Transition metals (Groups 3–12) have the configuration (n−1)$d^{1-10}$ $ns^{0-2}$ and show variable oxidation states because both d and s electrons participate in bonding. Two critical anomalies — Cr ([Ar]3$d^{5}$ 4$s^{1}$) and Cu ([Ar]3$d^{10}$ 4$s^{1}$) — arise from the extra stability of half-filled and fully-filled d-subshells. $KMnO_{4}$ is reduced to $Mn^{2+}$ in acidic medium (purple → colourless, 5$e^{-}$), $MnO_{2}$ in neutral medium (brown, 3$e^{-}$), and $MnO_{4}^{2-}$ in basic medium (green, 1$e^{-}$). $K_{2}Cr_{2}O_{7}$ in acidic medium is a strong oxidant reducing $Cr_{2}O_{7}^{2-}$ (orange) to $Cr^{3+}$ (green), while $CrO_{4}^{2-}$ (yellow) is the stable form in basic conditions. Lanthanoid contraction — the steady shrinkage of $Ln^{3+}$ radii from La to Lu due to poor 4f shielding — causes 4d and 5d congeners such as Zr and Hf to be nearly identical in size. Werner's coordination theory defines primary valence (oxidation state, ionisable) and secondary valence (coordination number, non-ionisable) of the central metal. Ligands are classified by denticity: monodentate ($H_{2}O$, $CN^{-}$), bidentate (en), hexadentate (EDTA); ambidentate ligands ($NO_{2}^{-}$, S$CN^{-}$) cause linkage isomerism. Crystal Field Theory explains d-orbital splitting: octahedral gives t_{2}g (lower) and eg (higher) with gap $\Delta o$; tetrahedral is inverted (e lower, t_{2} higher) with $\Delta t$ = 4/9 $\Delta o$. The spectrochemical series ranks ligand field strength: $I^{-}$ < $Br^{-}$ < $Cl^{-}$ < $F^{-}$ < $OH^{-}$ < $H_{2}O$ < $NH_{3}$ < en < $NO_{2}^{-}$ < $CN^{-}$ < CO, determining whether a complex is high or low spin and hence its colour and magnetic moment. Biologically important coordination compounds include haemoglobin ($Fe^{2+}$), chlorophyll ($Mg^{2+}$), vitamin $B_{12}$ ($Co^{3+}$), and carbonic anhydrase ($Zn^{2+}$), while cisplatin ($Pt^{2+}$, cis geometry) is a landmark anticancer drug.