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Chapter 21

Transition Metals and Coordination Chemistry

Shaun Williams, PhD

Transition Metals

The Postion of the Transition Elements on the Periodic Table

The transition elements are located between the main group elements (between columns 2 and 13) as well as the inner transition elements located at the bottom of the periodic table.

Forming Ionic Compounds

The Complex Ion \(\chem{Co(NH_3)_6^{3+}}\)

The arrangement of the octahedral complex ion formed by a central cobalt atom bound to six octahedrally arranged ammonia atoms.

Ionic Compounds with Transition Metals

Electron Configurations

Concept Check

What is the expected electron configuration of \(\chem{Sc^+}\)?

\(\chem{[Ar]3d^2}\)

Plots of the First (Red Dots) and Third (Red Dots) Ionization Energies for the First-Row Transition Metals

The first ionization enegeries of the first-row transition metals risely slightly left to right across the row (from about 6 to about 9 electron volts per atom). The third ionization energy of the same atoms shows a much more pronounced increase from left to right across the row (from about 24 to about 39 electron volts per atom).

Atomic Radii of the \(3d\), \(4d\), and \(5d\) Transition Series

The first row transition elements (\(3d\)) has smaller radii that the second and third row elements. The second and third row elements have quite similar radii even though they are different rows.

The First-Row Transition Metals

Oxidation States and Species for Vanadium in Aqueous Solution

Oxidation Stae of Vanadium Species in Aqueous Solution
+5 \(\chem{VO_2^+}\) (yellow)
+4 \(\chem{VO^{2+}}\) (blue)
+3 \(\chem{V^{3+}(aq)}\) (blue-green)
+2 \(\chem{V^{2+}(aq)}\) (violet)

Typical Chromium Compounds

Oxidation State of Chromium Examples of Compound (\(\chem{X}\) is a halogen)
+2 \(\chem{CrX_2}\)
+3 \(\chem{CrX_3}\) (green)
\(\chem{Cr_2O_3}\) (blue-green)
+6 \(\chem{K_2Cr_2O_7}\) (orange)
\(\chem{Na_2CrO_4}\) (yellow)
\(\chem{CrO_3}\) (red)

Some Compounds of Manganese in Its Most Common Oxidation States

Oxidation State of Manganese Examples of Compound
+2 \(\chem{Mn(OH)_2}\) (pink)
\(\chem{MnS}\) (salmon)
\(\chem{MnSO_4}\) (reddish)
\(\chem{MnCl_2}\) (pink)
+4 \(\chem{MnO_4}\) (dark brown)
+7 \(\chem{KMnO_4}\) (purple)

Typical Iron Compounds

Oxidation State of Iron Examples of Compounds
+2 \(\chem{FeO}\) (black)
\(\chem{FeS}\) (brownish black)
\(\chem{FeSO_4\cdot 7H_2O}\) (green)
\(\chem{K_4Fe(CN)_6}\) (yellow)
+3 \(\chem{FeCl_3}\) (brownish black)
\(\chem{Fe_2O_3}\) (reddish brown)
\(\chem{K_3Fe(CN)_6}\) (red)
\(\chem{Fe(SCN)_3}\) (red)
+2, +3 (mixture) \(\chem{Fe_3O_4}\) (black)
\(\chem{KFe[Fe(CN)_6]}\) (deep blue, "Prussian blue")

Typical Cobalt Compounds

Oxidation State of Cobalt Examples of Compounds
+2 \(\chem{CoSO_4}\) (dark blue)
\(\chem{[Co(H_2O)_6]Cl_2}\) (pink)
\(\chem{[Co(H_2O)_6](NO_3)_2}\) (red)
\(\chem{CoS}\) (black)
\(\chem{CoO}\) (greenish brown)
+3 \(\chem{CoF_3}\) (brown)
\(\chem{Co_2O_3}\) (charcoal)
\(\chem{K_3[Co(CN)_6]}\) (yellow)
\(\chem{[Co(NH_3)_6]Cl_3}\) (yellow)

Typical Nickel Compounds

Oxidation State of Nickel Examples of Compounds
+2 \(\chem{NiCl_2}\) (yellow)
\(\chem{[Ni(H_2O)_6]Cl_2}\) (green)
\(\chem{NiO}\) (greenish black)
\(\chem{NiS}\) (black)
\(\chem{[Ni(H_2O)_6]SO_4}\) (green)
\(\chem{[Ni(NH_3)_6](NO_3)_2}\) (blue)

Typical Copper Compounds

Oxidation State of Copper Examples of Compounds
+1 \(\chem{Cu_2O}\) (red)
\(\chem{Cu_2S}\) (black)
\(\chem{CuCl}\) (white)
+2 \(\chem{CuO}\) (black)
\(\chem{CuSO_4\cdot 5H_2O}\) (blue)
\(\chem{CuCl_2\cdot 2H_2O}\) (green)
\(\chem{[Cu(H_2O)_6](NO_3)_2}\) (blue)

