1. Coordination Chemistry
Third Edition
Julia Burdge
Lecture PowerPoints
Chapter 22
Coordination Chemistry
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2. 22 Coordination Chemistry 22.1 Coordination Compounds 22.2
Structure of Coordination Compounds
22.3
Bonding in Coordination Compounds: Crystal Field Theory
22.4
Reactions of Coordination Compounds
22.5
Applications of Coordination Compounds
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3. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
Ligands
Nomenclature of Coordination Compounds
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4. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
Coordination compounds contain coordinate covalent bonds formed by the reactions of metal ions with groups of anions or polar molecules.
A coordinate covalent bond is a covalent bond in which one of the atoms donates both of the electrons that constitute the bond.
A coordination compound often consists of a complex ion and one or more counter ions.
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5. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
Use square brackets to separate the complex ion from the counter ion.
Most of the metals in coordination compounds are transition metals.
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6. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
Transition metals (green) have incompletely filled d subshells or form ions with incompletely filled d subshells.
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7. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
The Group 2B metals—Zn, Cd, and Hg — are d-block metals, but they are not transition metals.
Incompletely filled d subshells give rise to several properties:
distinctive colors
formation of paramagnetic compounds
catalytic activity
tendency to form complex ions
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8. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
The most common transition metals are Sc through Cu.
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9. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
Transition metals have
higher densities
higher melting points and boiling points
higher heats of fusion and vaporization
than the main group and Group 2B metals
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10. Coordination Compounds
22.1
Coordination Compounds
Properties of Transition Metals
Transition metals exhibit variable oxidation states.
All these metals can exhibit the oxidation state +3 and nearly all can exhibit the oxidation state +2.
The highest oxidation state for a transition metal is +7.
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11. Coordination Compounds
22.1
Coordination Compounds
Ligands
The molecules or ions that surround the metal in a complex ion are called ligands.
To be a ligand, a molecule or ion must have at least one unshared pair of valence electrons.
Examples include:
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12. Coordination Compounds
22.1
Coordination Compounds
Ligands
The ligand acts as a Lewis base while the transition metal acts as a Lewis acid.
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13. Coordination Compounds
22.1
Coordination Compounds
Ligands
The atom in a ligand that is bound directly to the metal atom is known as the donor atom.
Nitrogen is the donor atom in the [Cu(NH3)4]2+ complex ion.
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14. Coordination Compounds
22.1
Coordination Compounds
Ligands
The coordination number in a coordination compound refers to the number of donor atoms surrounding the central metal atom in a complex ion.
The coordination number of Cu2+ in [Cu(NH3)4]2+ is 4.
The most common coordination numbers are 4 and 6.
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15. Coordination Compounds
22.1
Coordination Compounds
Ligands
Depending on the number of donor atoms a ligand possesses, it is classified as
monodentate (1 donor atom)
bidentate (2 donor atoms)
polydentate (> 2 donor atoms)
Ethylenediamine forms two bonds to a metal atom (bidendate).
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16. Coordination Compounds
22.1
Coordination Compounds
Ligands
Bidentate and polydentate ligands are also called chelating agents because of their ability to hold the metal atom like a claw.
EDTA is a polydentate ligand ‒ 6 donor atoms.
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17. Coordination Compounds
22.1
Coordination Compounds
Ligands
The oxidation state of a transition metal in a complex ion is determined using the known charges of the ligands and the known overall charge of the complex ion.
In the complex ion [PtCl6]2‒, each chloride ion ligand has an oxidation number of ‒1.
For the overall charge of the ion to be ‒2, the Pt must have an oxidation number of +4.
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18. 22.1
Determine the oxidation state of the central metal atom in each of the following compounds:
[Ru(NH3)5(H2O)]Cl2
[Cr(NH3)6](NO3)3
Fe(CO)5
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19. The complex ion is [Ru(NH3)5(H2O)]2+. Each ligand is neutral.
22.1
Setup
The complex ion is [Ru(NH3)5(H2O)]2+. Each ligand is neutral.
The complex ion is [Cr(NH3)6]3+. Each ligand is neutral.
Fe(CO)5 does not contain a complex ion. The ligands are CO molecules, which have a zero charge.
