Transition metals consist of the elements in groups 3–12 of the periodic table (Sc through Zn). In their elemental states, they display metallic luster (shiny appearance) and are good conductors of heat and electricity. Transition metals generally act as Lewis acids, accepting electron pairs from ions or polyatomic compounds (molecules or ions) that often function as Lewis bases called ligands. These Lewis acid/base donor-acceptor interactions lead to the formation of covalent bonds between transition metals and ligands. These are called coordinate covalent bonds or dative bonds. The entire molecule (or ion) is called a coordination complex. The simplest transition metal complexes contain a central metal center, usually a cation, that has one or more covalent bonds to several ligands. The three-dimensional shape of a transition metal complex can often be predicted using valence shell electron repulsion theory concepts. The metal ion with attached ligands is a coordination complex, and its shape depends on how ligands bond. The same ligands can bond with the central metal ion in different geometries, forming isomers, with distinct properties. Hemoglobin and chlorophyll are examples of two coordination complexes found in biological systems.
The crystal field theory description of bonding is a simple electrostatic model that is used to predict numbers of unpaired electrons and in some cases the colors of transition metal complexes. Electrons occupy d orbitals according to Aufbau filling principles. Most complexes maximize the number of unpaired electrons due to electron-electron repulsion; pairing of electrons is generally avoided unless all other d orbitals contain one (or more) electrons, according to Hund’s rule (electrons first singly occupy empty orbitals before pairing in an orbital).
At A Glance
- Transition metals have electrons in d orbitals. They have incomplete d subshells and form cations. Inner transition elements have 4f or 5f electrons in their valence shell.
- Transition metals and inner transition metals form bonds by sharing electron pairs with ligands. These occur between a metal and one or more ions or molecules. The number of coordinated ligands in a complex is called its coordination number.
- Transition metal–ligand bonding leads to the formation of numerous coordination complexes, whose molecular shapes can often be predicted using VSEPR concepts.
- Isomers are molecules that have the same chemical formula but different arrangement of atoms.
- The naming of coordination complexes follows five rules: name the cation, name the ligands, use prefixes for multiple ligands, name the central metal, and give the central metal's oxidation state.
- Coordination compounds are used in a variety of ways, including as biological agents, as pigments and dyes, as industrial catalysts, and for medicinal purposes.
- Crystal field theory is a model used to predict bonding in octahedral and tetrahedral complexes considering a central cation surrounded by anionic (ligand) charge points in 3D space using simple VSEPR concepts.
- The magnetic properties of a coordination complex are affected by the arrangement of electrons, ligand field strength, electron-electron repulsion, and pairing energies.
- The arrangement of electrons in a coordination complex or transition metal ion and their interaction with light results in the bright colors associated with them.