Irving-Williams series
Encyclopedia
The Irving-Williams Series refers to the relative stabilities of complexes formed by a metal ion. For high-spin complexes of the divalent
ions of first-row transition metal
s, the stability constant
for the formation of a complex
follows the order
This order was found to hold for a wide variety of ligand
s.
There are three explanations that are quoted frequently to explain the series.
The actual CFSE values for octahedral systems are 0.4Δ (4 Dq); for iron, 0.8Δ (8 Dq) for cobalt and 1.2Δ (12Dq) for nickel. Δ (10 Dq) is the crystal field splitting energy (the energy gap between the metal-based t2g and e.g. orbitals). When the stability constants are quantitatively adjusted for these values they follow the trend that is predicted, in the absence of crystal field effects, between manganese and zinc. This was an important factor contributing to the acceptance of crystal field theory, the first theory to successfully account for the thermodynamic, spectroscopic and magnetic properties of complexes of the transition metal ions and precursor to ligand field theory
.
However, none of the above three explanations can satisfactorily explain the broad scope of validity of Irving-Williams series (both octahedral
and tetrahedral complexes containing different ligands). The recent study of metal-ligand binding in M(II)-thiolate series (M = Mn-Zn) revealed that the interplay between the covalent and electrostatic contributions to metal-ligand binding energies result in Irving-Williams series.
The series is named after H. Irving and Robert Williams from Oxford University who discovered this relationship and subsequently published a paper on it.
Divalent
In chemistry, a divalent ion or molecule has a valence of two and thus can form two bonds with other ions or molecules. An older term for divalent is bivalent....
ions of first-row transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s, the stability constant
Stability constants of complexes
A stability constant is an equilibrium constant for the formation of a complex in solution. It is a measure of the strength of the interaction between the reagents that come together to form the complex...
for the formation of a complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
follows the order
- Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)
This order was found to hold for a wide variety of ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s.
There are three explanations that are quoted frequently to explain the series.
- The ionic radiusIonic radiusIonic radius, rion, is the radius of an atom's ion. Although neither atoms nor ions have sharp boundaries, it is important to treat them as if they are hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice...
is expected to decrease regularly for Mn2+ to Zn2+. This is the normal periodic trend and would account for the general increase in stability. - The crystal field stabilization energy (CFSE) increases from zero for manganese(II) to a maximum at nickel(II). This makes the complexes increasingly stable. CFSE for zinc(II) is zero.
- Although the CFSE of copper(II) is less than that of nickel(II), octahedral copper(II) complexes are subject to the Jahn-Teller effectJahn-Teller effectThe Jahn–Teller effect, sometimes also known as Jahn–Teller distortion, or the Jahn–Teller theorem, describes the geometrical distortion of non-linear molecules under certain situations. This electronic effect is named after Hermann Arthur Jahn and Edward Teller, who proved, using group theory,...
, which affords a complex extra stability.
The actual CFSE values for octahedral systems are 0.4Δ (4 Dq); for iron, 0.8Δ (8 Dq) for cobalt and 1.2Δ (12Dq) for nickel. Δ (10 Dq) is the crystal field splitting energy (the energy gap between the metal-based t2g and e.g. orbitals). When the stability constants are quantitatively adjusted for these values they follow the trend that is predicted, in the absence of crystal field effects, between manganese and zinc. This was an important factor contributing to the acceptance of crystal field theory, the first theory to successfully account for the thermodynamic, spectroscopic and magnetic properties of complexes of the transition metal ions and precursor to ligand field theory
Ligand field theory
Ligand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...
.
However, none of the above three explanations can satisfactorily explain the broad scope of validity of Irving-Williams series (both octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
and tetrahedral complexes containing different ligands). The recent study of metal-ligand binding in M(II)-thiolate series (M = Mn-Zn) revealed that the interplay between the covalent and electrostatic contributions to metal-ligand binding energies result in Irving-Williams series.
The series is named after H. Irving and Robert Williams from Oxford University who discovered this relationship and subsequently published a paper on it.