Which of these ions will have a zero crystal field splitting energy in an octahedral complex?
Answers
Right from the start, you can eliminate options (1) and (3) because those ions form a low spin complex.
Now, I will not go into too much detail about crystal field theory in general because I assume you're familiar with it.
So, you know that transition metal ions placed in symmetric fields have degenerate d-orbitals that are higher in energy than they would have been in an isolated cation.
When you place such a cation in a field with octahedral symmetry, the five degenerate d-orbitals will split into two
eg orbitals, which are higher in energy, and three t2g orbitals, which are lower in energy.
The crystal field stabilization energy, or CFSE,
Δ
, is defined as the stability gained by the ion after placing it in a crystal field.
The idea here is that electrons placed in the
t
2
g
orbitals will increase the stability of the ion because these orbitals are lower in energy than the degenerate d-orbitals.
On the other hand, electrons placed in the
e
g
orbitals will reduce the stability of the ion because the orbitals are higher in energy than the degenerate d-orbitals.
A low field complex is characterized by the fact that the electrons found in the d-orbitals are all placed in the lower energy t2g orbitals. This means that the CFSE for such an ion cannot be equal to zero, since the ion is gaining stability relative to the initial energy level of the d-orbitals.