Coordination Complexes and Transition Metals in Action

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Coordination Complexes and Transition Metals in
Action
Al2O3 crystal with traces of Cr3 (ruby)
Spring and summer chlorophyll and xanthophyll
Fall xanthophyll colors dominate
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Plants and Animals
chlorophyll
heme
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Colors of Chromium
Cr(NO3)3
CrCl3
K2CrO4
K2Cr2O7
Cr3
Cr6
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Coordination Compound and Complex
Coordination Compound is Co(NH3)6Cl3

• Co(NH3)6Cl3 ? Co(NH3)63 3 Cl-

Coordination Complex is Co(NH3)63
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Components of Complex (Coordination Sphere)
Co(NH3)63

• Metal ion usually transition metals with empty
  valence orbitals
• Specifically empty d orbitals
• Act as Lewis acid (electron pair acceptor)

• Ligand complexing agent bound to (surrounding)
  the metal ion (Lewis base)
• Normally ligands are anions or polar molecules
• Anion (CN-)
• Polar molecule (NH3)
• Donor atom

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Characteristics of Complex (Coordination Sphere)

• Metal ion
• Oxidation number
• Co ?

• Ligand
• Charge on ligand
• NH3 ?

Charge of Complex sum of charges on the central
metal ion and the surrounding ligands
What is the charge of the complex?
Co(NH3)6Cl3
Coordination number The number of donor atoms
attached to the metal.
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Example Problem

• Indicate the coordination number of the metal and
  the oxidation number of the metal in each of the
  following complexes
• Na2CdCl4 Co(NH3)4Cl2Cl
• K2MoOCl4 Zn(en)2Br2

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Types of Ligands

• Monodentate ligand NH3, H2O, Cl-
• Bidentate ligand ethylenediamine (en)
• Polydentate ligand ethylenediaminetetraacetate
  ion (EDTA)4-

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Chelating Agent

• Polydentate ligands (including bidentate) are
  called chelating agents because they appear to
  grasp the metal between donor atoms

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Example Ligands
What kind of ligands are these examples?
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Chelate Effect

• Chelating agents form more stable complexes with
  metal ions than monodentate ligands
• Ni2 (aq) 6NH3 (aq) ? Ni(NH3)62 (aq) Kf 4
  x 108
• Ni2 (aq) 3 en (aq) ? Ni(en)32 (aq) Kf
  2 x 1018
• Sequestering agents because the chelating
  agents can be used to remove or separate ions
• removal of ions from hard water
• removal of trace metals from food
• removal of heavy metal ions from blood

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Geometry of Complex

• Common coordination of 4

square planar Transition metal ions with 8 d
electrons
Tetrahedral Most common
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Geometry of Complex

• Coordination 6

Octahedral
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Effect of Ligand on Coordination Number

• The larger the size of the ligand, the fewer
  ligands that can get close enough to bind to the
  central metal.
• FeF63- FeCl4-
• Ligands which provide negative charge to the
  complex reduce the coordination number.
• Ni(NH3)62 Ni(CN)42-

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Metal Complexes

• Distinct chemical properties different from the
  metals and ligands from which they were formed.
• Different colors
• Different electrochemical properties
• Different solubility properties

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Nomenclature

• In naming salts, the name of the cation is given
  before the name of the anion.
• Mo(NH3)3Br3NO3

Cation
Anion nitrate
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Nomenclature

• Within a complex ion or molecule the ligands are
  named before the metal. Ligands are listed in
  alphabetical order, regardless of charge on the
  ligand. Prefixes that give the number of ligands
  are not considered part of the ligand name in
  determining alphabetical order.
• Mo(NH3)3Br3NO3

Ammonia, bromide, molybdenum
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Nomenclature

• The names of the anionic ligands end in the
  letter o, whereas neutral ones ordinarily bear
  the name of the molecules.
• Mo(NH3)3Br3NO3

Example ligand names NH3 ammine CO carbonyl
NO - nitrosyl H2O aqua CN- cyano en -
ethylenediammine
Ammine, bromo, molybdenum
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Nomenclature

• Greek prefixes (di, tri, tetra, penta, hexa) are
  used to indicate the number of each kind of
  ligand when more than one is present.
• Mo(NH3)3Br3NO3
• If the ligand itself contains a prefix, then
  these prefixes are used for the ligand name
  (bis-, tris-, tetrakis-, pentakis-, etc.).
• Ru(bipy)3Cl3

triamminetribromomolybdenum
tris-bipyridineruthenium
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Nomenclature

• If the complex is an anion, its name ends in
  ate.
• K3V(C2O4)3

cation potassium
anion trioxalatevanadate
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Nomenclature

• The oxidation number of the metal is given in
  parentheses in Roman numerals following the name
  of the metal.
• Mo(NH3)3Br3NO3
• Ru(bipy)3Cl3
• K3V(C2O4)3

triamminetribromomolybdenum(IV) nitrate
tris-bipyridineruthenium(III) chloride
Potassium trioxalatevanadate(III)
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Isomers
Hydrate isomer
Ionization isomer
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Structural Isomerism have the same numbers and
kinds of atoms, but differ in the bonds that are
present.

