Chemistry of First-Row Transition Elements

A transition element is one for which an atom has an incomplete d-subshell, or which gives rise to a cation with an incomplete d-subshell. These elements are found in the d-block of the periodic table between Groups 2 and 3.

The first transition series consists of the elements from scandium to zinc. These ten d-block metals are found in period 4.

After calcium, the 3d shell is filled before the 4S subshell, showing that the 4s orbital has lower energy than the 3d orbital.

Chromium and copper have one electron in their 4s orbital because half-filled or completely filled subshells are more stable than partially filled ones.

Transition elements are generally hard, have high tensile strength, are ductile, have high melting and boiling points, possess strong metallic bonding, and are good conductors of heat and electricity.

Even within the same period, transition metals have less variation in chemical properties because electrons are added to inner d-orbitals, not the outermost.

Transition elements have variable oxidation states because electrons in the 3d and 4s orbitals are available for bond formation.

A complex ion has a central positive ion linked to several atoms, ions or molecules called ligands. They can be formed by electrovalent or coordinate covalent bonding.

Transition metals form colored compounds due to the partially filled 3d orbitals. The nature of the ligands also impacts the color.

Transition elements and their compounds act as efficient catalysts because they can easily change oxidation states and their partially filled d-orbitals allow electron exchange.

Paramagnetism is the property of a substance to be attracted to a magnet. It is common in d-block metals due to the presence of unpaired electrons.

A ligand is a molecule or ion that can bind to a central atom through a covalent bond. It can be anionic, cationic, or neutral, and can donate one or more electron pairs.

Monodentate ligands bind to the central atom through only one atom. Examples include fluoride, chloride, bromide, and ammonia.

Bidentate ligands are molecules or ions that bind to the central atom through two coordinate covalent bonds. Examples include ethylenediamine and oxalate.

Tridentate and polydentate ligands have three or more donor groups that bind to the central metal. Examples include diethylenetriamine, triethylenetetraamine, and EDTA.

Ambidentate ligands can bind to the central metal through two different donor atoms. Examples include nitrito, cyano, and thiocyanato.

Bridging ligands connect two or more central atoms. They are named with a 'µ' prefix. Examples include CO in Fe2(CO)9 and hydride in B2H6.

Chelating ligands have two or more points of attachment to the same central atom or ion. The compound formed with the metal is called a chelate.

The chelate effect refers to the increased affinity of chelating ligands for the central metal ion compared to monodentate ligands. This effect is due to the formation of chelate rings.

The coordination number of a complex is the number of atoms or ions bonded to the central metal. It's determined by the size of the metal, the d-electron count, and steric effects from the ligands.

Metal complexes are chemical compounds consisting of a central metal atom or ion surrounded by one or more ligands. They can be cationic, anionic, neutral, homoleptic, heteroleptic, mononuclear, or polynuclear.

The coordination sphere is the non-ionizable part of a complex, consisting of the central metal, the ligands attached to it, and the overall charge. It is typically enclosed in square brackets.

IUPAC rules for naming coordination compounds dictate the order of naming, prefixes for ligand counts, naming of anionic ligands, and the indication of oxidation states.

Crystal field theory is a model based on electrostatic interactions between the ligands (as point charges) and d-orbitals of the central metal. It explains the splitting of d-orbitals and the magnetic properties of complexes.

In an octahedral complex, the six ligands are placed at the vertices of an octahedron. The d orbitals are split into two sets, eg (dz², dx²-y²) and t2g (dxy, dyz, dxz), with eg being destabilized more.

Tetrahedral complexes have four ligands placed at the corners of a tetrahedron. The d-orbital splitting is inverted compared to octahedral, with t2 being destabilized more.

The Jahn-Teller theorem explains why a non-linear molecule with a degenerate electronic state will undergo distortion to remove the degeneracy and lower the energy.

Square planar complexes have four ligands arranged in a square plane. The d-orbital splitting is again different, with the dz² orbital greatly stabilized.

Crystal field theory explains several properties of transition metal complexes that other bonding theories failed to account for, such as the splitting of d-orbitals, color, magnetic properties, and distorted geometries.

Crystal field theory has limitations in explaining why certain ligands are placed where they are in the spectrochemical series. The theory doesn't provide insight into the nature of the metal-ligand interactions.