The d-block elements, also called transition metals, occupy Groups 3–12 of the periodic table and have the general outer electronic configuration (n−1)$d^{1-10}$ $ns^{0-2}$. Two critical anomalies dominate NEET questions: chromium adopts [Ar]3$d^{5}$ 4$s^{1}$ instead of the expected [Ar]3$d^{4}$ 4$s^{2}$ because the half-filled 3$d^{5}$ subshell carries maximum exchange energy, which provides greater stabilisation than a doubly occupied 4s. Similarly, copper adopts [Ar]3$d^{10}$ 4$s^{1}$ instead of [Ar]3$d^{9}$ 4$s^{2}$ because a fully filled 3$d^{10}$ subshell is maximally symmetric and stable. In both cases, nature sacrifices one 4s electron to achieve a more stable d-configuration.

Transition metals exhibit five defining characteristics that repeatedly appear in NEET questions. First, they show variable oxidation states because both (n−1)d and ns electrons have similar energies and participate in bonding — manganese displays the widest range from +2 to +7. Second, their ions are coloured: when the d-subshell is partially filled ($d^{1}$ to $d^{9}$), visible light promotes an electron from a lower d-orbital (t_{2}g) to a higher d-orbital (eg) in a d–d transition; the complementary colour of the absorbed wavelength is what we observe. Ions with $d^{0}$ ($Sc^{3+}$) or $d^{10}$ ($Zn^{2+}$) configurations are colourless because d–d transitions cannot occur. Third, unpaired d-electrons make these ions paramagnetic, with the spin-only magnetic moment given by $\mu = \sqrt{n(n+2)}\text{ BM}$. Fourth, variable oxidation states and available d-orbitals make transition metals excellent catalysts (iron in the Haber process, $V_{2}O_{5}$ in the Contact process). Fifth, small ionic size and high charge density favour coordination compound formation.

Potassium permanganate ($KMnO_{4}$) behaves as a powerful oxidising agent in all media, but its reduction products depend on the medium. In acidic medium ($H_{2}SO_{4}$), $MnO_{4}^{-}$ is reduced to $Mn^{2+}$ (colourless), with Mn going from +7 to +2 — a gain of 5 electrons. In neutral medium, the product is $MnO_{2}$ (brown precipitate), with Mn going from +7 to +4 — a gain of 3 electrons. In basic medium (NaOH), the product is $MnO_{4}^{2-}$ (manganate, green), with Mn going from +7 to +6 — a gain of only 1 electron. The colour changes — purple to colourless, brown, or green — are the key identifiers in NEET questions. Potassium dichromate ($K_{2}Cr_{2}O_{7}$) in acidic medium reduces $Cr_{2}O_{7}^{2-}$ (orange, +6) to $Cr^{3+}$ (green, +3). The chromate-dichromate equilibrium is pH-dependent: in basic solution, the stable form is $CrO_{4}^{2-}$ (yellow); acidification converts it to $Cr_{2}O_{7}^{2-}$ (orange).

The f-block elements (lanthanoids and actinoids) fill the 4f and 5f subshells respectively. Lanthanoids predominantly show the +3 oxidation state, though $Ce^{4+}$ and $Eu^{2+}$ are exceptions. The most tested f-block concept is lanthanoid contraction: as electrons are added to 4f orbitals from La to Lu, the poor shielding efficiency of 4f electrons causes the effective nuclear charge felt by outer electrons to increase progressively. This causes a steady decrease in ionic radius from $La^{3+}$ (116 pm) to $Lu^{3+}$ (85 pm). The critical consequence is that 4d and 5d elements in the same group become nearly identical in size — Zr (72 pm) and Hf (71 pm) in Group 4 are the classic example, making them the most difficult pair of elements to separate chemically. Actinoids fill the 5f subshell, show more variable oxidation states (+3 to +6), and are all radioactive.

Coordination compounds are the largest section of this topic. Werner's theory distinguishes primary valence (equal to oxidation state, ionisable, satisfied by anions) from secondary valence (equal to coordination number, non-ionisable, directional, satisfied by ligands). Ligands are classified by denticity: monodentate ligands ($H_{2}O$, $NH_{3}$, $Cl^{-}$, $CN^{-}$, CO) donate one lone pair; bidentate ligands (ethylenediamine/en, oxalate/$ox^{2-}$) donate two; EDTA is hexadentate with four carboxylate and two amine donor groups. Ambidentate ligands ($NO_{2}^{-}$, S$CN^{-}$) can donate through two different atoms, giving rise to linkage isomerism.

IUPAC nomenclature requires ligands to be named alphabetically (ignoring multiplying prefixes), anionic ligands receive the suffix -o, and neutral ligands use special names (aqua for $H_{2}O$, ammine for $NH_{3}$, carbonyl for CO). The metal follows with its oxidation state in Roman numerals. For anionic complexes, the metal takes a Latin root and the suffix -ate. The overall cation is named before the anion.

Four types of isomerism arise in coordination compounds: geometrical (cis/trans arrangements in square planar $MA_{2}B_{2}$ and octahedral $MA_{4}B_{2}$), optical (non-superimposable mirror images in chelate complexes such as [Co(en)_{3}]^{3+}), linkage (different donor atom of an ambidentate ligand), and ionisation (exchange of a ligand and a counter-ion).

Crystal Field Theory (CFT) explains colour and magnetism quantitatively. In an octahedral field, d-orbitals split into a lower t_{2}g set (dxy, dxz, dyz) and a higher eg set ($dz^{2}$, $dx^{2}$-$y^{2}$), with splitting energy $\Delta o$. In a tetrahedral field, the splitting is inverted — the e set is lower and t_{2} is higher — and the magnitude is $\Delta t$ ≈ 4/9 $\Delta o$. If $\Delta o$ exceeds the pairing energy (P), electrons pair in t_{2}g → low spin; if $\Delta o$ < P, electrons distribute maximally → high spin. The spectrochemical series ranks ligands by their ability to produce $\Delta o$: $I^{-}$ < $Br^{-}$ < $Cl^{-}$ < $F^{-}$ < $OH^{-}$ < $H_{2}O$ < $NH_{3}$ < en < $NO_{2}^{-}$ < $CN^{-}$ < CO. Strong field ligands ($CN^{-}$, CO) cause large $\Delta o$ and pairing; weak field ligands ($Cl^{-}$, $F^{-}$, $H_{2}O$) cause small $\Delta o$ and no pairing. Valence Bond Theory (VBT) offers a complementary picture: inner orbital complexes use (n−1)d orbitals (d^{2}$$sp^{3} hybridisation, strong field, low spin), while outer orbital complexes use nd orbitals (sp^{3}$$d^{2} hybridisation, weak field, high spin).

Biologically, coordination chemistry sustains life: haemoglobin ($Fe^{2+}$) transports oxygen, chlorophyll ($Mg^{2+}$) drives photosynthesis, vitamin $B_{12}$ ($Co^{3+}$) enables DNA synthesis, and carbonic anhydrase ($Zn^{2+}$) catalyses $CO_{2}$ hydration. Cisplatin (cis-[Pt($NH_{3}$)_{2}$Cl_{2}$]) is a coordination compound used as an anticancer drug — its cis geometry uniquely allows formation of intrastrand DNA crosslinks that block replication and trigger apoptosis.