Bipyridyl complex isomers
These are examples of enantiomerism or optical isomerism. If you imagine a mirror placed between the two images you will see that they are mirror images of each other. If you try to rotate them to match up (easiest to do with a model, either physical or virtual) you will find that they are non-superimposable and this is what gives them their isomeric properties. The two isomers rotate plane polarised light in opposite directions (this is where the name, optical isomerism, comes from).

The $\Delta$ and $\Lambda$ labels distinguish the two isomers by their absolute stereochemical configuration.

An decent explanation of this type of isomerism can be found here.