The Lewis Acid-Base Concept:Coordination Compounds

So far we have only considered acid base chemistry in the narrow framework of H+ and OH-. G.N. Lewis introduced broader definitions of the terms acid and base:

Lewis acid => an electron acceptor
Lewis base => an electron donor

In a Lewis acid-base neutralization, the base donates a pair of electrons forming a coordinate covalent bond which joins the two species together into the reaction product.

Species which donate electrons are termed Lewis bases, donors or ligands (the organic chemist's nucleophile!) Any atom bearing a lone pair of electrons is capable of being a Lewis base or donor atom.

Species which accept electrons are termed Lewis acids or acceptors (the organic chemist's electrophile!)

The product can be called an acid-base adduct, a coordination complex or coordination compound if it is neutral in charge or a complex ion if there is a resulting charge.

Classification of Ligands

monodentate a Lewis base which can form only one bond to a Lewis acid name means "one tooth"
chelating has more than one donor atom than can form more than one coordinate covalent bond to the same metal ion
classified according to the number of donor atoms correctly positioned for potential binding to a Lewis acid
non-linear, often with 2 or 3 atoms separating the donor atoms
bridging a can donate more than one pair of electrons to more than one Lewis acid simultaneously  
ambidentate has more than one element that can serve as a donor atom possesses bridging capability but tends to be monodentate; often linear in geometry
macrocyclic large ring compound with several donor atoms that can bind a Lewis acid inside the ring  
pi-donor donates electrons from a pi bond to a metal ion  

Charged Complexes vs Uncharged Complexes

Charged complex ions and their counterions attract hydration spheres. Since these ions are usually quite large, they typically fall into the nonacidic cation and nonbasic anion categories.
Complex cations are expected to form soluble salts with basic anions and are best precipitated from solution using nonbasic anions.

Ligands in complex ions are often organic or other easily oxidized species. Although perchlorates of these complex cations crystallize nicely, the resulting crystals are dangerously explosive.

Halo anions such as TiCl6-, BiCl4-,SnCl6-,and SbCl5- can be used for precipitation.

BF4- and PF6- resemble perchlorate in size and basicity but do not involve extremely gihg oxidatin states and are nonoxidizing.

SiF6- resembles sulfate and can be used to precipitate feebly acidic cations.

Uncharged complexes do not attract waters of hydration and are generally insoluble in water unless they contain ligands us as -OH or -NH2 that can H-bond to water.

Uncharged complexes that contain hydrophobic ligands are soluble in nonpolar solvents.

These solubility characteristics can be used in analytical chemistry.

A metal ion present in very low concentration in a natural water is complexed with an organic ligand to give an uncharged complex which is then extracted into a much smaler volume of nonpolar solvent (separation from impurities and concentrated simultaneously)

Uncharged complexes tend to accumulate in fatty tissue and lipids and are not excreted in the aqueous urine allowing the complex to build up. When an animal is eaten by another animal higher up in the food chain, the uncharged complex is passed on to the predator. Since the predator eats many of the lower-chain animals, the concentration of the complex builds up (bioamplification of a toxin).

The Chelate and Macrocyclic Effects

Although the number of possible complex ions and compounds formed in a natural water system are very large, there are two generalizations which allow prediction of some of the most likely complexes that will form in a complicated mixture of metal ions and ligands.
  1. Chelate Effect
    Chelate ligands form more stable complexes than analogous monodentate ligands
  2. Macrocyclic Effect
    Macrocyclic ligands form more stable complexes than chelate ligands
Stability follows the order:

Macrocyclic > Chelate > Monodentate

The Chelate Effect

Consider the competition between 1 mole of an n-dentate ligand and n moles of a very similar monodentate ligand for a metal ion by mixing 1 mole of Ni2+ and 6 moles of ammonia and 3 moles of ethylenediamine.

[Ni(NH3)6]3+ + 3 NH2CH2CH2NH2 [Ni(NH2CH2CH2NH2)3]3+ + 6 NH3

For this equilibrium:
DG = -67 kJ/mol
DH = -13 kJ/mol
      (small change since each complex ion involves 6 similar Ni-N bonds)
T DS = -54 kJ/mol
      (responsible for the shift favoring complex of chelated ligand)

Macrocyclic Effect

Consider a competition between a noncyclic chelating ligand and a macrocyclic chelating ligand having the same number and type of donor atoms.

[K(CH3O(CH2CH2O)5CH3)]+ + cyclo-(CH2CH2O)6[K(CH2CH2O)6]+ + CH3O(CH2CH2O)5CH3

For this equilibrium, Keq = 104.

Thermodynamics of the Lewis Acid-Base Reaction

The enthalpy change for a Lewis acid-base reaction can be predicted using the Drago-Wayland equation.

DH (in kJ/mol) = -4.184 (CACB+EAEB)

Predictions using this equation typically agree with experimental values (for uncharged species) unless there is spacial interference of groups attahed to the donor or acceptor atoms.

Calculate the enthalpy changes for the following Lewis acid-base reactions:

(a) I2 + ethylacetate

(b) SbCl5 + tetrahydrofuran

The Drago-Wayland equation contains two parameters for each acid and for each base. Neither parameter can be clearly identified with strength in a manner analogous to the Z2/r acid strength relationship.

Both parameters are strength parameters measuring strength of different types.
  • E parameters reflect the ability to participate in electrostatic interactions
  • C parameters reflect covalent (orbital overlap) tendencies
E values may be compared to other E values but not the C values.

Comparing CA of I2 to CA of SbCl5 shows SbCl5 is better at covalent bonding than I2.

Comparing EA of I2 to EA of SbCl5 shows SbCl5 is better at covalent bonding than I2.

In this case SbCl5 can be concluded to be an overall stronger Lewis acid than I2 because it is superior in both CA and Ea