Alloys Containing Copper

Alloy Composition (% by mass)
Brass \(\chem{Cu}\) (20-97), \(\chem{Zn}\) (2-80), \(\chem{Sn}\) (0-14), \(\chem{Pb}\) (0-12), \(\chem{Mn}\) (0-25)
Bronze \(\chem{Cu}\) (50-98), \(\chem{Sn}\) (0-35), \(\chem{Zn}\) (0-29), \(\chem{Pb}\) (0-50), \(\chem{P}\) (0-3)
Sterling silver \(\chem{Cu}\) (7.5), \(\chem{Ag}\) (92.5)
Gold (18-karat) \(\chem{Cu}\) (5-15), \(\chem{Au}\) (75), \(\chem{Ag}\) (10-20)
Gold (14-karat) \(\chem{Cu}\) (12-28), \(\chem{Au}\) (58), \(\chem{Ag}\) (4-30)

A Coordination Compound

Ligands

The Bidentate Ligand Ethylenediamine and the Monodentate Ligand Ammonia

The size of the ethylenediamine molecule allows it to make two bonds to the metal atom. Ammonia is much smaller and can only make one bond to the metal atom. In both cases, the bonds are made between the nitrogren atoms and the metal atom.

The Coordination of EDTA with a 2+ Metal Ions

ethylenediaminetetraacetate

The complex struction of ethylenediaminetetraacetate. Four acetates are acting as bidentate ligands. One ethylenediamine is also acting as a bidentate ligand.

Rules for Naming Coordination Compounds

$$ \chem{[Co(NH_3)_5Cl]Cl_2} $$

  1. Cation is named before the anion.
    • "chloride" goes last (the counterion)
  2. Ligands are named before the metal ion.
    • ammonia (ammine) and chlorine
    • (chloro) named before cobalt
  3. For negatively charged ligands, an "o" is added to the root name of an anion (such as fluoro, bromo, chloro, etc.).
  4. The prefixes mono-, di-, tri-, etc., are used to denote the number of simple ligands.
    • penta ammine

Rules for Naming Coordination Compounds (cont.)

$$ \chem{[Co(NH_3)_5Cl]Cl_2} $$

  1. The oxidation state of the central metal ion is designated by a Roman numeral:
    • cobalt (III)
  2. When more than one type of ligand is present, they are named alphabetically:
    • pentaamminechloro
  3. If the complex ion has a negative charge, the suffix "ate" is added to the name of the metal.
    • The correct name is:
    • pentaamminechlorocobalt(III) chloride

Exercise

Name the following coordination compounds.

  1. \(\chem{[Co(H_2O)_6]Br_3}\)
  2. hexaaquacobalt(III) bromide

  3. \(\chem{Na_2[PtCl_4]}\)
  4. sodiumtetrachloro-platinate(II)

Some Classes of Isomers

Isomers have the same formula but different properties. Structural isomers have different bonds while stereoisomers have the same bonds but different spatial arrangements. Structural isomers are made up of coordination isomerism and linkage isomerism. Stereoisomers are made up of geometric (cis and trans) isomerism and optical isomerism.

Structural Isomerism

Linkage Isomerism of \(\chem{NO_2^-}\)

Linkage isomers where the nitrite ion changes whether it is attached to the cobalt center through its nitrogen or its oxygen.

Stereoisomerism

Geometrical (cis-trans) Isomerism for a Square Planar Compound

In the cis isomer, the two ammonia are side-by-side and the chlorine atoms are side-by-side. In the trans isomoer, the two ammonias are across from each other as are the two chlorine atoms.

Geometrical (cis-trans) Isomerism for an Octahedral Complex Ion

In the cis isomer, the chlorine atoms are side-by-side. In the trans isomer, the chlorine atoms are across from each other.

Steroisomerism

Unpolarized Light Consists of Waves Vibrating in Many Different Planes

Light from a light source has its waves rising and falling in all directions. Passing that light through a polarizing filter removes all of the direction except on so that we create plane polarized light.

The Rotation of the Plane of Polarized Light by an Optically Active Substance

When plane polarized light is passed through an optically active substance causes the angle of the light plane to roate. The angle between the initial plane polarized light and the rotated polarized light is measured.

Optical Activity

A Human Hand Exhibits a Nonsuperimposable Mirror Image

Consider first your left hand facing you (the thumb is to the left). Holding your right hand in front of a mirror with its palm facing the mirror yields an image of your palm with the thumb facing the left, just like looking at you left hand. Your left hand and right hand are not superimposable.

