Isomerism - Real 3D Effects

Only in the case where all ligands are identical monodentates with a single common donor atom, is the situation simple and the prospect of positional isomers removed for 

Despite first appearances, the molecules at the left and right are identical, since simple rotation by 120° around the marked axis as shown in the central view converts left view into right view. They are not isomers, merely different orientations of the same molecule in space. The need to consider orientation issues in three dimensions complicates decisions on isomers. 

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