Properties of Transition Metals

Physical Properties and Atomic Size

Due to partially-filled d subshells, transition metals possess a number of unique properties.

Transition Metal Properties

There are a number of properties shared by the transition elements that are not found in other elements, which result from the partially filled d subshell. These include the formation of compounds whose color is due to d–d electronic transitions and the formation of many paramagnetic compounds due to the presence of unpaired d electrons. Color in transition-series metal compounds is generally due to electronic transitions of two principal types: charge-transfer transitions and d-d transitions.

Charge Transfer Transitions

An electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge-transfer (LMCT) transition. These can most easily occur when the metal is in a high oxidation state. For example, the color of chromate, dichromate, and permanganate ions is due to LMCT transitions. In each case the metals (Cr and Mn) have oxidation states of +6 or higher.

A metal-to ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is easily reduced.

d-d Transitions

In a d-d transition, an electron jumps from one d-orbital to another. In complexes of the transition metals, the d orbitals do not all have the same energy. The pattern of splitting of the d orbitals can be calculated using crystal field theory. The extent of the splitting depends on the particular metal, its oxidation state, and the nature of the ligands.

In centrosymmetric complexes, such as octahedral complexes, d-d transitions are forbidden. Tetrahedral complexes have a somewhat more intense color because mixing d and p orbitals is possible when there is no center of symmetry, so transitions are not pure d-d transitions.

Some d-d transitions are spin forbidden. An example occurs in octahedral, high-spin complexes of manganese(II) in which all five electrons have parallel spins. The color of such complexes is much weaker than in complexes with spin-allowed transitions. In fact, many compounds of manganese(II) appear almost colorless.

Transition metal compounds are paramagnetic when they have one or more unpaired d electrons. In octahedral complexes with between four and seven d electrons, both high spin and low spin states are possible. Tetrahedral transition metal complexes, such as [FeCl₄]²⁻, are high-spin because the crystal field splitting is small. This means that the energy to be gained by virtue of the electrons being in lower energy orbitals is always less than the energy needed to pair up the spins.

Paramagnetic vs. Diamagnetic

Some compounds are diamagnetic. In these case all of the electrons are paired up. Ferromagnetism occurs when individual atoms are paramagnetic and the spin vectors are aligned parallel to each other in a crystalline material. Metallic iron is an example of a ferromagnetic material involving a transition metal. Anti-ferromagnetism is another example of a magnetic property arising from a particular alignment of individual spins in the solid state.

As implied by the name, all transition metals are metals and conductors of electricity. In general, transition metals possess a high density and high melting points and boiling points. These properties are due to metallic bonding by delocalized d electrons, leading to cohesion which increases with the number of shared electrons. However, the Group 12 metals have much lower melting and boiling points since their full d subshells prevent d–d bonding. In fact, mercury has a melting point of −38.83 °C (−37.89 °F) and is a liquid at room temperature.

Transition Metals and Atomic Size

In regards to atomic size of transition metals, there is little variation. Typically, when moving left to right across the periodic table, there is a trend of decreasing atomic radius. However, in the transition metals, moving left to right, there is a trend of increasing atomic radius which levels off and becomes constant. In the transition elements, the number of electrons are increasing but in a particular way. The number of electrons increase going across a period, thus, there is more pull of these electrons towards the nucleus. However, with the d−electrons, there is some added electron-electron repulsion. For example, in chromium, there is a promotion of one of the 4s electrons to half fill the 3d sublevel; the electron-electron repulsions are less and the atomic size is smaller. The opposite holds true for the latter part of the row.

Lanthanides and Actinides

Lanthanides and actinides are elements of the inner transition series of the periodic table.

The lanthanides and actinides form a group that appears almost disconnected from the rest of the periodic table. This is the f block of elements, known as the inner transition series. This is due to the proper numerical position between Groups 2 and 3 of the transition metals.

Electron Configuration

The 15 elements (numbers 58 to 71) of the lanthanide series are rare earth elements. Most lanthanides are formed when uranium and plutonium undergo nuclear reactions. Atomic bombs charged with plutonium (actinoid) were used in World War II. Plutonium was a power source for Voyager spacecrafts launched in 1977 and is also used in artificial heart pacemakers.

The f sublevel contains seven orbitals, each of which will hold two electrons. Therefore, it is possible to place 14 electrons in the 4f sublevel. Generally speaking, the lanthanides have electron configurations that follow the Aufbau rule, and the 4f sublevel is filled as atomic number increases from cerium (Ce) to lutetium (Lu). However, there are three lanthanide metals that have properties similar to the d block: cerium (Ce), lutetium (Lu), and gadolinium (Gd). All of these metals contain a d electron in their electron configuration.

A similar overall trend holds for the 14 elements in the actinide series (numbers 90 to 103): from thorium (Th) to Lawrencium (Lr), the 5f sublevel is progressively filled.

Elemental Properties

The chemistry of the lanthanides differs from main group elements and transition metals because of the nature of the 4f orbitals. These orbitals are "buried" inside the atom and are shielded from the atom's environment by the 4d and 5p electrons. As a consequence, the chemistry of the elements is largely determined by their size, which decreases gradually with increasing atomic number. This phenomenon is known as the lanthanide contraction. All the lanthanide elements exhibit the oxidation state +3.

Actinides are typical metals. All of them are soft, have a silvery color (but tarnish in air), and have relatively high density and plasticity. Some of them can be cut with a knife. The hardness of thorium is similar to that of soft steel, so heated pure thorium can be rolled in sheets and pulled into wire. Thorium is nearly half as dense as uranium and plutonium but is harder than both of them.

Unlike the lanthanides, most elements of the actinide series have the same properties as the d block. Members of the actinide series can lose multiple electrons to form a variety of different ions. All actinides are radioactive, paramagnetic, and, with the exception of actinium, have several crystalline phases. All actinides are pyrophoric, especially when finely divided (i.e., they spontaneously ignite upon exposure to air).

The melting point of actinides does not have a clear dependence on the number of f electrons. The unusually low melting point of neptunium and plutonium (~640 °C) is explained by hybridization of 5f and 6d orbitals and the formation of directional bonds in these metals. Like the lanthanides, all actinides are highly reactive with halogens and chalcogens; however, the actinides react more easily. Actinides, especially those with a small number of 5f electrons, are prone to hybridization. This is explained by the similarity of the electron energies at the 5f, 7s, and 6d subshells. Most actinides exhibit a larger variety of valence states.