Chapter 7 and 19-Acid and Base Theories Continuation

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2

Review of the Previous Lecture

1. Solubility principles▪ Like dissolves like▪ Solvent overcome solute interactions▪ Simple dissolution vs dissolution with dissociation

2. Acid and Base Theories▪ Defined by taste▪ Arrhenius: Defined for aqueous solutions▪ Brønsted-Lowry: Defined for all solvents

-The Leveling Effect

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1. Lewis Acids and BasesA. Definition

Acid: e- pair acceptorBase: e- pair donor

▪ Encompasses the Brønsted-Lowry definition:

H+ accepts an e- pair from a Brønsted base

▪ Includes reactions involving metal adduct formation

Ag+ + 2 :NH3 H3N-Ag- NH3

Dative Bond: e- pair comes from Lewis Base

Lewis acid-base adducts involving metal ions are called coordination compounds.

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1. Lewis Acids and BasesB. Adduct formation and frontier orbitals

Consider the following reaction:

▪ Involves a change in geometry

▪ Involves the interaction of frontier orbitals

H+ + :NH3 H4N+

+

Point Group: C3v Td

5

Molecular orbital diagram of NH3

*

*

*

*

E

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▪ Orbitals that will interact are those ofcomparable energy and symmetry

a1 LUMO of H+ and

a1 HOMO of NH3

▪ New set of frontier orbitals are generatedNew LUMO and HOMO

▪ Driving force: stabilization of the e- in thedonor HOMO

H+ + :NH3 H4N+

HOMO

LUMO

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1. Lewis Acids and BasesC. Thermodynamics of binding

Consider the following reaction:

Use affinity scales to measure relative strengths:▪ Compare strength of Lewis acids or bases to a standard

For instance, compare the strength of different Lewis Bases to Boron-based Lewis Acids

:B + A BA

KBA =[BA]

[B] [A]KBA , the stronger the interaction

BF3 + LB LB-BF3KLB-BF3

or-∆H⁰ = BF3 Affinity

SetConstant

Vary

Affinity , Stronger Lewis Basicity

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2. Hard and Soft Acids and BasesA. Expands the Lewis Acid-Base concept by identifying the “why” behind the affinity

Pearson (1968) and Chatt recognized that the polarizability of species was correlated with the preference of species to interact with other species.

Consider a random base with a nonpolarizable and polarizable Lewis Acid:

H+ + :B HB+KH+

CH3Hg+ + :B CH3HgB+KCH3Hg+

Affinity ScalesNonpolarizable

Polarizable

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2. Hard and Soft Acids and BasesFor a given base:

H+ + :B HB+KH+

CH3Hg+ + :B CH3HgB+KCH3Hg+

if KH+ >> KCH3Hg+ B is “Hard”, Nonpolarizable, High charge/size ratio

if KH+ << KCH3Hg+ B is “Soft”, Polarizable, Low charge/size ratio

if KH+ ~ KCH3Hg+ B is “Intermediate” or “Borderline”

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2. Hard and Soft Acids and Bases

B. Locating Hard and Soft Acids on the Periodic Table

Hard acids▪ Alkali metals▪ Alkaline earth metals▪ 1st row transition metals in high oxidation states

Soft acids▪ 2nd row or lower transition metals in low oxidation states

Note:

▪ As you go down the periodic table of the main group elements, the elements are less Hard

▪ The Hard- or Softness of a metal can depend on oxidation state

Oxidation State, Hardness Fe3+ vs Fe2+

Hard Intermediate

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B. Locating Hard and Soft Acids on the Periodic Table

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2. Hard and Soft Acids and BasesC. Binding preferences

Hard acids prefer Hard bases

Soft acids prefer Soft bases

These interactions are stronger than those between Hard-Soft species.

LiF + CsI LiI + CsF

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2. Hard and Soft Acids and BasesC. Binding preferences

Hard acids prefer Hard bases

Soft acids prefer Soft bases

These interactions are stronger than those between Hard-Soft species.

LiF + CsI LiI + CsF∆Gformation = -573 -335 -343 -502 kJ/mol

∆Gformation = Σ ∆Gf (products) - Σ ∆Gf (reactants)

∆Gformation = 59 kJ/mol Not favorable

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2. Hard and Soft Acids and Bases

Hard-Hard/Soft-Soft matching of acids and bases represents a stabilization that isadditional to other factors that contribute to the strength of donor and acceptor bonds:

I. Orbital overlap

II. ∆ Electronegativities

III. In solution competition with solvent binding

Coordination Chemistry:History and Introduction to Structure

Chapter 7 and 19

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1. Coordination Compounds

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Composed of:

▪ Metal atoms or ions▪ One or more ligands (atoms, ions, or molecules) that donate e- to the metal▪ Chemistry of the metal d-orbitals

Consist of the formation of coordinate covalent (dative) bonds:

▪ Lewis Acid-Base Adduct

Metal is the Lewis AcidLigand is the Lewis Base

M + L M-L

2. History

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A. A tale as old as time (Prehistoric)

B. Formally introduced by Alfred Werner (late 19th Century)

▪ Introduced new bonding concepts

▪ The famous Werner Cobalt Compounds

Compound Elemental Formula Color

A CoN6H18Cl3 Yellow

B CoN5H15Cl3 Red

C CoN4H12Cl3 Green or Purple

D CoN3H9Cl3

Complimentary Color Wheel

Absorb

See

C. Werner Compounds

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▪ Early bonding theories allowed only three atoms to be attached to cobalt because of its valence of 3 (Co3+) for charge balance.

▪ Jørgensen proposed that for the above compounds• N could form chains because of its valence of 5• Chloride (Cl-) could be bound to N or to Co3+

▪ Werner proposed something very radical for the time• As many as 6 N (as NH3) could bond directly to Co3+

• Cl- could bond to Co3+ or associate loosely; two kinds of Cl-

Compound Elemental Formula Color

A CoN6H18Cl3 Yellow

B CoN5H15Cl3 Red

C CoN4H12Cl3 Green or Purple

D CoN3H9Cl3

D. The Werner Titrations

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Complex + AgNO3 n AgCl + Complexn+

AgCl equivs

3

2

1

0

Compound

A

B

C

D

Jørgensen Werner

Note: 2 Isomers

Note: 0 Cl?

Compound ElementalFormula

Color

A CoN6H18Cl3 Yellow

B CoN5H15Cl3 Red

C CoN4H12Cl3 Greenor

Purple

D CoN3H9Cl3

3 Clloosely

associated

2 Clloosely

associated

1 Clloosely

associated

0 Clloosely

associated

E. Werner’s Theory

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A. Primary Bonding

▪ The positive charge of the metal ion is balanced by negative ions.▪ Does not have to involve direct bonding to the metal ion.

• Today when direct bonding is not involved, we refer to it as the secondary coordination sphere.

B. Secondary Bonding

▪ Ligands (molecules or ions) directly attached to the metal ion.▪ This interaction constitutes the coordination sphere; the complex ion.

• Today we refer to it as the primary coordination sphere.▪ Defines the coordination number.▪ Defines a specific geometry; “directed in space”.

Remember these are older theories

Primary Sphere

[Co(NH3)6]Cl3

Secondary Sphere

F. Determining the coordination geometry

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Which coordination mode would give 2 isomers for complex C?

a b dc

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Model A

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Model A

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Model B

Model C produces similar results

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cis trans

Yes!

Model D

Cis and Trans Isomers

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You can have optical isomers.

Ethylenediamine (en)

Nonsuperimposable