Transition Metals

Elements where the last electron enters a d subshell. They are located in groups 3-12 and periods 4-7.

The elements where the d subshell is not completely filled in their ground or any common oxidation state.

The electronic configuration of d-block elements follows the pattern (n-1)d1-10 ns1-2, where n is the principal quantum number and s represents the shell number.

The 3d series, also known as the first transition series, includes elements from Scandium (Sc) to Zinc (Zn).

The d-block elements in the fourth period (Sc-Zn) are called the first transition series.

The stability of half-filled and completely-filled orbitals leads to unusual configurations for Chromium (Cr) and Copper (Cu).

Zinc (Zn), Cadmium (Cd), and Mercury (Hg) are not considered transition elements because their d orbitals are fully filled both in their ground state and common +2 oxidation state.

For an element in the d-block, the period number is the same as the principal quantum number of its valence shell. The group number equals the sum of (n-1)d electrons and ns electrons.

The sum of the oxidation numbers of all atoms in a neutral compound is zero.

The oxidation number of a monatomic ion is equal to its charge.

The oxidation number of an element in its elemental form is zero. For example, the oxidation number of O2 or S8 is zero.

The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion. For example, in SO42-, the oxidation number of S is +6.

A complex ion is a species where a central metal atom or ion is coordinated to multiple electron-rich species called ligands.

Complex compounds are substances containing at least one complex ion, retaining their identity in both solid and solution states.

A complex ion with a net positive charge is a cationic complex ion, while a complex ion with a net negative charge is an anionic complex ion.

A neutral complex is a complex ion that has no charge and does not ionize in solution.

The metal atom or ion in the center of a complex, surrounded by ligands.

Electron-rich species (ions or molecules) that bind to the central metal atom/ion via coordinate bonds.

The atom in the ligand that donates an electron pair to the central metal.

Ligands with one donor atom, like Cl-, Br-, OH-, CN-, H2O, NH3, CO.

Ligands with two donor atoms, like ethylene diamine (en) and oxalate ion (ox).

Ligands with multiple donor atoms, like diethylene triamine (dien) and EDTA.

The total number of ligands (or their donor atoms) directly attached to the central metal atom or ion.

The central metal atom or ion and its directly attached ligands, enclosed in square brackets.

In a free atom or ion, all d-orbitals have the same energy level. However, when ligands approach a metal ion in a complex, they create a field that splits the d-orbitals into two energy levels: t2g and eg.

The energy gap between the t2g and eg energy levels caused by crystal field splitting.

A series of ligands ordered by their ability to increase crystal field splitting energy.

Ligands that cause a smaller energy difference between t2g and eg orbitals, leading to unpaired electrons in higher eg orbitals .

Ligands that cause a larger energy difference between t2g and eg orbitals, leading to paired electrons in lower t2g orbitals.

Complexes with unpaired electrons due to weak field ligands.

Complexes with paired electrons due to strong field ligands.

The ability of electrons to pair up in the lower energy orbitals.

The electronic transition of an electron from one d-orbital to another within the same d-subshell, caused by the absorption of light.

The colour of a transition metal compound is determined by the wavelength of light that is not absorbed during the d-d transition.

A solution of Cu2+ ions appears blue because it absorbs orange light during the d-d transition. The complementary colour of orange is blue, hence the blue appearance.

The energy difference between the d-orbitals, which determines the colour, is influenced by the type of ligands surrounding the metal ion and the geometry of the complex formed.

Transition metal ions with completely filled or empty d-orbitals (d10 or d0) are colourless because no d-d transition can occur.

In the absence of ligands, the energy difference between the d-orbitals disappears, leading to no d-d transition and therefore no colour. For example, anhydrous CuSO4 is colourless, while CuSO4.5H2O is blue due to the presence of water ligands.

In some compounds, like KMnO4, the colour arises from a charge transfer spectrum. An electron from the oxygen atom temporarily moves to the manganese atom, absorbing light in the visible region.

The charge transfer spectrum occurs when an electron is temporarily transferred between the metal atom and a surrounding ligand, resulting in the absorption of light of a specific colour.

The colour of a transition metal compound is determined by the d-orbital electronic configuration of the metal ion. The presence of unpaired electrons in d orbitals results in absorption of specific wavelengths of light, leading to complementary colours being reflected. Compounds with a d10 configuration generally appear colourless due to the lack of d-d transitions.

Zinc sulfate heptahydrate (ZnSO4.7H2O) is white because Zn2+ has a d10 configuration, meaning all its d orbitals are filled. This prevents d-d transitions and absorption of light, resulting in a colourless compound. On the other hand, copper sulfate pentahydrate (CuSO4.5H2O) is blue because Cu2+ has incompletely filled d orbitals, allowing for d-d transitions and the absorption of specific wavelengths of light, reflecting blue.

TiO2 is white because Ti4+ has no d electrons, preventing any d-d transitions and resulting in a colourless compound. TiCl3, on the other hand, has Ti3+ with one d electron, leading to d-d transitions and absorption of certain wavelengths, ultimately displaying a violet colour.

CuSO4.5H2O is blue due to the presence of water molecules coordinated to the Cu2+ ion. These water molecules influence the ligand field around the Cu2+ ion, leading to a specific energy difference between d orbitals and resulting in the absorption of wavelengths corresponding to blue colour. In anhydrous CuSO4, the absence of water molecules alters the ligand field, reducing d-d transitions and leading to a colourless compound.

Transition metals and their compounds often act as catalysts due to their vacant d orbitals and variable oxidation states. They can form unstable intermediate compounds with reactants, accelerating the reaction by providing an alternative lower-energy pathway. The intermediate compounds readily decompose into the final products, regenerating the catalyst.