Complex Ion Formation Question

[betterfuture]

So I am reading up on complex ion formation and it is basically a coordination compound where a cation is bonded to an electron pair (ligand). The cation acts as a Lewis acid and the ligand acts as a Lewis base.

My question is do the cations form a lot of bonds to a ligand because these cations are usually transition metals. Meaning, they have d orbitals and can therefore form extra bonds? Cause I am not understanding how a cation can have like 6 bonds. I know this isn't like a normal metal non-metal ionic bonding. It's a coordinant covalent bond. But is that the reason for the multiple bonds? Thanks!!

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[aldol16]

In metal-ligand bonds, the metal uses d-orbitals for bonding. So coordinate covalent bonds are sigma bonds (you can also have pi donation or back-donation but that's beyond the scope of the MCAT). So for sigma bonds, you need to have head-on overlap of orbitals. So say you have six ligands, each on the three coordinate axes. You can use the s, px, py, pz, d(z^2), d(x^2-y^2) orbitals for bonding. The others are non-bonding. That allows you to have six ligands maximum.

 

[betterfuture]

Thanks @aldol16. So basically to get back to my other question, are they almost always transition metals?

 

[aldol16]

Are what always transition metals?

 

[betterfuture]

The cations.

 

[aldol16]

Most of them you will see will be transition metals. Nature uses a lot of first-row transition metals so those will appear in the context of enzymes.

Formally, all you need are d-orbitals.

 

[betterfuture]

Oh, alright so like in the n=4 shell. Gotcha! Thanks.