These magnetic
fields tend to cancel
each other.
Transition
Metals
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Ferrimagnetism
• Ferrimagnetic
substances have spins
that align opposite each
other, but the spins are
not equal, so there is a
net magnetic field.
• Examples are NiMnO3,
Y3Fe5O12, and Fe3O4.
Transition
Metals
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23.2 Transition-Metal Complexes
• Commonly, transition
metals can have
molecules or ions that
bond to them.
• These give rise to
complex ions or
coordination
compounds.
Transition
Metals
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Ligands
The molecules or ions that bind to the central
metal are called ligands (from the Latin ligare,
meaning “to bind”).
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Metals
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Coordination
One of the properties that has led to the
fascination with complexes and transition
metals is the wide range of stunning colors
found in them.
Transition
Metals
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Werner Theory
• Alfred Werner (Swiss) deduced that there was a
difference between the oxidation number of a metal
and the number of ligands it took on, which he called
the coordination number.
• Thus, although the first two complexes in the table
above each have 3 chlorines, in the first all three
serve as anions, while in the second one of the
chlorines is tightly bound to the cobalt and the other
two are counterions.
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Transition
Metals
Sample Exercise 23.1 Identifying the Coordination Sphere of a
Complex
Palladium(II) tends to form complexes with coordination number 4. A compound
has the composition PdCl2 · 3 NH3. (a) Write the formula for this compound that
best shows the coordination structure. (b) When an aqueous solution of the
compound is treated with excess AgNO3(aq), how many moles of AgCl(s) are
formed per mole of PdCl2 · 3 NH3?
Solution
(a)[Pd(NH3)3Cl]Cl
(b)[Pd(NH3)3Cl]Cl(aq) + AgNO3(aq) → [Pd(NH3)3Cl]NO3(aq) + AgCl(s)
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Sample Exercise 23.2 Determining the Oxidation Number of a Metal
in a Complex
What is the oxidation number of the metal in [Rh(NH3)5Cl](NO3)2?
Solution
x + 5(0) + (–1) + 2(–1) = 0
[Rh(NH3)5Cl](NO3)2
The oxidation number of rhodium, x, must therefore be +3.
Practice Exercise
What is the charge of the complex formed by a platinum(II) metal ion
surrounded by two ammonia molecules and two bromide ions?
Answer: zero
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The Metal–Ligand Bond
• The reaction between a metal and a ligand is a
reaction between a Lewis acid (the metal) and
Lewis base (the ligand).
• The new complex has distinct physical and chemical
properties.
– Ag+(aq) + e–  Ag(s)
E° = +0.799 V
– [Ag(CN)2]–(aq) + e–  Ag(s) + 2CN– (aq) E° = –0.31 V
Transition
Metals
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Coordination Numbers
• The coordination number of a
metal depends upon the size of
the metal and the size of the
ligands.
• CN=4 or 6
– CN=6 for Cr3+ and Co3+
– CN=4 for d8 metal ions such as
Pt2+ and Au3+.
• While iron(III) can bind to 6
fluorides, it can only
accommodate 4 of the larger
chlorides.
Transition
Metals
– [FeF6]3– vs. [FeCl4] –
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23.3 Common Ligands in Coordination
Chemistry
Monodentate
ligands
coordinate to
one site on
the metal,
bidentate to
two, and so
forth.
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Metals
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chelating agents
Bi and polydentate ligands are also called
chelating agents.
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Metals
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Chelate effect
• [Ni(H2O)6]2+(aq) + 6NH3  [Ni(NH3)6]2+(aq) + 6H2O(l)
Kf = 1.2  109
• [Ni(H2O)6]2+(aq) + 3en  [Ni(en)3]2+(aq) + 6H2O(l)
Kf = 6.8  1017
Cf) sequestering agents are used to selectively
remove toxic metal ions (e.g., Hg2+ and Pb2+) while
leaving biologically important metals.
.
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Chelates in Biological Systems
• There are many transition
metals that are vital to
human life.
Myoglobin
– V, Cr, Mn, Fe, Co, Cu, Zn,
Mo, Cd, and Ni
• Several of these are
bound to chelating agents.
– porphine
– porphyrin : a metal
complex derived from
porphine.
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Transition
Metals
Chelates in Biological Systems
• For instance, the iron in
hemoglobin carries O2
and CO2 through the
blood.
• CO and CN- are
poisonous because they
will bind more tightly to
the iron than will oxygen.
