Chapter 7 and 19-Nomenclature of Coordination

Compounds

Chapter 7 and 19-Nomenclature of Coordination

Compounds1

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Review of the Previous Lecture

1. Acid and Base Theories

Lewis Definition: Includes adduct formation reactions

Hard and Soft Acids and Bases: -Defining species based on their polarizability-Helps identify the “why” behind the affinity of species

2. Introduction to Coordination Chemistry

Metals as Lewis Acids and Ligands as Lewis Bases

Alfred Werner began the field with his “Werner Cobalt Complexes”

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1. DenticityDefines the number of bonds that a ligand can form with a metal. We use prefixes to distinguish the denticity of ligands.

Prefix Coordinating AtomsMono 1

Bi 2Tri 3

Tetra 4Penta 5Hexa 6

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1. DenticityLet’s use a coordination number 6 to establish ligand denticity.

Monodentate Bidentate Tridentate

Tetradentate Pentadentate Hexadentate

A ligand that can bind at two or more sites is polydentate and is called a chelator.

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2. Common Ligand Names

A. MonodentateI. Anions

Halides Pseudohalides OthersF- Fluoro CN- Cyano OH- HydroxoCl- Chloro NCO- Cyanato H- HydridoBr- Bromo NCS- Thiocyanato NO2

- NitroI- Iodo N3

- Azido

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2. Common Ligand Names

A. MonodentateII. Neutral Molecules

Amines OthersNH3 Ammine H2O Aqua

NH2CH3 Methylamine CO CarbonylNH(CH3)2 Dimethylamine NO NitrosylN(CH3)3 Trimethylamine P(CH3)3 Trimethylphosphine

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2. Common Ligand NamesB. Polydentate

I. 5-membered ring

Oxalic Acid Oxalato Anionic ligand

2-

II. 6-membered ring

Acetylacetone Acetylacetonato Anionic ligand

1-

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2. Common Ligand NamesB. Polydentate

III. Neutral Bidentate

Ethylenediamine (en)IV. Neutral Tridentate

Diethylenediamine (dien)

3. Nomenclature Rules

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A. For charged molecules, the cation comes first followed by the anion.

The following rules apply to both neutral and charged molecules:

B. The elemental formulation has the primary coordination sphere in brackets.

[Pt(NH3)4]Cl2

3. Nomenclature Rules

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A. For charged molecules, the cation comes first followed by the anion.

The following rules apply to both neutral and charged molecules:

B. The elemental formulation has the primary coordination sphere in brackets.

[Pt(NH3)4]Cl2

When writing the name, the ligands within the coordination sphere are written before the metal and are listed in alphabetical order.

tetrakisammineplatinum(II) chloride

C. Ligand names (as we have already discussed). Monodentate: Ligands with one point of attachment Chelates (Bidentate…multidentate): Ligands with two or more points of attachment

Nomenclature Rules

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D. The number of ligands of each kind is indicated by prefixes using the following table.

A B Use prefixes in column A for simple cases.

Use prefixes in column B for ligands with names that already use prefixes from column A.

[Co(en)2Cl2]+

Dichlorobis(ethylenediamine)cobalt(III)

Always use prefixes in column B when the name of a ligandbegins with a vowel.

[Rh(aqua)6]3+

Hexakis(aqua)rhodium(III)

Nomenclature Rules

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E. Ligands are written in alphabetical order-according to the ligand name, not the prefix.

F. Special:

Anionic ligands are given an o suffix.

Neutral ligands retain their usual name

Coordinated water is called aqua

Coordinated ammonia is called ammine

Nomenclature Rules

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G. Designate the metal oxidation state after the metal.

[PtClBr(NH3)(H2O)] Ammineaquabromochloroplatinum(II)

[Pt(NH3)4]2+

Tetrakisammineplatinum(II)

If the molecule is negatively charged, the suffix –ate is added to the name

[Pt(NH3)Cl3]-

Amminetrichloroplatinate(II)

Ammineaquabromochloroplatinum(II)

Nomenclature Rules

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Special names for metals when in a negatively charged molecule:

Copper (Cu): Cuprate

Iron (Fe): ferrate

Silver (Ag): argentate

Lead (Pb): Plumbate

Tin(Sn): Stannate

Gold(Au): Aurate

Nomenclature Rules

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H. Prefixes designate adjacent (cis-) and opposite (trans-) geometric locations

cis-bisamminedichloroplatinum(II) is an anticancer agent. The trans isomer is not.

