Find tetrahedral and square planar geometries helps to determine the low versus high spin of complexes. Here we can say that if you have a square planar geometry that is associated with a large crystal field splitting energy or Δ. Now that just means that the energy level between your two rows of orbitals is pretty high. So it would be a high cost for electrons to fill in the first row and then try to go up to the second row. They'd rather stay there on the bottom row and fill in as much of the orbitals as they can.
In this case, they're going to stay in the lower level. So we're going to say this is a low spin complex. We're going to say because it's a low spin complex for squared planar, this will give us a diamagnetic electron orbital diagram. Now if you're a tetrahedral, so we can assume since square planar is that way, tetrahedral would be what the opposite. So here with tetrahedral, that's associated with a small Δ.
A small Δ here would mean that there's not much difference between the energy levels, so you would treat all the orbitals as degenerate having the same energy. So that would give us a high spin complex, and we'd say with a high spin complex we typically are a pair of magnetic species. So the take away from this is if you have square planar, we have low spin complexes and they are diamagnetic, and if you have tetrahedral they tend to be high spin complexes that lead to paramagnetic species, right?
So keep this in mind when we're looking for the geometry of different types of complex ions.