[Fe(CN)6]4- is a low spin complex.
Understanding Spin States in Coordination Complexes
In coordination chemistry, the "spin state" of a transition metal complex refers to the arrangement of electrons in the d-orbitals of the central metal ion. This arrangement is governed by the principles of Crystal Field Theory or Ligand Field Theory, which describe how the ligands interact with the metal's d-orbitals, causing them to split into different energy levels. For an octahedral complex like [Fe(CN)6]4-, the five degenerate d-orbitals split into two sets:
- t2g orbitals: Three lower-energy orbitals.
- eg orbitals: Two higher-energy orbitals.
The difference in energy between these two sets is known as the crystal field splitting energy (Δo). The spin state depends on the balance between Δo and the pairing energy (P), which is the energy required to pair two electrons in the same orbital.
- Low Spin: Occurs when Δo is large (strong field ligands). In this case, it is energetically more favorable for electrons to pair up in the lower-energy t2g orbitals before occupying the higher-energy eg orbitals, even if it means pairing electrons.
- High Spin: Occurs when Δo is small (weak field ligands). Here, it is energetically more favorable for electrons to occupy all available orbitals (t2g and eg) singly before pairing up, maximizing the number of unpaired electrons.
Analysis of [Fe(CN)6]4-
To determine the spin state of [Fe(CN)6]4-, we first identify the central metal ion's oxidation state and d-electron count, and then evaluate the ligand's strength.
1. Oxidation State and d-electron Count
In the complex [Fe(CN)6]4-, each cyanide (CN-) ligand carries a -1 charge, and there are six of them, contributing a total charge of -6. Since the overall charge of the complex is -4, the iron (Fe) must have an oxidation state of +2.
Fe(II) has an electron configuration of [Ar] 3d6. Therefore, [Fe(CN)6]4- is a d6 complex.
2. Ligand Field Strength
Cyanide (CN-) is known to be a strong field ligand. This means it causes a large crystal field splitting energy (Δo).
3. Spin State Conclusion
For a d6 metal ion, a strong field ligand like cyanide results in a large Δo that is greater than the pairing energy (Δo > P). Consequently, electrons will preferentially fill the lower-energy t2g orbitals first, pairing up before populating the higher-energy eg orbitals. This leads to a low spin configuration.
The electron configuration for a low spin d6 complex is t2g^6 eg^0, meaning all six d-electrons occupy the three t2g orbitals, with two electrons in each. This configuration results in zero unpaired electrons, making the complex diamagnetic. This specific arrangement means all bonding orbitals are filled, contributing to the complex's stability.
Key Properties and Related Complexes
The filled bonding orbitals in [Fe(CN)6]4- contribute to strong Fe-CN bonds. However, this complex can undergo interesting photochemistry. Irradiation with visible light can lead to the population of the eg* antibonding orbital. When electrons move into these antibonding orbitals, the Fe-CN bond strength is weakened, making the cyanide ligands more labile (easily replaced).
In contrast, consider [Fe(CN)6]3−. This complex contains Fe(III), which is a d5 system. Although it also features strong field cyanide ligands, it has only five electrons in the t2g bonding orbital (t2g^5 eg^0). This configuration results in a slightly weaker Fe-CN bond compared to [Fe(CN)6]4-, making [Fe(CN)6]3− more labile even without photoexcitation.
Comparison: High Spin vs. Low Spin for d6 Complexes
The table below summarizes the key differences for d6 complexes based on their spin state:
| Feature | Low Spin d6 Complex (e.g., [Fe(CN)6]4-) | High Spin d6 Complex (e.g., [Fe(H2O)6]2+) |
|---|---|---|
| Ligand Field Strength | Strong field ligands | Weak field ligands |
| Δo vs. P | Δo > P (large splitting) | Δo < P (small splitting) |
| Electron Configuration | t2g^6 eg^0 | t2g^4 eg^2 |
| Number of Unpaired Electrons | 0 (diamagnetic) | 4 (paramagnetic) |
| Bond Strength/Stability | Generally stronger, more stable bonds | Generally weaker, less stable bonds |
In conclusion, [Fe(CN)6]4- is unequivocally a low spin complex due to the d6 configuration of Fe(II) and the strong field nature of the cyanide ligand.
