High Spin and Low Spin Complexes

Transition metal complexes can exist as high spin or low spin depending on the strength of the ligands.
Let's understand how the strength of ligands affect the spin of the complex.
This concept involving high spin and low spin complexes is not in A Level Chemistry syllabus but has appeared in some Prelim questions.
1. Gaseous Fe(III) cation
The electronic configuration for Fe3+ is given as 1s2 2s2 2p6 3s2 3p6 3d5
We can also determine the electron in box diagram for 3d subshell.
Notice there are 5 unpaired electrons in 3d subshell for Fe3+.
2. Hexaaquairon(III) complex
Since oxidation state of iron is still +3, there are still 5 electrons in 3d subshell in [Fe(H2O)6]3+ complex.
In a complex the ligands will interact with the d orbitals to different extent depending on the shape of the complex.
Therefore the d orbitals that interact more with the ligands will have a higher d* energy level, while the d orbitals that interact less will have a lower d energy level.
This is known as d-d* orbital splitting.
For octahedral complexes, the splitting pattern is 2 orbitals at higher d* level and 3 orbitals at lower d level.
Water is a weak ligand and the energy gap between d to d* level is small.
Hence the d electrons will ignore the small energy difference and be filled in the same way as in gaseous Fe3+ cation, where electrons will occupy orbitals singly and with parallel spins.
Notice there are 5 unpaired electrons, hence hexaaquairon(III) complex is considered a high spin complex.
3. Hexacyanoferrate(III) complex
CN- is a strong ligand and will cause the energy gap between d to d* level to be larger.
It requires too much energy to put the d electrons at the higher d* level, so electrons will pair up at the lower d level first.
Notice there is now only 1 unpaired electron, hence hexacyanoferrate(III) complex is considered a low spin complex.
Comparing both high spin and low spin complexes:
a weak ligand such as H2O will cause a smaller d-d* energy gap and tend to form high spin complexes
a strong ligand such as CN- will cause a larger d-d* energy gap and tend to form low spin complexes
Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore
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Thank you for helping me understand this concept. This specifically was asked in my exam and the examiner was pleased by my answer because there were no follow-up questions (big praise where I am from😂)

[@apratimtewari4288]

Hi, can you explain what is the minimum amount of unpaired electrons needed for complex to be called a high spin one?

[@nikunjabiharisarkar6747]

I am from India! Was struggling about this concept for a week.... Thanks for helping.

[@ChemistryGuru]

Thanks! Glad you found it helpful 😄😄

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[@pabitrakumargouda6119]

Hexaaquachromium(3) is a low spin or high spin complex ?

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Thank you for the help. I have been struggling with this topic for a while and your video helped me understand it. Thank you.

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[@ChemistryGuru]

You are most welcome Kokila! Glad you found the video useful :)

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[@AS-mm4pn]

Very helpful sir.👍Love from India🇮🇳. But I have doubt. Is it always that a weak field ligand results into a high spin complex and a strong field ligand into a low spin complex, irrespective of the central metal atom?Can I take it as a general rule?How can we relate this with hybridisation and shape?

[@ChemistryGuru]

Hi Aritra thank you! Yes I think in general stronger ligands will lead to bigger difference in energy gap so will tend to form low spin complexes. We should be able to use this as a general rule but in Chemistry there will always be exceptions so if we encounter something unusual then we can try to explain it using other concepts. 😄

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[@ChemistryGuru]

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[@garvsenshivpuri9891]

[Cr (NH3)6]+3 is not low spin

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[@audreyx_x]

Hello! This video is amazing it definitely enlightens me a lot! But one doubt will be, how do you know if one is in the tetrahedral form or octahedral?

[@ChemistryGuru]

Hi Audrey deducing the shape of complexes is not in A Level Chemistry Syllabus. So the default shape if coordination number is 6 will be octahedral. If coordination number is 4, the shape usually will be tetrahedral eg CuCl4^2-, but sometimes can be square planar eg PtCl4^2-. But the question will give us the shape or formula of the complex so we can deduce from there :)

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Chemistry Guru - #1 JC, A Level, H2 Chemistry Tuition okay understood! thank you for the super prompt and clear reply!! :D

[@ChemistryGuru]

@@audreyx_x you are welcome Audrey! :)

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[@brambeekmans]

i have a question and i can't find an answer anywhere. what determines the strength of the ligand? please help

[@ChemistryGuru]

I think the strength of ligand is related to how well the ligand can interact with the d-orbitals and therefore can stabilise/destabilise the d-orbitals to different extent. But we don't need to explain that. For monodentate ligands we just need to know CO and CN- are very strong ligands compared to others like H2O, NH3 and OH-. Polydentate ligands are stronger ligands than monodentate ligands due to the chelate effect - the first lone pair donated will increase the chances of the subsequent donations as the donor atoms of the polydentate ligand will be in closer proximity to the metal centre.

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From india

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