– Kf (-CO) > 210 Kf(-O2)
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Metals
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Chlorophylls
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
Photosynthesis: cholophyll-containing
pigments in the plant
leaves absorbs photons  glucose formation
•
•
•
•
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porphyrins that contain Mg(II)
Chlorophyll a
Mg2+ is in the center
conjugated double bonds
23.4 Nomenclature and Isomerism
in Coordination Chemistry
Nomenclature
1. In naming complexes that are salts, the
name of the cation is given before the name
of the anion: cation + anion
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Metals
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Nomenclature in Coordination Chemistry
2. In naming complex ions or molecules,
the ligands are named before the metal.
Ligands are listed in alphabetical order,
regardless of their charges. :
Ligand (alphabetical order)+ metal
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Nomenclature in Coordination Chemistry
3. The names of anionic ligands end in the
letter o, but electrically neutral ligands
ordinarily bear the name of the molecules.
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Metals
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Nomenclature in Coordination Chemistry
4. Greek prefixes (di-, tri-, tetra-, etc.) are
used to indicate the number of each kind
of ligand when more than one is present.
If the ligand contains a Greek prefix or
is polydentate, the prefixes bis-, tris-,
tetrakis-, etc. are used and the ligand
name is placed in parentheses.
–
[Co(en)3]Cl3 tris(ethylenediamine)cobalt(III)
chloride.
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Metals
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Nomenclature in Coordination Chemistry
5. If the complex is an anion, its name ends
in -ate.
6. The oxidation number of the metal is
given in parentheses in Roman numerals
following the name of the metal.
Transition
Metals
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Sample Exercise 23.4 Naming Coordination Compounds
Name the compounds (a) [Cr(H2O)4Cl2]Cl, (b) K4[Ni(CN)4].
Solution
+3 + 4(0) + 2(–1) + (–1) + 0
[Cr(H2O)4Cl2]Cl
tetraaquadichlorochromium(III)
chloride
4(+1) + 0 + 4(–1) = 0
K4[Ni(CN)4]
potassium tetracyanonickelate(0)
Practice Exercise
Name the compounds (a) [Mo(NH3)3Br3]NO3, (b) (NH4)2[CuBr4]. (c) Write the formula for
sodium diaquabis(oxalato)ruthenate(III).
Answer: (a) triamminetribromomolybdenum(IV) nitrate, (b) ammonium tetrabromocuprate(II)
(c) Na[Ru(H2O)2(C2O4)2]
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Sample Exercise 23.4 Naming Coordination Compounds
Continued
Practice Exercise
Name the compounds (a) [Mo(NH3)3Br3]NO3, (b) (NH4)2[CuBr4]. (c) Write the
formula for sodium diaquabis(oxalato)ruthenate(III).
Answer: (a) triamminetribromomolybdenum(IV) nitrate, (b) ammonium
tetrabromocuprate(II) (c) Na[Ru(H2O)2(C2O4)2]
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Isomers
• Isomers have the same molecular formula
but a different arrangement of atoms.
• There are two main subgroupings:
structural isomers and stereoisomers.
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Metals
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Linkage Isomers
In linkage isomers the ligand is bound to the
metal by a different atom.
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Metals
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Coordination Sphere Isomers
• Coordination sphere isomers differ
in what ligands are bound to the metal
and which fall outside the coordination
sphere.
• For example, CrCl3(H2O)6 exists as
– [Cr(H2O)6]Cl3,
– [Cr(H2O)5Cl]Cl2H2O, or
– [Cr(H2O)4Cl2]Cl2H2O.
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Metals
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Geometric Isomers
• In geometric isomers, the ligands have
a different spatial relationship.
• In the complexes above, the chlorines
can be adjacent to each other (cis) or
opposite each other (trans).
Transition
Metals
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Sample Exercise 23.5 Determining the Number of Geometric Isomers
The Lewis structure
indicates that the CO molecule has two lone pairs of electrons.
When CO binds to a transition-metal atom, it nearly always does so by using the C lone pair.
How many geometric isomers are there for tetracarbonyldichloroiron(II)?
Solution
two isomers possible: one with the Cl– ligands across the metal from each other,
trans-[Fe(CO)4Cl2], and one with the Cl– ligands adjacent to each other, cis[Fe(CO)4Cl2].
Practice Exercise
How many isomers exist for the square-planar molecule [Pt(NH3)2ClBr]?
Answer: two
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Optical Isomers
Optical isomers, or enantiomers, are
non-superimposable mirror images of
one another.
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Metals
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Detection of Optical Activity
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Sample Exercise 23.6 Predicting Whether a Complex Has Optical
Isomers
Does either cis-[Co(en)2Cl2]+ or trans-[Co(en)2Cl2]– have optical isomers?
