Error 1 — $F^{-}$ is NOT a strong field ligand. Students assume that because fluorine is the most electronegative element, $F^{-}$ must cause the greatest crystal field splitting. Wrong. The spectrochemical series is not based on electronegativity. $F^{-}$ is a weak field ligand that causes small $\Delta o$. Strong field ligands ($CN^{-}$, CO) achieve large splitting through both σ-donation AND π-back-bonding.

Error 2 — Tetrahedral splitting is NOT inverted in magnitude, it's inverted in orbital labels AND smaller. The key pair of facts: (a) orbital order is inverted — e is lower, t_{2} is higher in tetrahedral (vs t_{2}g lower, eg higher in octahedral); (b) $\Delta t$ = 4/9 $\Delta o$. Students sometimes invert the order but forget the 4/9 factor, or know the 4/9 but forget the orbital inversion.

Error 3 — $KMnO_{4}$ in basic medium does NOT give $MnO_{2}$. $MnO_{2}$ (brown) is the neutral medium product. Basic medium gives $MnO_{4}^{2-}$ (manganate, green). The colour green ↔ basic is the cue: Base = Green = $MnO_{4}^{2-}$.

Error 4 — Not all f-block elements are radioactive. All actinoids are radioactive. Most lanthanoids are NOT radioactive (only Pm is radioactive among lanthanoids). A question asking "which f-block elements are all radioactive" → answer is actinoids only.

Error 5 — Linkage isomers vs ionisation isomers are confused. Linkage isomerism: different donor atom of an ambidentate ligand (same ions inside and outside, different bonding). Ionisation isomerism: same atoms total, but a ligand and a counter-ion swap positions. To distinguish: ask whether the complex ion changes. If yes → ionisation. If the complex ion is the same formula but bonds differently → linkage.

Error 6 — Werner's primary vs secondary valence. Primary valence = oxidation state = ionisable (these are the counter-ions). Secondary valence = coordination number = NON-ionisable. Students frequently flip this and write that coordination number is ionisable.