Arranging d6 Electrons in t2g and eg Orbitals
In the Coordination Compounds chapter, Crystal Field Theory (CFT) is one of the most critical concepts for JEE and NEET. When ligands approach a central metal ion in an octahedral complex, the 5 degenerate d-orbitals split into two distinct energy levels: the lower-energy t2g set and the higher-energy eg set.
The real challenge arises when we have to arrange electrons. For d1 to d3, it's simple—they all go into the t2g level. But what happens at the 4th, 5th, and 6th electron? Does it pair up below, or does it jump to the higher level?
Video Tutorial: The d6 Configuration
Watch Abhishek Sengar sir from CHEMCA explain exactly how the competition between Crystal Field Splitting Energy (Δo) and Pairing Energy (P) dictates the fate of a d6 complex.
High Spin vs. Low Spin (The CFT Battle)
The arrangement of electrons from the 4th electron onwards depends entirely on the strength of the ligand attached to the central metal ion.
Electrons are lazy. They want to exist in the lowest possible energy state. They must choose between paying the "fee" to jump up to the eg level (Δo), or paying the "fee" to squeeze into an already occupied t2g orbital (Pairing Energy, P). They will always choose the cheaper option!
1. Weak Field Ligand → High Spin Complex
- Weak ligands (like Halogens, H2O) create a very small energy gap. So, Δo < P.
- Because jumping up is "cheaper" than pairing up, the 4th and 5th electrons jump to the eg level.
- The 6th electron has no choice but to pair up in the t2g level.
- Final Configuration: t2g4 eg2
2. Strong Field Ligand → Low Spin Complex
- Strong ligands (like CN-, CO, NH3) create a massive energy gap. So, Δo > P.
- Because pairing up is "cheaper" than jumping across the massive gap, the 4th, 5th, and 6th electrons all pair up in the t2g level. The eg level remains completely empty.
- Final Configuration: t2g6 eg0
Practice Questions for JEE & NEET
Ligand strength doesn't just dictate electron placement; it fundamentally alters the magnetic properties of the complex.
Question 1: Compare the magnetic behavior of [Fe(H2O)6]2+ and [Fe(CN)6]4-. Both central metals have a d6 configuration.
Answer:
- [Fe(H2O)6]2+: H2O is a weak field ligand. It forms a high spin complex (t2g4 eg2). As seen in the diagram above, there are 4 unpaired electrons. Therefore, it is highly paramagnetic.
- [Fe(CN)6]4-: CN- is a strong field ligand. It forms a low spin complex (t2g6 eg0). As seen in the diagram, all electrons are paired up. Therefore, it is diamagnetic.
Question 2: What will be the configuration of a d5 ion placed in an octahedral field with a very strong ligand?
Answer: t2g5 eg0
Reasoning:
- A strong ligand means Δo > P (Low Spin complex).
- Electrons will prefer to pair up in the lower energy t2g level rather than crossing the large energy gap.
- Electrons 1, 2, and 3 fill the t2g singly. Electrons 4 and 5 pair up in the first two t2g orbitals. The eg level remains empty.
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