word_count: 220

The d-block elements occupy groups 3-12 with general configuration (n-1)d^(1-10) ns^(0-2). The 3d series (Sc to Zn) has two key anomalies: Cr = [Ar]3d^5 4s^1 (half-filled d stability) and Cu = [Ar]3d^10 4s^1 (fully-filled d stability).

When forming cations, 4s electrons are lost before 3d despite 4s filling first. This occurs because 3d orbitals contract below 4s in energy once d-electrons are added. Fe^2+ = [Ar]3d^6, NOT [Ar]3d^4 4s^2. Cu^2+ = [Ar]3d^9.

The maximum oxidation state increases from Sc(+3) to Mn(+7), equalling the total number of (ns + (n-1)d) electrons. After Mn, the maximum achievable OS decreases because electrons are too tightly held. The +2 state (loss of ns^2) is nearly universal across the series. Stability of +2 increases across the series, with Mn^2+ (d^5) and Zn^2+ (d^10) being exceptionally stable.

Higher oxidation states are stabilised by electronegative ligands (F^-, O^2-) forming covalent bonds. Lower oxidation states are stabilised by pi-acceptor ligands (CO, CN^-) through backbonding. This interplay between oxidation state and ligand type is fundamental to understanding transition metal chemistry.