For the Fe-II atom, we have a 4-fold rotation axis that goes through the Fe-II atom and the N atom at the center. We also have two 2-fold rotation axes that are perpendicular to two of the sides of the face of the cubus if that makes sense? As a result we can take the 4-fold rotation axis as the z-axis and take eta = 0.
For the Fe-I atom, we have three 4-fold rotation axes that go through the Fe-I atom and the three other nearest Fe-I atoms. Hence the EFG tensor is zero.