Nomenclature Rules

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I. Bridging ligands between two metal ions have the prefix μ

μ-amido-μ-hydroxobis(tetrakisamminecobalt(III))

Chapter 7 and 19-Thermodynamics of

Metal-Ligand Binding

Chapter 7 and 19-Thermodynamics of

Metal-Ligand Binding17

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1. Metal-ligand complexation

M + L M-L

M + L

M-L

ΔG≠

1 2

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1. Metal-ligand complexation

Kinetic Standpoint:

Tells how slow or fast the complexation event is

Later we will discuss inertness (slow) vs lability (rapid) ligand exchange

The ΔG≠ values correspond to the activation barriersfor the forward and reverse reactions and define the rate constants (k)

ΔG≠ ,   rate of the reaction

M + L M-Lkforward

kreverse

M + L

M-L

ΔG≠1 2

kforward kreverse

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1. Metal-ligand complexation

Thermodynamic Standpoint:

Tells about the relative ratio of product to reactants at equilibrium

The ΔG⁰ values corresponds to the stability of the M-L complex

ΔG⁰ ,   stability of the complex

M + L M-L

M + L

M-L

ΔG≠1 2

K K [M-L][M] [L]

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2. Defining AffinityA. Single-step metal ligand interactions

When considering a single metal ligand interaction, we treat the interaction as a single ligand addition event regardless of the denticity of the ligand

M + L M-L

K K [M-L][M] [L]

Formation (Stability) Constant =

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Say a second ligand can coordinate, now we would have more thana single-step process…

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2A. Single-step metal ligand interactionsThe first ligand binding step:

M + L M-L

K1 K1 [M-L][M] [L]

The second ligand binding step:

M-L + L M-L2

K2 K2 [M-L2]

[M-L] [L]

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Let’s consider the metal ligand binding process in a cumulative manner.

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2B. Cumulative M-L interactionsWhen considering a metal ligand interaction in a cumulative process, we introduce the term β

M + L M-L

β 1 β 1 [M-L][M] [L]

Now consider both ligands binding to the metal:

M + 2L M-L2

β 2 β 2 [M-L2][M][L]2

= K1 x K2

= K1

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2B. Cumulative M-L interactionsGeneral expression for describing the cumulative process of metal ligand interactions:

M + x L M-Lx

β x β x [M-Lx][M] [L]x

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2C. Metal ions are never “free” in solutionI. In aqueous solutions, metal ions are water bound:

M M(H2O)xH2O

Some metals interact so strongly with water that they cause hydrolysis

M(H2O) M(OH)- + H+

Metal as Brønsted-Lowry Acid

M(H2O) M(OH)- + H+

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2C. Metal ions are never “free” in solution

Metal as Brønsted-Lowry AcidBrønsted-Lowry Acid Relative Acidity: Alkali metal cations- Not acids Alkaline earth- Slight acidity 2+ Transition metals: Weak acidity 3+ Transition metals: Moderate acidity 4+ and higher: Strong acidity, typically oxygenated ions

• Vanadium(V) usually exists as dioxovanadium (VO2+)

Metal ions lower the effective pKw of water and pKa of ligands.

M(H2O)x (aq) + L (aq) M(H2O)x-1L (aq) + H2O (l)

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2C. Metal ions are never “free” in solutionII. Ligand binding can be seen as a competition with solvent binding

Considering water as your solvent: When you write formation constants (K) you ignore the bound water

K K [M-L]

[M] [L]

K

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2D. Metal-ligand binding preferencesI. Hard Soft Acid Base Theory

log K1

Metal ion F‐ Cl‐ Br‐ I‐

Fe3+ (aq) 6.04 1.41 0.5 ‐Hg2+ (aq) 1.0 6.7 8.9 12.7

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2D. Metal-ligand binding preferencesII. The Chelate EffectWhen coordinating through the same type of atom, a chelating ligand willoutcompete a monodentate ligand for metal binding.

Chelates are favorable when they form five/six membered rings when metalbound

Smaller rings are strained Larger rings result in unfavorable ligand distortion

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2D. Metal-ligand binding preferencesLet’s consider thermodynamic factors for the stability afforded by chelators.

