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Ionic or covalent in tetrahydroxoaluminate ion?

by Bipolarity
Tags: covalent, ionic
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Feb19-12, 10:04 AM
P: 783
Consider the metal-ligand complex of the tetrahydroxoaluminate ion. [tex][Al(OH)_{4}]^-[/tex]

In this ion, the aluminium is bonded with four hydroxides where each hydroxide is a ligand.

My question:
Is the aluminium-hydroxide bond ionic or covalent?

I can think of the ionic one working in someway like this:

[tex]Al^{+3} + 4(OH)^- \rightarrow [Al(OH)_{4}]^- [/tex]

I can also think of it as being covalent:

[tex]Al + 3(OH) + (OH)^- \rightarrow [Al(OH)_{4}]^- [/tex]

Which would be more "correct"?
Aluminium is a metal, so you would expect it to form ionic bonds right? But I heard of instances where metals form covalent bonds, especially in metal complexes. So I hope I can clear this confusion in my head somehow. Also, will a complete octet be formed in both cases?


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Feb19-12, 12:42 PM
P: 15
I would go for covalent bonding it seems more appropriate, Hydroxide ions do posses a lone pair of electrons. Hence they can provide both the electrons to form a dative covalent bond.

Metal complexes do commonly form covalent bonds, especially transition metals. the metal complex will form a tetrahedral structure in this case hydroxide ions act as ligands...

click on the link and look for equations 27-29

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