Concept Check

Does \(\chem{[Co(en)_2Cl_2]Cl}\) exhibit geometrical isomerism?

Yes

Does it exhibit optical isomerism?

trans form - no
cis form - yes

Bonding in Complex Ions

  1. The VSEPR model for predicting structure generally does not work for complex ions.
    • However, assume a complex ion with a coordination number of 6 will have an octahedral arrangement of ligands.
    • And, assume complexes with two ligands will be linear.
    • But, complexes with a coordination number of 4 can be either tetrahedral or square planar.
  2. The interaction between a metal ion and a ligand can be viewed as a Lewis acid–base reaction with the ligand donating a lone pair of electrons to an empty orbital of the metal ion to form a coordinate covalent bond.

The Interaction Between a Metal Ion and a Ligand Can Be Viewed as a Lewis Acid-Base Reaction

The metal atom has empty hybrid orbital that accepts electrons from a long pair on the ligand in order to make the convalent bond.

Hybrid Orbitals on \(\chem{Co^{3+}}\) Can Accept an Electron Pair from Each \(\chem{NH_3}\) Ligand

In hexaammonia cobalt(III), the ammonia molecules bond onto the cobalt ion with their lone pair attaching to the empty d2sp3 hybrid orbitals on the cobalt ions.

The Hybrid Orbitals Required for Tetrahedral, Square Planar, and Linear Complex Ions

sp3 hybrid creates a tetrahedral arrangement of orbitals. The dsp3 hybrid creates a square planar arrangement of orbitals. The sp hybrid creates a linear arrangment of orbitals.

The Crystal Field Model

Assumptions

  1. Ligands are negative point charges.
  2. Metal–ligand bonding is entirely ionic:
    • strong-field (low–spin):
    • large splitting of \(d\) orbitals

    • weak-field (high–spin):
    • small splitting of \(d\) orbitals

Octahedral Complexes

An Octahedral Arrangement of Point-Charge Ligands and the Orientation of the 3d Orbitals

Diagrams of the five \(d\) orbitals. Each of the orbitals lines in a different plane spacially.

Which Type of Orbital is Lower in Energy?

The Energies of the \(3d\) Orbitals for a Metal Ion in an Octahedral Complex

Diagram showing the five 3d orbital energy being the same when it is a free metal atom. When the metal atom complexes with ligands the orbitals break into a lower energy group and a higher energy group (as discussed on the previous slide). Both of these new energies are higher than the initial energy of the orbitals in the free atoms.

Possible Electron Arrangements in the Split \(3d\) Orbitals in an Octahedral Complex of \(\chem{Co^{3+}}\)

In a strong ligand field, the energy separation of the two d-orbital groups is large whereas in a weak ligand field, the energy separation is much smaller.

Magnetic Properties

Spectrochemical Series

Complex Ion Colors

Absorbtion of Visible Light by the Complex Ion \(\chem{Ti(H_2O)_6^{3+}}\)

When the molecule absorbs visible light, one electron is promoted from the t2g ground state to the eg excited state. This needs visible light because the energy splitting has an energy corresponding to the energy of visible light.

Concept Check

Which of the following are expected to form colorless octahedral compounds? \(\chem{Zn^{2+},\,Fe^{2+},\,Mn^{2+},\,Cu^{+},\,Cr^{3+},\,Ti^{4+},\,Ag^{+},\,Fe^{3+},\,Cu^{2+},\,Ni^{2+}}\)

\(\chem{Zn^{2+},\,Cu^{+},\,Ti^{4+},\,Ag^{+}}\)

Tetrahedral Arrangement

The \(d\) Orbitals in a Tetrahedral Arrangement of Point Charges

Showing the five d orbitals and their planes lining up with a tetrahedral arrangement.

The Crystal Field Diagrams for Octahedral and Tetrahedral Complexes

The difference in energy between the high and low orbital energies of the d-orbitals in an octahedral arrangement is larger than the difference in a tetrahedral arrangement. In the tetrahedral arrangment, the identity of the high and low energy orbitals are reversed (three high, two low).

Concept Check

Consider the Crystal Field Model (CFM).

  1. Which is lower in energy, \(d\)–orbital lobes pointing toward ligands or between?
  2. between

  3. The electrons in the \(d\)–orbitals – are they from the metal or the ligands?
  4. metal

  5. Why would electrons choose to pair up in \(d\)–orbitals instead of being in separate orbitals?
  6. Since some orbitals are higher in energy than others (see "1"), electrons may actually be lower in energy by pairing up than by jumping up in energy to be in a separate orbital.

  7. Why is the predicted splitting in tetrahedral complexes smaller than in octahedral complexes?
  8. In an octahedral geometry there are some orbitals pointing directly at ligands. Thus, there is a greater energy difference between these (larger splitting).

