Transcript Complexation Reactions

Chapter 23
Sections 2-4
Transition Metal
Complexes
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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”).
Transition
Metals
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A. Yes. Ammonia is the Lewis acid.
B. Yes. The metal cation is the Lewis acid.
C. No. Ammonia is a neutral molecule and cannot
can not accept electron density from a Lewis
base.
D. No. The metal cation cannot accept electron
density from a Lewis base.
Coordination
Werner
Theory
The Swiss
Alfred
Werner
One
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two
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have 3 chlorines,
and transition
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and the number.
other two are counterions.
Transition
Metals
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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?
(a) By analogy to the ammonia complexes of cobalt(III) shown in Figure 23.7,
we predict that the three NH3 are ligands attached to the Pd(II) ion. The fourth
ligand around Pd(II) is one chloride ion. The second chloride ion is not a
ligand; it serves only as a counterion (a noncoordinating ion that balances
charge) in the compound. We conclude that the formula showing the
structure best is [Pd(NH3)3Cl]Cl.
(b) Because only the non-ligand can react, we expect to produce 1 mol of AgCl(s) per mole of complex.
The balanced equation is
[Pd(NH3)3Cl]Cl(aq) + AgNO3(aq) → [Pd(NH3)3Cl]NO3(aq) + AgCl(s)
This is a metathesis reaction (Section 4.2) in which one of the cations is the [Pd(NH3)3Cl]+ complex ion.
Practice Exercise
Predict the number of ions produced per formula unit in an aqueous solution of CoCl2 · 6 H2O.
Answer: three: [Co(H2O)6]2+ and two Cl–
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.
Transition
Metals
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A.
B.
C.
D.
[Fe(H2O)4]3+(aq) + SCN–(aq)  [Fe(H2O)3NCS]2+(aq) + H2O(l)
[Fe(H2O)4]3+(aq) + S2+(aq) + CN–(aq)  [Fe(H2O)5NCS]2+(aq) + H2O(l)
Fe3+(aq) + 3SCN–(aq)  FeCN3 (aq) + 3S–(aq)
[Fe(H2O)6]3+ (aq) + SCN–(aq)  [Fe(H2O)5NCS]2+(aq) + H2O(l)
Coordination Numbers
• The number of atoms directly bonded to the
metal atom in a complex is the coordination
number.
Compound
Coordination number
[Ag(NH3)2]+
2
[Co(NH3)6]Cl3
6
[Co(NH3)5Cl]Cl2
6
trans -[Co(NH3)4Cl2]Cl
6
cis -[Co(NH3)5Cl2]Cl
6
Transition
Metals
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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?
Solve The NO3 group is the nitrate anion, which has a 1– charge. The NH3 ligands carry zero charge,
and the Cl is a coordinated chloride ion, which has a 1– charge. The sum of all the charges must be
zero:
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
Sample Exercise 23.3 Determining the Formula of a Complex Ion
A complex ion contains a chromium(III) bound to four water molecules and to two chloride ions.What is
the formula and charge of this ion?
+3 + 4(0) + 2(–1) = +1
Cr(H2O)4Cl2
The charge on the ion is 1+, [Cr(H2O)4Cl2]+.
Practice Exercise
Write the formula for the complex described in the Practice Exercise accompanying Sample Exercise
23.2.
Answer: [Pt(NH3)2Br2]
Coordination Numbers
• The coordination
number of a metal
depends upon the size
of the metal and the
size of the ligands.
• While iron(III) can bind
to 6 fluorides, it can
only accommodate 4 of
the larger chlorides.
Transition
Metals
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A.
B.
C.
D.
NH3—Zn—NH3
NH3—Pt—NH3
109.5°
120°
180°
120°
90°
109.5°
120°
90°
Geometries:
A.
B.
C.
D.
C.N. = 4
Octahedral
Tetrahedral
Octahedral
Square planar
C.N. = 6
Tetrahedral
Octahedral
Square planar
Tetrahedral
A.
B.
C.
D.
Two for both
Three for both
Four for both
Six for both
A. Bidentate ligand
B. Monodentate ligand
Common Ligands
The table above contains some ligands
commonly found in complexes.
Monodentate ligands coordinate to one site
on the metal, bidentate to two, and so forth. Transition
Metals
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Common Ligands
Bi and polydentate ligands are also called
chelating agents.
Transition
Metals
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Chelates in Biological Systems
• There are many
transition metals that
are vital to human life.
• Several of these are
bound to chelating
agents.
Transition
Metals
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Chelates in Biological Systems
• For instance, the
iron in hemoglobin
carries O2 and CO2
through the blood.
Transition
Metals
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A.
B.
C.
D.
On the protein group
On the heme group
On the NH group
Where O2 binds
Chelates in Biological Systems
• For instance, the
iron in hemoglobin
carries O2 and CO2
through the blood.
• Carbon monoxide
and cyanide are
poisonous because
they will bind more
tightly to the iron
than will oxygen.
Transition
Metals
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A.
B.
C.
D.
420 nm
500 nm
610 nm
650 nm
A. It has a planar structure that enables the C-H
bonds to absorb light in the visible region.
B. The C-C bonds absorb light in the ultraviolet
region.
C. It has conjugated double bonds that absorb light
in the visible region.
D. It has C-H bonds that absorb light in the infrared
region.
Nomenclature in Coordination
Chemistry
1. In naming complexes that are salts, the
name of the cation is given before the name
of the anion.
Transition
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.
Transition
Metals
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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.
Transition
Metals
© 2012 Pearson Education, Inc.
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.
Transition
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].
Solve
(a) The ligands are four water molecules—
tetraaqua—and two chloride ions—dichloro. By
assigning all the oxidation numbers we know for
this molecule, we see that the oxidation number
of Cr is +3:
Thus, we have chromium(III). Finally, the anion is
chloride. The name of the compound is
(b) The complex has four cyanide ion ligands,
CN–, which means tetracyano, and the oxidation
state of the nickel is zero:
Because the complex is an anion, the metal is
indicated as nickelate(0). Putting these parts
together and naming the cation first, we have
+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]