Figure 2.4.1 – Formation of a Lewis acid-base complex.
Figure 2.4.1 – Formation of a Lewis acid-base complex.
An example of a coordination complex is hexaaquo cobalt dichloride, Co(H2O)6Cl2. This compound contains a Co2+ ion. This electrophilic metal ion is coordinated by six nucleophilic water ligands. Because the water molecules are neutral, the complex still has a 2+ charge overall. There are two chloride counterions to balance the charge, but the chlorides are not attached to the complex ion.
Notice that the usual Lewis conventions are usually abandoned in drawing coordination complexes of transition metals. With so many different nucleophiles sticking to the Lewis acid, the number of formal charges that must be drawn becomes very cumbersome. Usually the oxidation state of the metal cation is denoted. The oxidation state essentially means the charge on the metal cation and it is written in Roman numerals beside the metal atom. In addition, the overall charge on the Lewis acid-base complex is given, with square brackets indicating that the charge belongs to the entire complex within the brackets. Exactly where that charge resides is up to the reader to consider.
Coordination complexes are frequently useful in mining and metallurgy. For example, nickel can be extracted from nickel ore by converting the nickel into Ni(CO)4 via addition of carbon monoxide. Ni(CO)4, or tetracarbonyl nickel, is a gas that can be easily separated from the solid ore. When removed from the presence of carbon monoxide, the coordination complex decomposes back into Ni and CO.
Exercise 2.4.1
Problem 1. These questions concern the formation of tetracarbonyl nickel.
a) Based on what you know about transition metals, is the nickel most likely a Lewis acid or a Lewis base?
b) Draw the Lewis structure for carbon monoxide, CO. Make sure each atom has an octet. Make sure you add formal charges.
c) Based on formal charge considerations, which atom in carbon monoxide will bind to the nickel?
d) Using arrow notation, show the coordination of CO to Ni. Do this one step at a time, showng each CO molecule binding to the nickel and the new complex that results after each step.
e) Why does nickel bind four carbon monoxide ligands, and not three or five?
- Answer a):
- Lewis acid
- Answer b):
- Answer c):
- Carbon will be nucleophilic due to the lone pair electrons and the negative charge.
- Answer d):
- Answer e):
- Ni0 is d10 and with 4 CO ligands the nickel tetracarbonyl complex has 18 electrons.
Another example is seen in the liquid extraction of metals from ores. Often, a metal is coordinated by some kind of ligand that changes the solubility of the metal atom, so that it can be extracted from the ore with a solvent. The solvent is usually water, but other liquids could be used. One of the easiest cases to picture on paper is the leaching of gold from gold ores with cyanide.
Often, coordination of a ligand to a metal changes many properties of the metal, in addition to changing its solubility. Transition metals have different oxidation