7.8 Lewis Acids, Bases, and Coordinate Covalent Bonding

Introduction to the Lewis Model of Acid-Base Chemistry

• Conceptual Overview: The Lewis model of acid-base chemistry is a fundamental framework used to describe chemical reactions based on electron pair movement rather than proton transfer. This section outlines the definition of Lewis acids and bases, the formation of coordinate covalent bonds, and the creation of adducts and complex ions.

• Goal of the Section:

  • Introduce and define the terms Lewis acids and Lewis bases.
  • Explain the Lewis model of acid-base chemistry.
  • Write chemical equations for the formation of adducts and complex ions.

Understanding Coordinate Covalent Bonds

• General Definition: A coordinate covalent bond is a specific type of covalent bond where one atom or species provides both of the bonding electrons found in the complex.

• Contrast with Standard Covalent Bonds:

  • In introductory chemistry courses (e.g., Chemistry 01/2003), a standard covalent bond is described as the result of two atoms coming together to share their individual electrons, with each atom contributing one electron to form a bonding pair.
  • In a coordinate covalent bond, one species provides the entire lone pair of electrons required to establish the bond.

• Example 1: Formation of the Hydronium Ion ($H_3O^+$):

  • Reactants: A water molecule ($H_2O$) and an individual proton ($H^+$).
  • Mechanism: The proton ($H^+$) has no electrons. The central oxygen atom in the water molecule has two lone pairs. The proton approaches the water molecule, and a new covalent bond is formed using both electrons from the oxygen's lone pair.
  • Result: The resulting bond is covalent but classified as coordinate because both electrons originated from the oxygen.

• Example 2: Formation of the Ammonium Ion ($NH_4^+$):

  • Reactants: An ammonia molecule ($NH_3$) and a proton ($H^+$).
  • Mechanism: The ammonia molecule features a central nitrogen atom with one lone pair of electrons. This pair is not involved in bonding until it comes into contact with a proton in solution.
  • Structure Representation: While structural diagrams may explicitly show the lone pair on the nitrogen, this is often considered redundant once the bond is formed. In the final structure, the line connecting the nitrogen to the hydrogen represent the shared pair of electrons that originated from the nitrogen.

Definitions of Lewis Acids, Lewis Bases, and Adducts

• Lewis Acid-Base Chemistry Definition: This field of chemistry is defined by reactions involving the formation of coordinate covalent bonds. This definition differs significantly from the Brønsted-Lowry definition (introduced in previous modules), which focuses on proton ($H^+$) donation and acceptance.

• Lewis Base: A species that donates the electron pair in the formation of a coordinate covalent bond.

• Lewis Acid: A species that accepts the electron pair in the formation of a coordinate covalent bond.

• Lewis Base Acid Adduct (LBAA):

  • Definition: The product resulting from the combination of a Lewis acid and a Lewis base.
  • Characteristics: The adduct contains the newly formed coordinate covalent bond.
  • Example: In the reaction between water and a proton, water acts as the Lewis base (electron pair donor) and the proton acts as the Lewis acid (electron pair acceptor). The resulting hydronium ion ($H_3O^+$) is the Lewis base acid adduct, which carries a positive charge.

Examples of Lewis Acid-Base Interactions

• Example: Boron Tetrafluoride ($BF_4^-$):

  • Lewis Base: A fluoride ion ($F^-$), which is surrounded by four pairs of electrons.
  • Lewis Acid: Boron trifluoride ($BF_3$).
  • Process: When the fluoride ion comes into close proximity with $BF_3$, it donates a pair of electrons to form a coordinate covalent bond.
  • Resulting Adduct: The boron tetrafluoride ($BF_4^-$) ion, which carries an overall charge of $-1$.

• Example: Silver-Ammonia Complex:

  • Lewis Base: Two ammonia molecules ($NH_3$).
  • Lewis Acid: A central silver ion ($Ag^+$).
  • Process: The two ammonia molecules each donate a lone pair to form two coordinate covalent bonds (one on each side) with the central silver ion.
  • Result: An acid-base adduct involving a central metallic silver atom experiencing two coordinate covalent bonds.

Displacement Reactions in Lewis Acid-Base Chemistry

• Concept: In some instances, a Lewis acid-base reaction involves the displacement of one substance by another. A Lewis base can replace another Lewis base, or a Lewis acid can replace another Lewis acid.

• Base Displacement Example:

  • Starting Adduct: A central silver atom bonded to two ammonia molecules ($[Ag(NH_3)_2]^+$).
  • Incoming Base: Cyanide ion ($CN^-$).
  • Process: The cyanide ions replace the ammonia molecules.
  • Result: A new adduct is formed with a central silver atom bonded to two cyanide ions ($[Ag(CN)_2]^-$), while the displaced ammonia molecules go off to potentially donate electron pairs to other species.

• Acid Displacement Example:

  • Reactants: Carbonate ion ($CO_3^{2-}$) and the Lewis acid sulfur trioxide ($SO_3$).
  • Process: A new coordinate covalent bond is formed where an oxygen (doubly bonded to the central sulfur) is added, transitioning the structure from carbonate to sulfate.
  • Result: The formation of a new adduct, the sulfate ion ($SO_4^{2-}$), and a new acid in the form of carbon dioxide ($CO_2$).

Complex Ions and Ligands

• Complex Ion:

  • Definition: A structure composed of a central atom surrounded by ions or molecules known as ligands.
  • Central Atom: Typically a transition metal cation (e.g., the silver ion seen in previous examples).
  • Bonding: The central atom is surrounded by substances that act as Lewis bases, providing the electron pairs for coordinate covalent bonding.

• Ligand:

  • Definition: An ion or molecule that is attached to a metal atom through coordinate covalent bonding.
  • Role: Ligands typically act as Lewis bases, donating the necessary electron pairs to the central metal (the Lewis acid).

• Example: Hexaamminecobalt(III) Ion ($[Co(NH_3)_6]^{3+}$):

  • Nomenclature:
    • "Hexa-": A prefix representing the number six.
    • "Ammine": Represents the functional group ($NH_3$).
    • "Cobalt": The central metal atom.
  • Structure: A central cobalt atom is surrounded by six ammonia complexes and carries an overall charge of $+3$.