d-orbital

Oxidation States of d-Block Elements: d-block elements exhibit variable oxidation states due to the involvement of (n-1)d and ns electrons in bonding.

Important Oxidation States and Colors:

• Cr: +3 (violet/green), CrO42− (yellow), Cr2O72− (orange)

• Mn: +2, +4, +7, MnO4− (purple)

• Fe: +2 (pale green), +3 (yellow/brown)

• Co: +2 (pink)

• Cu: +2 (blue)

Bonding in Complexes:

• Tetrahedral and octahedral complexes involve coordinate bonds between metal ions and ligands.

Origin of Color in Transition Metal Complexes:

• Color arises from d-d transitions, influenced by crystal field splitting (e.g., $[Cu(H2O)6]^{2+}$ and $[Fe(H2O)6]^{3+}$).

Ligand Exchange and Coordination Number:

• Ligand exchange can alter coordination number (e.g., reactions of $[Cu(H2O)6]^{2+}$ and $[Co(H2O)6]^{2+}$ with concentrated HCl).

Colors and Formulae of Complex Ions:

• Octahedral: $[Cu(H2O)6]^{2+}$ (blue), $[Cu(NH3)4(H2O)2]^{2+}$ (deep blue), $[Co(H2O)6]^{2+}$ (pink)

• Tetrahedral: $[CuCl4]^{2−}$ (yellow/green), $[CoCl4]^{2−}$ (blue)

Catalytic Properties:

• Transition metals exhibit catalytic properties; heterogeneous catalysis involves surface adsorption, while homogeneous catalysis involves variable oxidation states.

Catalysts in Industrial Processes:

• Ni (hydrogenation of alkenes), Fe (Haber process).

Examples of Catalysis:

• $V2O5$ (contact process), $MnO2$ (decomposition of $H2O2$).

Reactions with Excess Aqueous OH-:

• $Cr^{3+}$: green precipitate of $Cr(OH)3$

• $Fe^{2+}$: green precipitate of $Fe(OH)2$

• $Fe^{3+}$: brown precipitate of $Fe(OH)3$

• $Cu^{2+}$: blue precipitate of $Cu(OH)2$