Solution
The trans isomer of [Co(en)2Cl2]+ and its mirror image are:
The mirror image of the cis isomer cannot be superimposed on the original: Thus, the two cis
structures are optical isomers (enantiomers). We say that cis-[Co(en)2Cl2]+ is a chiral complex.
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Sample Exercise 23.6 Predicting Whether a Complex Has Optical
Isomers
Continued
Practice Exercise
Does the square-planar complex ion [Pt(NH3)(N3)ClBr]– have optical isomers?
Explain your answer.
Answer: no, because the complex is flat. This complex ion does, however,
have geometric isomers (for example, the Cl and Br ligands could be cis or
trans).
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Racemic mixtures
• Dextrorotatory solutions rotate the plane of
polarized light to the right.
– dextro or d isomer.
• Levorotatory solutions rotate the plane of
polarized light to the left.
– levo or l isomer.
• Racemic mixtures contain equal amounts of l
and d isomers.
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23.5 Color and Magnetism in Coordination
Chemistry
• The complex [Ti(H2O)6]3+
at the right appears
red-violet because those
are the wavelengths of
visible light not absorbed
by the complex.
• Many complexes are
pigmented because they
absorb in the visible part
of the spectrum.
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Metals
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Crystal-Field Theory
• As was mentioned earlier, ligands are
Lewis bases that are attracted to a Lewis
acid (the metal).
• But d electrons on the metal would repel
the ligand.
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Metals
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23.6 Crystal-Field Theory
• Therefore, the d orbitals on a metal in a
complex would not be degenerate.
• Those that point toward ligands would be
higher in energy than those that do not.
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Metals
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Crystal-Field Theory
The energy gap between d orbitals often
corresponds to the energy in a photon of
visible light.
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Metals
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Spectrochemical Series
The spectrochemical series ranks ligands in
order of their ability to increase the energy
gap between d orbitals.
• Weak-field ligands : low-∆ end
• Strong-field ligands : high-∆ end
• Example: [CrF6]3- [Cr(CN)6]3–
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Metals
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Crystal-Field Theory
The stronger the crystal-field strength of the
ligand, the larger the energy gap between
d orbitals, and the shorter the wavelength
of light absorbed by the complex.
Transition
Metals
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Sample Exercise 23.8 Using the Spectrochemical Series
[Ti(H2O)6]3+, [Ti(en)3]3+, and [TiCl6]3– all absorb visible light. Which one absorbs at the shortest
wavelength?
Solution
[Ti(en)3]3+.
Practice Exercise
The absorption spectrum of a Ti(III) complex containing the ligand L, [TiL6]3–, shows a peak
maximum at a wavelength intermediate between the wavelengths of the absorption
maxima for [TiCl6]3– and [TiF6]3–.What can we conclude about the place of L in the
spectrochemical series?
Answer: It lies between Cl– and F–.
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Electron Configurations in Octahedral
Complexes
∆ vs. spin-pairing energy
high-spin complexes
low-spin complexes
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high-spin complexes vs.
low-spin complexes
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Sample Exercise 23.9 Predicting the Number of Unpaired Electrons
in an Octahedral Complex
Predict the number of unpaired electrons in high-spin and low-spin Fe3+
complexes that have a coordination number of 6.
Solution
Fe3+ is a d5 ion. In a high-spin complex, all five electrons are unpaired, with
three in the t2 orbitals and two in the e orbitals. In a low-spin complex, all five
electrons reside in the t2 set, so there is one unpaired electron:
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Tetrahedral Complexes
• Because there are only four ligands, ∆Td <∆O
• This causes all tetrahedral complexes to be
high-spin.
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Crystal Field Splitting
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Square Planar Complexes
•
•
•
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Most d8 metal ions form square-planar complexes.
The majority of complexes are low-spin (i.e., diamagnetic).
Examples: Pd2+, Pt2+, Ir+, and Au3+.
Sample Exercise 23.10 Populating d Orbitals in Tetrahedral and
Square-Planar Complexes
Nickel(II) complexes in which the metal coordination number is 4 can have either squareplanar or
tetrahedral geometry. [NiCl4]2– is paramagnetic, and [Ni(CN)4]2– is diamagnetic. One of these
complexes is square planar, and the other is tetrahedral. Use the relevant crystalfield splitting diagrams
in the text to determine which complex has which geometry.
Solution
[NiCl4]2– is tetrahedral, and [Ni(CN)4]2– is square planar.
Practice Exercise
How many unpaired electrons do you predict for the tetrahedral [CoCl4]2– ion?
Answer: three
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Problems
• 4,12,20,28,32,38,40,56,60,70,72
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