There are a number of properties shared by the transition elements that are not found in other elements, which results from the partially filled d shell. These include • the formation of compounds whose colour is due to d–d electronic transitions • the formation of compounds in many oxidation states, due to the relatively low energy gap between different possible oxidation states • the formation of many
paramagnetic compounds due to the presence of unpaired d electrons. A few compounds of main-group elements are also paramagnetic (e.g.
nitric oxide,
oxygen) Most transition metals can be bound to a variety of
ligands, allowing for a wide variety of transition metal complexes.
Coloured compounds Colour in transition-series metal compounds is generally due to electronic transitions of two principal types. •
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 colour of
chromate,
dichromate and
permanganate ions is due to LMCT transitions. Another example is that
mercuric iodide, HgI2, is red because of a LMCT transition. 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. In general charge transfer transitions result in more intense colours than d–d transitions. • d–d transitions. 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. The actual energy levels are shown on
Tanabe–Sugano diagrams. In
centrosymmetric complexes, such as octahedral complexes, d–d transitions are forbidden by the
Laporte rule and only occur because of
vibronic coupling in which a
molecular vibration occurs together with a d–d transition. Tetrahedral complexes have somewhat more intense colour because mixing d and p orbitals is possible when there is no centre of symmetry, so transitions are not pure d–d transitions. The
molar absorptivity (ε) of bands caused by d–d transitions are relatively low, roughly in the range 5–500 M−1cm−1 (where
M = mol dm−3). Some d–d transitions are
spin forbidden. An example occurs in octahedral, high-spin complexes of
manganese(II), which has a d5 configuration in which all five electrons have parallel spins; the colour of such complexes is much weaker than in complexes with spin-allowed transitions. Many compounds of manganese(II) appear almost colourless. The
spectrum of shows a maximum molar absorptivity of about 0.04 M−1cm−1 in the
visible spectrum.
Oxidation states A characteristic of transition metals is that they exhibit two or more
oxidation states, usually differing by one. For example, compounds of
vanadium are known in all oxidation states between −1, such as , and +5, such as .
Main-group elements in groups 13 to 18 also exhibit multiple oxidation states. The "common" oxidation states of these elements typically differ by two instead of one. For example, compounds of
gallium in oxidation states +1 and +3 exist in which there is a single gallium atom. Compounds of Ga(II) would have an unpaired electron and would behave as a
free radical and generally be destroyed rapidly, but some stable radicals of Ga(II) are known. Gallium also has a formal oxidation state of +2 in dimeric compounds, such as , which contain a Ga-Ga bond formed from the unpaired electron on each Ga atom. Thus the main difference in oxidation states, between transition elements and other elements is that oxidation states are known in which there is a single atom of the element and one or more unpaired electrons. The maximum oxidation state in the first row transition metals is equal to the number of valence electrons from
titanium (+4) up to
manganese (+7), but decreases in the later elements. In the second row, the maximum occurs with
ruthenium (+8), and in the third row, the maximum occurs with
iridium (+9). In compounds such as and , the elements achieve a stable configuration by
covalent bonding. The lowest oxidation states are exhibited in
metal carbonyl complexes such as (oxidation state zero) and (oxidation state −2) in which the
18-electron rule is obeyed. These complexes are also covalent. Ionic compounds are mostly formed with oxidation states +2 and +3. In aqueous solution, the ions are hydrated by (usually) six water molecules arranged octahedrally. A solid circle represents a common oxidation state, and a ring represents a less common oxidation state. -->
Magnetism 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 are
high spin because the crystal field splitting is small so 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. Some compounds are
diamagnetic. These include octahedral, low-spin, d6 and square-planar d8 complexes. In these cases,
crystal field splitting is such that all 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 and the alloy
alnico are examples of ferromagnetic materials involving transition metals.
Antiferromagnetism is another example of a magnetic property arising from a particular alignment of individual spins in the solid state.
Catalytic properties The transition metals and their compounds are known for their homogeneous and heterogeneous
catalytic activity. This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes.
Vanadium(V) oxide (in the
contact process), finely divided
iron (in the
Haber process), and
nickel (in
catalytic hydrogenation) are some of the examples. Catalysts at a solid surface (
nanomaterial-based catalysts) involve the formation of bonds between reactant molecules and atoms of the surface of the catalyst (first row transition metals utilize 3d and 4s electrons for bonding). This has the effect of increasing the concentration of the reactants at the catalyst surface and also weakening of the bonds in the reacting molecules (the activation energy is lowered). Also because the transition metal ions can change their oxidation states, they become more effective as
catalysts. An interesting type of catalysis occurs when the products of a reaction catalyse the reaction producing more catalyst (
autocatalysis). One example is the reaction of
oxalic acid with acidified
potassium permanganate (or manganate (VII)). Once a little Mn2+ has been produced, it can react with MnO4− forming Mn3+. This then reacts with C2O4− ions forming Mn2+ again.
Physical properties As implied by the name, all transition metals are
metals and thus 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, which again tends to differentiate them from the accepted transition metals. Mercury has a melting point of and is a liquid at room temperature. ==See also==