• Ionization isomer exchange of ligand with an
  anion or neutral molecule
  CoBr(NH3)5SO4 and CoSO4(NH3)5 Br
• Hydrate isomer the exchange of H2O molecule
  with another ligand CrBr(H2O)6Cl3 and
  CrClBr(H2O)5Cl2H2O
• Coordination isomers- differ due to the exchange
  of one or more ligands between a cationic complex
  and an anionic complex
  Cr(NH3)6Fe(CN)6
• Fe(NH3)6Cr(CN)6
• Linkage isomers- contain the same ligand
  coordinated to the metal through different donor
  atomsPd(NH3)3SCN and Pd(NH3)3NCS

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Linkage Isomerism
Nitro
Nitrito
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Stereoisomerism

• Geometric isomers have the same number and kinds
  of bonds, but differ in the relative positions of
  the ligands.
• Optical isomers rotate the plane of polarized
  light in opposite directions.

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Geometric Isomers
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Geometric Isomers
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Optical Isomers

• Rotate the plane of polarized light in opposite
  directions.

Levrorotatory rotate left
Dextrorotatory rotate right
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Optical Isomers

• Chiral molecules have mirror-image structures
  that cannot be superimposed.
• Only chiral molecules are optically active.
• Enantiomers are chiral molecules of each other.

Racemic Mixture occurs when equal amounts of each
enantiomer are mixed. When this happens, the
optical activity of each is canceled by the other.
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Enantiomers
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Properties of Coordination Complexes

• Color Many coordination complexes exhibit a
  wide variety of colors, that depend on the metal,
  its oxidation state, and the ligands present.
• The observed colors result from the absorption of
  light in the visible region by the complexes.

Color Exhibited
Colorless
Partially filled d orbital
Totally filled or empty d orbital (d0 and d10)
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Color Exhibited or Colorless

• Color Exhibited
• Cr(NH3)63
• Fe(SO4)(H2O)4

• Colorless
• Cd(NH3)4(NO3)2
• NaAlCl4

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Color of Complexes
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Crystal Field Theory

• Crystal field theory assumes electrostatic
  interactions between the negative or neutral
  ligands and the positive metal ion lower the
  energy of the system.
• Anionic ligands electrostatic attraction
• Neutral ligands ion dipole

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Lowered Energy of Metal/Ligand Complex
Crystal Field repulsive interaction between
electrons in the ligands and the d orbital
electrons in the metal
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Consequenced electron repulsion

• Crystal Field
• The negative ligands repel the electrons in the
  metal ion d orbitals.
• The repulsion energy of d electrons depends on
  the orientation of the orbital, relative to the
  location of the negative ligands.

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d Orbitals
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d Orbitals
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Octahedral complex
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d orbital splitting
D Energy gap the energy necessary for an
electron to move across the gap is similar to
energy of a visible light photon
Explains why d0 or d10 transition elements do not
show color
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Spectrochemical Series
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Electron Configurations
Co3 a d6 ion
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Electron Configurations

• High spin
• Low spin

Weak Field ligand
Spin pairing energy is the energy required to
pair 2 electrons in orbital
Strong Field ligand
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Properties of Coordination Complexes

• Paramagnetism - a property due to unpaired
  electrons, is common among transition metal
  complexes.
• Different complexes of the same metal ion, may
  have different numbers of unpaired electrons.
• Predict the magnetic properties of
• Fe(H2O)62 Fe(CN)64-

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Energy Calculation

• The complex Ti(H2O)63 absorbs light of
  wavelength 510 nm. What is the crystal field d
  orbital splitting energy (D) for the complex?

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Metal and Oxidation

• Color Many coordination complexes exhibit a
  wide variety of colors, that depend on the metal,
  its oxidation state, and the ligands present.
• Cr(H2O)63 V(H2O)62

The larger the charge on the metal ion involved
in the complex, the more metal-ligand
interaction. Therefore, D will be larger when
the oxidation state of the metal is larger.
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Increased Interaction Between Metal and Ligand
Increased Crystal Field
Crystal Field repulsive interaction between
electrons in the ligands and the d orbital
electrons in the metal
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Crystal Field Theory Tetrahedral
Tetrahedral complex always have high spin because
D is small
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Crystal Field Theory Square Planar
Square planar complexes always have high spin
because D is large
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Tetrahedral and Square Planar

• Draw the crystal field splitting diagrams for
  Ni(CN)42- and NiCl42- and predict the
  magnetic properties of each.

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Electron Configurations

• High spin
• Low spin

Weak Field ligand
Spin pairing energy is the energy required to
pair 2 electrons in orbital
Strong Field ligand
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Energy Calculation

• The complex Ti(H2O)63 absorbs light of
  wavelength 510 nm. What is the crystal field d
  orbital splitting energy (D) for the complex?
• c ln E hn
• c/l E (6.63 x 10-34 Js) (5.88 x
  1014 s-1)
• 3.00 x 108 m/s______ E 3.90 x 10-19 J
• (510 nm) ___1 m___ E (3.90 x 10-19
  J) _1 kJ_
• 1 x 109 nm 1000 J
• 5.88 x 1014 s-1 E 3.90 x 10-22 kJ/photon
• E (3.90 x 10-22 __kJ__) (6.022 x 1023 photons)
  235 _kJ_
• photon mole
  mole

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Spectroscopy
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Sources of Electromagnetic Radiation
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Window Material
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Wavelength Selector
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Detectors