# How to find the ligand field splitting of d orbitals in a square planar complex?

I am working with a Iron square planar complex with $$\ce{NH2-}$$ and $$\ce{OH-}$$ ligands having cis and trans conformation. My question is how to theoretically/computationally calculate the ligand filed splitting of the orbitals of Fe specifically?

• Chem+Math Expression formatting reference: MathJax Basics / Chem+Math expressions/formulas/equations / Upright vs italic / Math SE Mathjax tutorial Commented Jul 7, 2023 at 11:29
• Are you sure of the existence of this iron complex ? Is it Iron(II) or Iron(III) ? Commented Jul 7, 2023 at 19:44
• @Maurice the compound probably doesn’t exist. We are making a toy model for computational reasons only. So it’s a wholly neutral system Commented Jul 8, 2023 at 10:23
• Why do you play with non-existent compounds ? Why don't you try to study a cyclic compound made of 6 nitrogen atoms, for example ? Commented Jul 8, 2023 at 16:13

I'm not an expert in this field yet, but I think Using Orca you can calculate the orbital energy numerically. I have used CASSCF as method of calculation. The input file I've prepared is as following

! def2-TZVPP SlowConv PAtom
! Allpop
! printMOs
! PAl2

%casscf
nel     6        #Numeber of electrons
norb    5        #Number of Orbitals in active space
mult    1,3,5    #Multiplicities (I've got this from Term symbols)
nroots  11,11,5  #Number of micro states for each Multiplicity
end

*xyz -2 5
Fe       0.0000000000      0.0000000000      0.0000000000
O        1.8600000000      0.0000000000      0.0000000000
O       -1.8600000000      0.0000000000      0.0000000000
N        0.0000000000      1.9400000000      0.0000000000
N        0.0000000000     -1.9400000000      0.0000000000
H       -0.8407460951      2.2854839793      0.5145942634
H        0.8414809543      2.2855771457      0.5133281192
H        2.1097954426      0.0911972135     -0.9540592676
H       -2.1097621967      0.0917132162     -0.9538732789
H        0.8411680767     -2.2854165305      0.5139464472
H       -0.8411632857     -2.2854156190      0.5139547890
*


the xyz section has the coordination for atoms in your complex and the CASSCF block contain the information about the active space The output file has a section for orbital energy and occupation number of each orbital For the bond length I've just searched on google for bond length of those atoms with Fe.

By analyzing the combination coefficients of orbitals, it is possible to obtain the energies required for these orbitals and subsequently calculate the splitting phenomenon.

By simply modifying the coordination in the input file, you can explore various configurations and calculate their corresponding properties. To achieve more accurate and reliable results, you may consider enhancing the basis set, which can lead to improved energy estimations

• Did you optimise the structure at all? The whole calculation seems a bit aimless and possibly overkill. I'd expect dft to give better starting guesses. But basically you're right though, given enough time and effort, this is the way to evaluate this... Commented Dec 23, 2023 at 15:00