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20. 22.1 Solution [Ru(NH3)5(H2O)]Cl2 +2 [Cr(NH3)6](NO3)3 +3 Fe(CO)5 0 20
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21. Coordination Compounds
22.1
Coordination Compounds
Nomenclature of Coordination Compounds
The rules for naming ionic coordination compounds are as follows:
The cation is named before the anion. The rule holds regardless of whether the complex ion bears a net positive or a net negative charge.
Within a complex ion, the ligands are named first, in alphabetical order, and the metal ion is named last.
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22. Coordination Compounds
22.1
Coordination Compounds
Nomenclature of Coordination Compounds
The names of anionic ligands end with the letter o, whereas neutral ligands are usually called by the names of the molecules. The exceptions are H2O (aqua), CO (carbonyl), and NH3 (ammine).
When two or more of the same ligand are present, use Greek prefixes di, tri, tetra, penta, and hexa, to specify their number.
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23. Coordination Compounds
22.1
Coordination Compounds
Nomenclature of Coordination Compounds
The oxidation number of the metal is indicated in Roman numerals immediately following the name of the metal.
If the complex is an anion, its name ends in –ate.
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24. Coordination Compounds
22.1
Coordination Compounds
Nomenclature of Coordination Compounds
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25. Coordination Compounds
22.1
Coordination Compounds
Nomenclature of Coordination Compounds
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26. Write the names of the following coordination compounds:
22.2
Write the names of the following coordination compounds:
[Co(NH3)4Cl2]Cl
K3[Fe(CN)6]
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27. [Co(NH3)4Cl2]Cl Tetraamminedichlorocobalt(III) chloride
22.2
Setup
The cation is a complex ion with a charge of +1. The counter ion is Cl‒. The oxidation state of cobalt +3.
The cation is K+, and the anion is a complex ion with a charge of ‒3. The oxidation state of iron +3.
Solution
[Co(NH3)4Cl2]Cl Tetraamminedichlorocobalt(III) chloride
K3[Fe(CN)6] Potassium hexacyanoferrate(III)
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28. Write formulas for the following compounds:
22.3
Write formulas for the following compounds:
pentaamminechlorocobalt(III) chloride
(b) dichlorobis(ethylenediamine)platinum(IV) nitrate
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29. 22.3
Setup
There are five NH3 molecules and one Cl‒ ion. The overall charge on the complex ion +2. There are two chloride ions as counter ions.
There are two bidentate ethylenediamines and two Cl‒ ions. The overall charge on the complex ion +2. There are two nitrate ions as counter ions.
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30. [Co(NH3)5Cl]Cl2 (pentaamminechlorocobalt(III) chloride)
22.3
Solution
[Co(NH3)5Cl]Cl2 (pentaamminechlorocobalt(III) chloride)
[Pt(en)2Cl2](NO3)2 (dichlorobis(ethylenediamine)platinum(IV) nitrate)
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31. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
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32. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
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33. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
Compounds in which ligands are arranged differently around the central atom are known as stereoisomers.
Stereoisomers have distinctly different physical and chemical properties.
Coordination compounds may exhibit two types of stereoisomerism: geometric and optical.
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34. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
Geometrical isomers are stereoisomers that cannot be interconverted without breaking chemical bonds.
Geometric isomers come in pairs: cis and trans.
Cis means that two particular atoms are adjacent to each other. Trans means that the atoms are on opposite sides in the structural formula.
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35. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
The cis and trans isomers generally have different colors, melting points, dipole moments, and chemical reactivities.
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36. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
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37. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
Optical isomers are nonsuperimposable mirror images.
Optical isomers also come in pairs.
The optical isomers of a compound have identical physical and chemical properties.
Optical isomers differ from each other in their interactions with plane-polarized light.
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38. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
The structural relationship between two optical isomers is analogous to the relationship between your left and right hands.
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39. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
The cis and trans isomers of dichlorobis(ethylenediamine)cobalt(III) ion and the mirror image of each.
The cis isomer and its mirror image are optical isomers.
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40. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
Optical isomers are described as chiral.
Chiral molecules are nonsuperimposable.
Isomers that are superimposable with their mirror images are said to be achiral.
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41. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
Chiral molecules are optically active because of their ability to rotate polarized light as it passes through them.
Unlike ordinary light, which vibrates in all directions, plane-polarized light vibrates only in a single plane.