Concept Check

Using the Crystal Field Model, sketch possible electron arrangements for the following. Label one sketch as strong field and one sketch as weak field.

  1. \(\chem{Ni(NH_3)_6^{2+}}\)
  2. A \(d^8\) ion will look the same as strong field or weak field in an octahedral complex. In each case there are two unpaired electrons.

  3. \(\chem{Fe(CN)_6^{3-}}\)
  4. This is a \(d^5\) ion. In the weak field case, all five electrons are unpaired. In the strong field case, there is one unpaired electron.

  5. \(\chem{Co(NH_3)_6^{3+}}\)
  6. This is a \(d^6\) ion. In the weak field case, there are four unpaired electrons. In the strong field case, there are no unpaired electrons.

Concept Check

A metal ion in a high–spin octahedral complex has 2 more unpaired electrons than the same ion does in a low–spin octahedral complex.

What are some possible metal ions for which this would be true?

Metal ions would need to be \(d^4\) or \(d^7\) ions. Examples include \(\chem{Mn^{3+}}\), \(\chem{Co^{2+}}\), and \(\chem{Cr^{2+}}\).

Concept Check

Between \(\chem{[Mn(CN)_6]^{3–}}\) and \(\chem{[Mn(CN)_6]^{4–}}\) which is more likely to be high spin? Why?

\(\chem{[Mn(CN)_6]^{4-}}\) is more likely to be high spin because the charge on the \(\chem{Mn}\) ion is 2+ while the charge on the \(\chem{Mn}\) ion is 3+ in the other complex. With a larger charge, there is bigger splitting between energy levels, meaning strong field, or low spin.

The Crystal Field Diagrams for Octahedral and Tetrahedral Complexes

The d-orbitals in a square planar arrangment split differently than either of the previous two examples. Two orbitals are low energy. One immediately above those in energy. One slightly above that in energy. The final orbital is much higher in energy that the others.

The \(d\) Energy Diagrams for Linear Complexes Where the Ligands Lie Along the \(z\) Axis

The d-orbitals in a linear arrangment split differently than the others. Two orbitals are low energy. Two more are a little higher in energy than those. The final orbital is much higher in energy that the others.

The Biological Importance of Coordination Complexes

First-Row Transition Metals and Their Biological Significance

First-Row Transition Metals Biological Function(s)
Scandium None known.
Titanium None known.
Vanadium None known in humans.
Chromium Assists insulin in the control of blood sugar; may also be involved in the control of cholesterol.
Manganese Necessary for a number of enzymatic reactions.
Iron Component of hemoglobin and myoglobin; involved in the electron-transport chain.
Cobalt Component of vitamin B12, which is essential for the metabolism of carbohydrates, fats, and proteins.
Nickel Component of the enzymes urease and hydrogenase.
Copper Component of several enzymes; assists in iron storage; involved in the production of color pigments of hair, skin, and eyes.
Zinc Component of insulin and many enzymes.

Biological Importance of Iron

The Heme Complex

The heme complex contains a square planar iron atom at its section. Four nitrogens in the surrounding heme compound are bound to the iron center.

Myoglobin

  • The \(\chem{Fe^{2+}}\) ion is coordinated to four nitrogen atoms in the porphyrin of the heme (the disk in the figure) and on nitrogen from the protein chain.
  • This leaves a 6th coordination position (the \(\chem{W}\)) available for an oxygen molecule.

The diagram is explained in the text of this slide.

Hemoglobin

  • Each hemoglobin has two \(\alpha\) chains and two \(\beta\) chains, each with a heme complex near the center.
  • Each hemoglobin molecule can complex with four \(\chem{O_2}\) molecules.

The diagram is explained in the text of this slide.

Metallurgy

The Blast Furnace Used In The Production if Iron

In the blast furnace, iron ore, limestone, and coke a added from the top. Oxygen-enriched air is pumped in through the bottom. As the system is heated, the iron ore melts and converted to inquid iron which settles to the bottom under a layer of slag (waste). The liquid iron and the slag are extracted from the bottom.

A Schematic of the Open Hearth Process for Steelmaking

$$ \begin{align} & \chem{CaCO_3 \xrightarrow{Heat} CaO+CO_2} \\ & \chem{4Al + 3O_2 \rightarrow 2Al_2O_3} \end{align} $$

Air is pumped through the system from left to right. Gas or liquid fuel is aimed at the metal in the center and ignited melting the metal. Steel is extracted from the bottom of the system under the metal.

The Basic Oxygen Process for Steelmaking

  • Much faster.
  • Exothermic oxidation reactions proceed so rapidly that they produce enough heat to raise the temperature nearly to the boiling point of iron without an external heat source.

Into the chamber, oxygen is pumped in from the top and flux is poured into the system from the top. The metal is heated and liquified in the bottom of the container.

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