Transition metals exhibit variable oxidation states because the energy levels of the and subshells are very close. This allows varying numbers of electrons to be lost depending on the chemical environment and the lattice or hydration enthalpy available.
When transition metals form ions, electrons are always removed from the 4s subshell first, followed by the electrons. This is because, once the orbitals begin to fill, the electrons are effectively at a higher energy level and are more easily shielded.
Exceptions in filling: Chromium () and Copper () deviate from the Aufbau principle. A half-filled or fully-filled d-subshell provides extra stability due to symmetrical electron distribution and reduced electron-electron repulsion.
The color of transition metal complexes arises from d-d transitions. In an isolated gas-phase ion, all five d-orbitals are degenerate (equal in energy), but the presence of ligands causes them to split into different energy levels.
According to Crystal Field Theory (CFT), ligands approaching the metal ion create an electric field that repels electrons in the d-orbitals. Orbitals pointing directly at ligands (like and in an octahedral field) rise in energy more than those pointing between them.
When white light shines on a complex, an electron can absorb a photon of specific energy () to jump from a lower-energy d-orbital to a higher-energy one. The energy of the absorbed light is given by \Delta E = h u = rac{hc}{\lambda}.
The color perceived by the observer is the complementary color of the light absorbed. For example, if a complex absorbs red light, it appears blue-green.
Heterogeneous Catalysis involves the catalyst being in a different phase (usually solid) than the reactants (usually gas or liquid). Transition metals are excellent heterogeneous catalysts because they provide a surface for reactants to adsorb onto, weakening their internal bonds and lowering the activation energy.
Homogeneous Catalysis occurs when the catalyst and reactants are in the same phase. Transition metals excel here due to their ability to switch between oxidation states, allowing them to form intermediate compounds that provide an alternative reaction pathway with lower energy.
The efficiency of transition metal catalysts is often linked to their partially filled d-orbitals, which can accept or donate electrons to facilitate the breaking and forming of chemical bonds.
| Feature | Transition Metal | Main Group Metal |
|---|---|---|
| Oxidation States | Multiple, variable (e.g., ) | Usually fixed (e.g., Group 1 is always ) |
| Compound Color | Often highly colored due to d-d transitions | Usually white/colorless |
| Magnetism | Often paramagnetic (unpaired electrons) | Usually diamagnetic (paired electrons) |
| Catalysis | Highly active as catalysts | Rarely used as catalysts |
Electron Configuration: Always remember to remove electrons before electrons when writing ion configurations. A common mistake is removing electrons first because they are written last in the neutral atom sequence.
Predicting Color: If an ion has a or configuration, it will be colorless. Color requires at least one electron to move between split d-orbitals and at least one empty space in the higher energy level to receive it.
Coordination Number: Do not confuse the number of ligands with the coordination number. Polydentate ligands (like EDTA) can form multiple coordinate bonds per molecule; always count the total number of bonds to the central metal.
Sanity Check: When calculating oxidation states in a complex, ensure the sum of the metal's charge and the ligands' charges equals the overall charge of the complex ion.