We use a polarimeter to measure the rotation of polarized light by optical isomers.
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42. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
If the plane of polarization is rotated to the right, the isomer is dextrorotatory and is labeled d.
If the rotation is to the left, the isomer is levorotatory and is labeled l.
The d and l isomers of a chiral substance, called enantiomers, always rotate the plane of polarization by the same amount, but in opposite directions.
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43. Structure of Coordination Compounds
22.2
Structure of Coordination Compounds
Structure of Coordination Compounds
In an equimolar mixture of two enantiomers, called a racemic mixture, the net rotation is zero.
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44. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
Color
Magnetic Properties
Tetrahedral and Square Planar Complexes
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45. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
Crystal field theory accounts for the color and magnetic properties of many coordination compounds.
In a complex ion, two types of electrostatic interaction come into play.
the attraction between the positive metal ion and the negatively charged ligand
the electrostatic repulsion between the lone pairs on the ligands and the electrons in the d orbitals of the metals
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46. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
The d orbitals have different orientations but the same energy, in the absence of an external disturbance.
In an octahedral complex, a central metal atom is surrounded by six lone pairs of electrons, so all five d orbitals experience electrostatic repulsion.
The magnitude of this repulsion depends on the orientation of the d orbital that is involved. .
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47. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
The lobes of the dx2 ‒ y2 orbital point toward the corners of the octahedron along the x and y axes, where the lone-pair electrons are positioned.
An electron residing in this orbital would experience a greater repulsion from the ligands than an electron would in the dxy, dyz, or dxz orbitals.
The energy of the dx2 ‒ y2 orbital is increased relative to the dxy, dyz, or dxz orbitals.
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48. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
The dz2 orbital’s energy is also greater, because its lobes are pointed at the ligands along the z axis.
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49. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
The five d orbitals in an octahedral complex are split between two sets of energy levels.
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50. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Crystal Field Splitting in Octahedral Complexes
The crystal field splitting (Δ) is the energy difference between two sets of d orbitals in a metal atom when ligands are present.
The magnitude of Δ depends on the metal and the nature of the ligands.
It has a direct effect on the color and magnetic properties of complex ions.
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51. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Color
White light is a combination of all colors.
A substance appears black if it absorbs all the visible light that strikes it.
An object appears green if it absorbs all light but reflects the green component.
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52. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Color
An object also looks green if it reflects all colors except red, the complementary color of green.
The best way to measure crystal field splitting is to use spectroscopy to determine the wavelength at which light is absorbed.
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53. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Color
The [Ti(H2O)6]3+ ion absorbs light in the visible region of the spectrum with a
maximum absorption at 498 nm.
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54. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Color
To calculate the crystal field splitting energy, recall
Therefore,
This is the energy required to excite one [Ti(H2O)6]3+ ion.
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55. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Color
To express this energy difference in units of kJ/mol, we write
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56. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Color
Chemists have calculated the crystal field splitting for each ligand and established the following spectrochemical series,
These ligands are arranged in the order of increasing Δ.
CO and CN‒ are called strong-field ligands, because they cause a large splitting of the d orbital energy levels.
The halide ions and hydroxide ion are weak-field ligands, because they split the d orbitals to a lesser extent.
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57. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Magnetic Properties
The magnitude of the crystal field splitting also determines the magnetic properties of a complex ion.
The configuration of Fe3+ is [Ar]3d5, and there are two possible ways to distribute the five d electrons among the d orbitals.
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58. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Magnetic Properties
The actual arrangement of the electrons is determined by the amount of stability gained by having maximum parallel spins versus the investment in energy required to promote electrons to higher d orbitals.
Because F‒ is a weak-field ligand, the five d electrons enter five separate d orbitals with parallel spins to create a high-spin complex.
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59. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Magnetic Properties
The CN‒ is a strong-field ligand, so it is energetically preferable for all five electrons to be in the lower orbitals, thus forming a low-spin complex.
High-spin complexes are more paramagnetic than low-spin complexes
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60. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Magnetic Properties
A distinction between low- and high-spin complexes can be made only if the metal ion contains > 3 and < 8 d electrons
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61. Predict the number of unpaired spins in the [Cr(en)3]2+ ion.
22.4
Predict the number of unpaired spins in the [Cr(en)3]2+ ion.
