Question

Explain me all exceptions in Coordination Compounds

Answer 1

The exceptions in coordination compounds include:

1. Exceptions to the 18‑electron rule: Many stable complexes (especially with early transition metals) exist with electron counts less than 18.

2. Jahn–Teller distortions: Particularly in d⁹ complexes (e.g., Cu(II)), which do not conform to ideal geometries.

3. High‑spin vs. low‑spin anomalies: Occur when ligand strengths and pairing energies do not lead to standard predictions.

4. Geometrical isomerism anomalies: Kinetic and steric factors can result in fewer isomers than expected.

5. Oxidation state and non‑innocent ligand effects: These lead to ambiguity in electron counting and deviations from the simple ionic picture.

Answer 2

Below is a concise explanation covering the key “exceptions” encountered in coordination compounds that you should be aware of in JEE/NEET-level studies.

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Solution Explanation (Minimal Core Steps):

1. 18‐Electron Rule Exceptions:
   Although many organometallics stabilize by reaching an 18‑electron count, early transition metals (or highly oxidized late metals) often form stable complexes with fewer than 18 electrons. Thus, the 18‑electron rule is a useful guideline but not absolute.

2. Jahn–Teller Distortion:
   Complexes with an uneven distribution of electrons in degenerate orbitals (e.g., d⁹ configuration in Cu(II) complexes) may distort from the predicted geometry to lower energy. This is an exception from the idealized symmetry predicted by simple crystal field theory.

3. High‑Spin vs. Low‑Spin Exceptions:
   While ligand field theory predicts high‑spin or low‑spin states based on ligand strength, there are cases where factors such as pairing energy, metal oxidation state, and overall complex geometry cause deviations from expected configurations.

4. Geometrical Isomerism Exceptions:
   Although predictions about isomerism (cis–trans, facial–meridional, optical isomers) for octahedral and square‐planar complexes are often straightforward, kinetic inertness or additional stabilization from chelation sometimes results in fewer observable isomers than symmetry arguments suggest.

5. Non‑Innocent Ligands and Oxidation State Ambiguity:
   In certain complexes, the electron distribution between the metal and ligand can be ambiguous. Such “non‑innocent” ligands complicate the assignment of a unique oxidation state to the metal center, deviating from the ionic model.

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Final Answer:
The exceptions in coordination compounds include:

1. Exceptions to the 18‑electron rule: Many stable complexes (especially with early transition metals) exist with electron counts less than 18.
2. Jahn–Teller distortions: Particularly in d⁹ complexes (e.g., Cu(II)), which do not conform to ideal geometries.
3. High‑spin vs. low‑spin anomalies: Occur when ligand strengths and pairing energies do not lead to standard predictions.
4. Geometrical isomerism anomalies: Kinetic and steric factors can result in fewer isomers than expected.
5. Oxidation state and non‑innocent ligand effects: These lead to ambiguity in electron counting and deviations from the simple ionic picture.

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This summary highlights the main exceptions that deviates from the classical rules and predictions in coordination chemistry, which are important for a clear understanding in competitive exams.