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How many different possible structures (isomers) are there for the complex, $\ce{[Mn(OH2)3(NH3)3]^{3+}}$?

I really don't know how to begin with this. I know the answer is 2, but i don't have the faintest idea how this is the case. Since $\ce{Mn}$ has a charge of $3+$, I know that its valence electron configuration is $3d^4$. I also know that $\ce{NH3}$ and $\ce{H2O}$ are monodentate, which means they contribute 1 lone pair.

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Try sketching a few possible configurations. – Dissenter Aug 4 '14 at 1:22
up vote 5 down vote accepted

Metal complexes with three each of two types of ligands $(\ce{MX3Y3})$ have two isomers: mer- and fac-isomers. For example, for $\ce{Co(NH3)3Cl3}$ there are two isomers:

Fac for facial, in which all three ligands of each type are clustered on a single face of the octahedron and the three other three ligands are on the opposite face.

enter image description here

Mer for meridional, in which the three ligands of each type are arranged meridianally in a row around the metal and the other three ligands are arranged likewise on the opposite side in a perpendicular meridian.

enter image description here

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The compound you talked about is of the type $MA_3B_3$. I will illustrate a similar example here. There shall be 2 geometrical isomers, 'FACIAL' and 'MERIDONAL'

enter image description here

To identify which is which, remember that in MERIDONAL maximum bond angles between the groups is 180 whereas in facial it is lesser than that. Also none of them is optically active as both have a Plane of symmetry (merdional has diagonal POS whereas facial has horizontal).

Therefore the compound has total 2 STRUCTURAL ISOMERS (2 GEOMETRICAL + 0 OPTICAL).

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