d-Block, f-Block Elements & Coordination Compounds

This session covers the single highest-yield inorganic topic in NEET. d-Block elements (transition metals) occupy Groups 3-12 with general outer configuration (n-1)$d^{1}$-10 $ns^{0}$-2. Two critical configuration anomalies exist: Cr is [Ar]$3d^{5} \; 4s^{1}$ (not $3d^{4} \; 4s^{2}$) due to half-filled d-orbital stability, and Cu is [Ar]$3d^{10} \; 4s^{1}$ (not $3d^{9} \; 4s^{2}$) due to fully-filled d-orbital stability. Transition metals exhibit variable oxidation states, form colored ions (d-d electronic transitions in incompletely filled d-orbitals absorb visible light), are paramagnetic (unpaired electrons), act as catalysts (variable oxidation states and surface adsorption), and form coordination compounds.

KMnO₄ (potassium permanganate) behaves differently in each medium: in acidic medium, MnO₄^- → $Mn^{2}$+ (colorless, +7 to +2, gains 5 electrons); in neutral medium, MnO₄^- → MnO₂ (brown precipitate, +7 to +4, gains 3 electrons); in basic medium, MnO₄^- → $MnO4^{2}$- (green, +7 to +6, gains 1 electron). K₂Cr₂O₇ in acidic medium: $Cr2O7^{2}$- → $2Cr^{3}$+ (orange to green, +6 to +3).
The chromate-dichromate equilibrium is pH-dependent: $CrO4^{2}$- (yellow, basic) <=> $Cr2O7^{2}$- (orange, acidic).

f-Block elements: Lanthanoids (4f series) predominantly show +3 oxidation state. Lanthanoid contraction is the gradual decrease in ionic radii from $La^{3}$+ to $Lu^{3}$+ due to poor shielding by 4f electrons. This contraction makes 4d and 5d elements of the same group nearly identical in size (e.g., Zr and Hf). Actinoids (5f series) show more variable oxidation states (+3 to +6) and are all radioactive.

Coordination Compounds: Werner's theory distinguishes primary valence (oxidation state, ionizable) from secondary valence (coordination number, non-ionizable, directional). Ligands classified by denticity: monodentate (H₂O, NH₃, Cl^-, CN^-, CO), bidentate (en = ethylenediamine, $ox^{2}$- = oxalate), polydentate (EDTA = hexadentate), and ambidentate (NO₂^-/ONO^-, SCN^-/NCS^-). IUPAC nomenclature lists ligands alphabetically (anionic suffix -o, neutral as-is except aqua, ammine, carbonyl, nitrosyl), metal with Roman numeral oxidation state, cation before anion.

Isomerism types include: geometrical (cis/trans in square planar MA₂B₂ and octahedral MA₄B₂), optical (non-superimposable mirror images, e.g., [Co(en)₃]³⁺), linkage (NO₂ vs ONO), and ionization ([Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br).

VBT distinguishes inner orbital complexes (using (n-1)d orbitals — strong field, low spin) from outer orbital complexes (using nd — weak field, high spin). CFT explains crystal field splitting: in octahedral complexes, d-orbitals split into lower t2g and higher eg with splitting energy $\Delta$-o; in tetrahedral complexes, splitting is inverted (e lower, t2 higher) with $\Delta$-t approximately equal to $\frac{4}{9}$ $\Delta$-o. The spectrochemical series orders ligands: I^- < Br^- < Cl^- < F^- < OH^- < H₂O < NH₃ < en < NO₂^- < CN^- < CO. Strong field ligands (CN^-, CO) cause large $\Delta$-o, electron pairing, and diamagnetism.

Color arises from d-d transitions: energy of absorbed light equals $\Delta$-o, and the complementary color is observed.
Magnetic moment $\mu$ = $\sqrt{n(n+2}$) BM. Biological coordination compounds include hemoglobin ($Fe^{2}$+), chlorophyll ($Mg^{2}$+), vitamin B₁₂ ($Co^{3}$+), and carbonic anhydrase ($Zn^{2}$+).

The key testable concept is the crystal field splitting pattern in octahedral versus tetrahedral complexes, the spectrochemical series for predicting spin state, and KMnO₄ product identification based on the reaction medium.