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62. 22.4
Setup
The electron configuration of Cr2+ is [Ar]3d4; and en is a strong-field ligand.
Solution
Because en is a strong-field ligand, we expect [Cr(en)3]2+ to be a low-spin complex.
All four electrons will be placed in the lower-energy d orbitals and there will be a total of two unpaired spins.
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63. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Tetrahedral and Square Planar Complexes
The splitting pattern for a tetrahedral ion is the reverse of that for octahedral complexes.
Most tetrahedral complexes are high-spin complexes.
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64. Bonding in Coordination Compounds: Crystal Field Theory
22.3
Bonding in Coordination Compounds: Crystal Field Theory
Tetrahedral and Square Planar Complexes
The splitting pattern for square-planar complexes is the most complicated.
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65. Reactions of Coordination Compounds
22.4
Reactions of Coordination Compounds
Reactions of Coordination Compounds
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66. Reactions of Coordination Compounds
22.4
Reactions of Coordination Compounds
Reactions of Coordination Compounds
Complex ions undergo ligand exchange (or substitution) reactions in solution.
It is useful to distinguish between the stability of a complex ion and its tendency to react, which we call kinetic lability.
Stability in this context is a thermodynamic property, which is measured in terms of the species’ formation constant Kf .
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67. Reactions of Coordination Compounds
22.4
Reactions of Coordination Compounds
Reactions of Coordination Compounds
For example, the complex ion tetracyanonickelate(II) is stable because it has a large formation constant (Kf = 1 × 1030):
Complexes like the tetracyanonickelate(II) ion are termed labile complexes because they undergo rapid ligand exchange reactions.
Thus, a thermodynamically stable species (i.e., one that has a large formation constant) is not necessarily unreactive.
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68. Reactions of Coordination Compounds
22.4
Reactions of Coordination Compounds
Reactions of Coordination Compounds
A complex that is thermodynamically unstable in acidic solution is [Co(NH3)6]3+.
This is an example of an inert complex—a complex ion that undergoes very slow exchange reactions.
It shows that a thermodynamically unstable species is not necessarily chemically reactive.
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69. Reactions of Coordination Compounds
22.4
Reactions of Coordination Compounds
Reactions of Coordination Compounds
The rate of reaction is determined by the energy of activation.
Most complex ions containing Co3+, Cr3+, and Pt2+ are kinetically inert.
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70. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Applications of Coordination Compounds
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71. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Reactions of Coordination Compounds
Metallurgy
Examples of the use of coordination compounds in metallurgical processes:
extraction of silver and gold by the formation of cyanide complexes
purification of nickel
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72. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Reactions of Coordination Compounds
Chelation Therapy
Chelation therapy is used in the treatment of lead poisoning.
Arsenic and mercury can also be removed using chelating agents.
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73. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Reactions of Coordination Compounds
Chemotherapy
Several platinum-containing coordination compounds, including cisplatin [Pt(NH3)2Cl2] and carboplatin [Pt(NH3)2(OCO)2C4H6], can effectively inhibit the growth of cancerous cells.
The mechanism for the action of cisplatin is the chelation of DNA. This causes a bend in the double-stranded structure which inhibits replication. The damaged cell is then destroyed by the body’s immune system.
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74. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Reactions of Coordination Compounds
Chemical Analysis
Dimethylglyoxime forms an insoluble brick-red solid with Ni2+ and an insoluble bright-yellow solid with Pd2+.
These characteristic colors are used in qualitative analysis to identify nickel and palladium.
The quantities of ions present can be determined by gravimetric analysis.
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75. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Reactions of Coordination Compounds
Detergents
The cleansing action of soap in hard water is hampered by the reaction of the Ca2+ ions in the water with the soap molecules to form insoluble salts or curds.
The detergent industry used tripolyphosphate ion as a chelating agent to form stable complexes with Ca2+ ions .
However, wastewater containing phosphates discharged into rivers and lakes causes algae to grow, resulting in oxygen depletion.
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76. Applications of Coordination Compounds
22.5
Applications of Coordination Compounds
Reactions of Coordination Compounds
Sequestrants
EDTA is used as a food additive to sequester metal ions.
EDTA sequesters Cu, Fe, and Ni ions that would otherwise catalyze the oxidation reactions that cause food to spoil.
EDTA is a common preservative in a wide variety of consumer products.
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