# Ionic or covalent in tetrahydroxoaluminate ion?

1. Feb 19, 2012

### Bipolarity

Consider the metal-ligand complex of the tetrahydroxoaluminate ion. $$[Al(OH)_{4}]^-$$

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:

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

I can also think of it as being covalent:

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

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?

Thanks!

BiP

2. Feb 19, 2012

### xplosiv3s

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...

http://www.see.murdoch.edu.au/info/student/chemtutorials/equations.html

click on the link and look for equations 27-29