ΔG⁰ = ΔH ⁰ - TΔS⁰

Enthalpy (ΔH ⁰): Bond breaking/bond forming

Entropy (ΔS⁰): Tendency toward “disorder”

Compare metal binding by:

CH3NH2 vs

en

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2D. Metal-ligand binding preferencesOctahedral Cd2+ complexes:

A. [Cd(H2O)6]2+ + 4 CH3NH2 [Cd(CH3NH2)4(H2O)2]2+ + 4 H2OK (M)

3.3 x 106

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2D. Metal-ligand binding preferencesOctahedral Cd2+ complexes:

B. [Cd(H2O)6]2+ + 2 en [Cd(en)2(H2O)2]2+ + 4 H2OK (M)

4.0 x 1010

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2D. Metal-ligand binding preferencesOctahedral Cd2+ complexes:

A. [Cd(H2O)6]2+ + 4 CH3NH2 [Cd(CH3NH2)4(H2O)2]2+ + 4 H2O

B. [Cd(H2O)6]2+ + 2 en [Cd(en)2(H2O)2]2+ + 4 H2O

K (M)3.3 x 106

4.0 x 1010

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2D. Metal-ligand binding preferencesOctahedral Cd2+ complexes:

A. [Cd(H2O)6]2+ + 4 CH3NH2 [Cd(CH3NH2)4(H2O)2]2+ + 4 H2O

B. [Cd(H2O)6]2+ + 2 en [Cd(en)2(H2O)2]2+ + 4 H2O

5 molecules

3.3 x 106

4.0 x 1010

K (M)

5 molecules

3 molecules 5 molecules

2 more molecules¨.

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2D. Metal-ligand binding preferencesOctahedral Cd2+ complexes:

A. [Cd(H2O)6]2+ + 4 CH3NH2 [Cd(CH3NH2)4(H2O)2]2+ + 4 H2O

B. [Cd(H2O)6]2+ + 2 en [Cd(en)2(H2O)2]2+ + 4 H2O

5 molecules

3.3 x 106

4.0 x 1010

Rxn.

5 molecules

3 molecules 5 molecules

K (M)

ΔH⁰ (kJ/mol) ΔS⁰ (J/(K mol)) ΔG⁰ (kJ/mol)

A

B

-57.3

-56.5

-67.3

+47.1

-37.2

-60.7

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3. Redox Stability

The stability of the oxidation state of a metal ion in solution is highly dependent onthe biochemical conditions and the coordination environment. Both can dictate the“preference” of M-L interactions.

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3A. DefinitionsOxidation: Loss of one or more electrons

Reduction: Gain of one or more electrons

Oxidation-reduction (Redox) reactions are thermodynamically driven.

ΔG⁰ = - nFE⁰

n = # of electrons involved

F = Faraday’s constant (96,485 C/mol or J/V)

E⁰ = Electromotive force: The potential energy for electron (or charge) movement

If E⁰ is +, ΔG⁰ is - ; Spontaneous reaction

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3B. Ligand coordination shifts the E⁰ of metal ionsNOTE: Oxidation-reduction (Redox) reactions are written from the reduction perspective.

Consider Fe3+:

Half-reaction- Fe3+(aq) + e- Fe2+(aq) E⁰  = +0.77 V vs Standard Hydrogen Electrode(SHE)

In a reducing environment, ligand-free Fe has an oxidation state of 2+

But we live in an oxidizing environment, Fe3+ dominates

P = 1 atm; T = 25 ⁰C, pH =0

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3B. Ligand coordination shifts the E⁰ of metal ionsNOTE: Oxidation-reduction (Redox) reactions are written from the reduction perspective.

Consider Fe3+:

Half-reaction- Fe3+(aq) + e- Fe2+(aq) E⁰  = +0.77 V vs (SHE)

+ e- E⁰  = -0.53 V vs (SHE)pH = 7.0

Fe(III) Fe(II)

The ligand stabilized the oxidation state of Fe(III).

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3C. Redox potentials for metal ions can be found in tables or formatted in simplified diagrams.

Latimer Diagram: Summarizes a considerable amount of thermodynamic information about the oxidation states of an element.

• Diagrams are written for acidic, neutral, and basic conditions

• Omit H2O, H3O+, OH-

• Write oxidation state, highest to lowest, left to right

Fe3+(aq) Fe2+(aq) Fe(s) Acidic solution (pH 0)+0.77 V -0.44 V

-0.04 V

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3C. Redox potentials for metal ions can be found in tables or formatted in simplified diagrams.

Latimer Diagram: Summarizes a considerable amount of thermodynamic information about the oxidation states of an element.

• Diagrams are written for acidic, neutral, and basic conditions

• Omit H2O, H3O+, OH-

• Write oxidation state, highest to lowest, left to right

Fe3+(aq) Fe2+(aq) Fe(s) Acidic solution (pH 0)+0.77 V -0.44 V

Fe3+(aq) + e- Fe2+(aq)

Fe2+(aq) + 2e- Fe (s)

Fe3+(aq) + 3e- Fe (s)

-0.04 V