The Lewis Acid-Base Concept:Coordination Compounds
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
Charged Complexes vs Uncharged ComplexesCharged 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.
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.
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 EffectsAlthough 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.
The Chelate EffectConsider 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 EffectConsider 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 ReactionThe enthalpy change for a Lewis acid-base reaction can be predicted using the Drago-Wayland equation.
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.
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