Rank the crystal field splitting energy (Δo) of the following complexes in order from lowest to highest Δo. I. [CoBr6]3- II. [CoF6]3- III. [Co(NCS)6]3- IV. [Co(CN)6]3-

Question

BlackTom AI · Accepted Answer

For Co3+ in octahedral complexes, Δo is determined by the ligand’s position in the spectrochemical series (ability to produce σ- and π-interactions). Ligands that are strong π-acceptors (or strong σ-donors without π-donation) raise Δo; strong π-donors lower Δo. All complexes share the same metal and oxidation state (Co3+) and geometry, so only ligand effects matter.

Step-by-step reasoning:
1. [CoBr6]3- (Br−): Halides are generally weak-field ligands and are good π-donors (their lone pairs donate into metal t2g orbitals), which reduces the t2g–eg splitting. Among halides, the larger, more polarizable Br− donates more and is weaker-field than F−. Therefore [CoBr6]3- has the smallest Δo of the set.
2. [CoF6]3- (F−): Fluoride is a relatively small, highly electronegative ligand. It is a weaker π-donor than Br− (so it reduces t2g–eg splitting less), hence it produces a larger Δo than Br− but still is a relatively weak-field ligand compared with many others. So F− complex comes next.
3. [Co(NCS)6]3- (NCS−/SCN−): Thiocyanate is ambidentate but when bound through nitrogen it can act as a better π-acceptor/less π-donor than halides, leading to an intermediate-to-stronger field than F−. Consequently [Co(NCS)6]3- produces a larger Δo than the fluoride complex.
4. [Co(CN)6]3- (CN−): Cyanide is a very strong-field ligand and a strong π-acceptor, which greatly increases t2g–eg splitting; thus this complex has the largest Δo.

Why other choices are incorrect:
- b (II < IV < I < III) places F− lowest and CN− low, contradicting halide π-donor effects and CN−’s strong π-acceptor behavior.
- c (IV < III < II < I) makes CN− the weakest, which is opposite known spectrochemical order.
- d (III < I < IV < II) places NCS− as weakest and F− as strongest, inconsistent with NCS−’s greater ability to increase Δo vs halides.

Therefore the correct ordering from lowest to highest Δo is: [CoBr6]3- < [CoF6]3- < [Co(NCS)6]3- < [Co(CN)6]3-, which matches answer (a).

Rank the crystal field splitting energy (Δo) of the following complexes in order from lowest to highest Δo. I. [CoBr6]3- II. [CoF6]3- III. [Co(NCS)6]3- IV. [Co(CN)6]3-Single choice

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