Transition Elements

Defining the d-block

A transition element is one whose atom or commonly occurring ion has a partially filled d-subshell. The first row spans Sc to Zn, but Zn ([Ar]3d¹⁰4s², Zn²⁺ [Ar]3d¹⁰) is technically not a transition metal because its d-shell is full in all stable oxidation states. Sc³⁺ ([Ar]) is similarly excluded. The FSc Punjab Textbook Chemistry XII Chapter 2 lists the canonical properties: variable oxidation states, formation of coloured ions, paramagnetism, catalytic activity, and complex ion formation.

Variable oxidation states

Because 4s and 3d orbitals are close in energy, electrons can be removed from both, giving multiple oxidation states. Manganese is the champion, with stable states from +2 to +7 (Mn²⁺, MnO₂, MnO₄²⁻, MnO₄⁻). Iron commonly shows +2 and +3, with Fe³⁺ being more stable due to its half-filled d⁵ configuration. Chromium ranges +2 to +6, with Cr₂O₇²⁻ a powerful oxidising agent in acidic medium. The +2 state is most common across the row because the 4s electrons are lost first.

Colour and the crystal field

Coloured ions arise from d-d electronic transitions: a ligand field splits the five degenerate d-orbitals into t_2g and e_g sets in an octahedral complex (energy gap Δ_o). The complex absorbs visible light at frequency ν = Δ_o/h, transmitting the complementary colour. [Ti(H₂O)₆]³⁺ is the textbook example: a single 3d¹ electron gives a violet solution by absorbing yellow-green light (~500 nm). d⁰ (Sc³⁺, Ti⁴⁺) and d¹⁰ (Zn²⁺, Cu⁺) ions are colourless. The spectrochemical series ranks ligands by Δ: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < CN⁻ < CO.

Complex ions and coordination geometry

A complex consists of a central metal ion surrounded by ligands donating lone pairs. Coordination numbers 4 (tetrahedral or square planar) and 6 (octahedral) dominate. [Cu(NH₃)₄]²⁺ is square planar and deep blue; [Fe(CN)₆]⁴⁻ is octahedral; [CoCl₄]²⁻ is tetrahedral. EDTA is a hexadentate chelating ligand used in titrations of Ca²⁺ and Mg²⁺. Werner's 1893 theory established the primary (ionic) and secondary (coordinate) valencies. Cotton, Wilkinson & Murillo treat this in canonical depth.

Catalysis and industrial relevance

Transition metals catalyse by providing variable oxidation states or surfaces for adsorption. Fe in the Haber process (N₂ + 3H₂ → 2NH₃); V₂O₅ in the Contact process; Pt-Rh in Ostwald; Ni in catalytic hydrogenation of alkenes (Sabatier reaction); Pd/C in Lindlar hydrogenation; Wilkinson's catalyst [RhCl(PPh₃)₃] for homogeneous hydrogenation. Ziegler-Natta TiCl₄/Et₃Al polymerises ethene to high-density polyethene. These named catalysts appear in MDCAT roughly every other paper.