Lecture 20: Metal Ions in Solution

Read section 4.4 from your textbook.

Element Oxides in Water

The main group element oxides have strong,covalent bonds to oxygen but the element oxygen bond is typically polar. The elements are electron poor because of the polar bonds and they react with the nucleophile, water. This generates an acidic solution.

The electropositive metals form oxides with a high ionic character. The oxygen anions react with the acid, water, to generate a basic solution.
Water and other Lewis bases coordinate to metal cation and form complexes.

Aquo Complexes in Water

Water is more acidic when it is coordinated to a metal because the electropositive metal stabilizes the negative charge of the conjugate base, HO-



The acidity of the coordinated water increases along with the size and charge of the metal.



Coordination Geometry

Coordination geometries from 1-7 are possible but 4, 5, and 6 are most common.
    2 coordinate
    3 coordinate
    4 coordinate
    5 coordinate
    6 coordinate
    7 coordinate

Electronic Bookkeeping

We calculate total electron count and oxidation state for metal complexes just as we did for simple molecules. It will be important to calulate the number of electrons donated by a ligand.

For the total electron count, we need to know how many electrons each ligand donates to the central atom. First, draw the Lewis structure of the ligand. Use only the sum of the valence electrons of the atoms on the ligand.



Let's look at the total electron count of this iron complex. Remember to ADD electrons if there is a negative charge and SUBTRACT electrons for a positive charge.



To calculate oxidation state of the central atom, break the bonds to give both electrons to the more electronegative element (always the ligand), then count the electrons around the central atom.