Question

the Al(H₂O)₆³⁺ ion is formed when an Al³⁺ ion acting as a Lewis acid picks up six pairs of electrons from neighboring water molecules acting as Lewis bases to give an acid-base complex, or complex ion. Question: Please explain in detail "why" AL³⁺ picks up "6" H2O's. where does the 6 come from?

Answer 1

Whenever most metallic ions becomes aqueous they like to suck up whatever free electrons are floatly around. Al³⁺ is an interesting ion since it is the smallest metal with a large positive ionic form (+3). This is where the term "high charge density" comes from, a very small element with a very high charge.

H₂O molecules come in and surround the Al³⁺, pointing the O ends toward it and the H ends away. Water is polar and it has a partial negative charge on the oxygen, so that extra electron density helps to stabilize the Al³⁺ ion.

Actually whats happens is

[Al(H₂O)₆]³⁺ (aq) [Al(H₂O)₅(OH)]²⁺ (aq) + H⁺ (aq)

Al³⁺ REALLY wants those electrons on O, and it pulls them closer to itself. That means the electrons from water get pulled even further toward the O end and away from the H end. (That's what it means when it says it polarizes the water.) But the O is losing elecrton density too, because the electrons are going right past it and into the Al³⁺. So the O pulls the electrons from one of the O-H bonds into it's orbit, making it O- (half of OH-). The H leaves as H+, making the solution acidic. At that point, the Al3+ has five neutral water molecules and one OH- molecule bound to it, so the overall complex is [Al(H₂O)₅OH]²⁺.