Textbook Question
The [Ti(NCS)6]3- ion exhibits a single absorption band at 544 nm. Calculate the crystal field splitting energy ∆ in kJ/mol. In NCS- a stronger or weaker field ligand than water? Predict the color of [Ti(NCS)6]3-.
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24. Transition Metals and Coordination Compounds
Strong-Field vs Weak-Field Ligands
Multiple Choice
Arrange the following complexes in an ascending order of the magnitude of Δ.
a) [Cr(NH3)6]2+
b) [Cr(en)3]2+
c) [Cr(NO2)6]4–
d) [Cr(H2O)4Cl2]
A
a < c < b < d
B
c < b < a < d
C
d < a < b < c
D
b < c < d < a
Verified step by step guidance1
Identify the ligands in each complex and their positions in the spectrochemical series. The spectrochemical series ranks ligands based on their ability to split the d-orbitals of the central metal ion, which affects the magnitude of Δ (crystal field splitting energy).
List the ligands in the given complexes: [Cr(NH3)6]2+ has NH3, [Cr(en)3]2+ has ethylenediamine (en), [Cr(NO2)6]4– has NO2-, and [Cr(H2O)4Cl2] has H2O and Cl-.
Consult the spectrochemical series: NO2- is a strong field ligand, en (ethylenediamine) is a strong field ligand, NH3 is a moderate field ligand, and H2O and Cl- are weak field ligands.
Rank the complexes based on the strength of their ligands: Stronger field ligands result in a larger Δ. Therefore, [Cr(NO2)6]4– should have the largest Δ, followed by [Cr(en)3]2+, then [Cr(NH3)6]2+, and finally [Cr(H2O)4Cl2] with the smallest Δ.
Arrange the complexes in ascending order of Δ: [Cr(H2O)4Cl2] < [Cr(NH3)6]2+ < [Cr(en)3]2+ < [Cr(NO2)6]4–.
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