For octahedral complexes having the general formula MA2B2, either A or B must be bidentate.
Bidentate ligands are coordinated to the central metal at two adjacent positions.
There are two ways in which two bidentate ligands can be coordinated in an octahedral complex.
The trans isomer is symmetrical as the two water molecules are on either side of a mirror plane passing through Cr and the oxalate ligands.
The cis isomer is asymmetric.
Asymmetric complexes exist as enantiomers. These are stereoisomers that are nonsuperimposable mirror images of one another.
Enantiomers have the same physical properties (colour, melting point.....) EXCEPT their solutions rotate plane-polarised light in opposite directions.
Bidentate ligands are coordinated to the central metal at two adjacent positions.
This is shown by the two complexes of chromium(III) involving the bidentate ligand oxalate (–O2C—CO2–).
There are two ways in which two bidentate ligands can be coordinated in an octahedral complex.
In one the second pair of ligands (H2O in the complex shown below) are at positions cis to one another.
in the other, the second pair of ligands occupies positions trans to one another.
in the other, the second pair of ligands occupies positions trans to one another.
 trans |
 cis - one enantiomer |  cis - the second enantiomer |
The trans isomer is symmetrical as the two water molecules are on either side of a mirror plane passing through Cr and the oxalate ligands.
The cis isomer is asymmetric.
Asymmetric complexes exist as enantiomers. These are stereoisomers that are nonsuperimposable mirror images of one another.
Rotating the second enantiomer about the vertical axis (O-Cr-OH2) puts the horizontal oxalate group in a position overlapping with one oxalate group in the first enantiomer BUT the rotation brings the water molecule from the back to the front (no overlap with the first enantiomer because this water is at the back).
Enantiomers have the same physical properties (colour, melting point.....) EXCEPT their solutions rotate plane-polarised light in opposite directions.