Question

A classmate says, "A weak-field ligand usually means the complex is high spin." Is your classmate correct? Explain.

Short answer

Yes, your classmate is correct. A weak-field ligand results in a smaller crystal field splitting energy (Δ), which is more likely to lead to a high-spin complex. This is because when Δ is smaller than the pairing energy (P), electrons prefer to occupy higher-energy orbitals with parallel spins rather than pairing up in lower-energy orbitals.

Step-by-step solution

Understanding Weak-Field Ligands and High-Spin Complexes

Weak-field ligands are ligands that do not produce a large crystal field splitting energy (Δ). They result in smaller differences between the energy levels of the d orbitals in a transition metal complex. This means that d electrons will not be strongly stabilized by the ligand, and the ligand has a weaker ability to create a field around the metal ion. On the other hand, high-spin complexes are those in which electrons prefer to occupy higher-energy orbitals with parallel spins, rather than pairing up in lower-energy orbitals. This occurs when the crystal field splitting energy (Δ) is smaller than the pairing energy (P), which is the energy required for an electron to pair up in a lower-energy orbital (Δ < P).

Examining the Correlation between Weak-Field Ligands and High-Spin Complexes

A weak-field ligand, as mentioned earlier, results in a smaller crystal field splitting energy (Δ). When the splitting energy is small, it is more likely that the complex will be high-spin because the electrons would prefer occupying the higher-energy orbitals with parallel spins rather than pairing up in the lower-energy orbitals. In other words, if the ligands are weak-field, the electrons will not be stabilized by their interaction with the ligands, so they will require less energy to move to the next energy level (as compared to moving to a higher orbit in a strong-field complex). Thus, it's more probable that the complex will be high-spin with weak-field ligands.

Conclusion

Based on this explanation, it can be said that the classmate's statement is indeed correct. A weak-field ligand usually means the complex will be high-spin, given that the crystal field splitting energy is smaller than the pairing energy, making it more favorable for electrons to occupy higher-energy orbitals with parallel spins.