Inert pair effect
Encyclopedia
The inert pair effect is the tendency of the outermost s electrons to remain nonionized or unshared in compounds of post-transition metal
s. The term inert pair effect is often used in relation to the increasing stability of oxidation state
s that are 2 less than the group valency for the heavier elements of groups 13, 14, 15 and 16. The term "inert pair" was first proposed by Nevil Sidgwick in 1927.
The same trend in stability is noted in groups 14, 15 and 16. As such the heaviest members of the groups, e.g. lead
, bismuth
and polonium
are comparatively stable in oxidation states +2, +3, and +4 respectively.
The lower oxidation state in each of the elements in question has 2 valence electrons in s orbitals. On the face of it a simple explanation could be that the valence electrons in an s orbital are more tightly bound are of higher energy than electrons in p orbitals and therefore less likely to be involved in bonding. Unfortunately this explanation does not stand up. If the total ionization potential
s (see below) of the 2 electrons in s orbitals (the 2d + 3d ionization potential
s), are examined it can be seen that they increase in the sequence:
The high IP(2nd + 3rd) of gallium is explained by d-block contraction
, and the higher IP(2nd + 3rd) of thallium relative to indium, has been explained by relativistic effects
.
An important consideration is that the compounds in the lower oxidation state are ionic, whereas in the higher oxidation state they tend to be covalent. Therefore covalency effects must also be taken into account. In fact an alternative explanation of the inert pair effect by Drago in 1958 attributed the effect to low M-X bond enthalpies for the heavy p-block elements and the fact that it requires less energy to oxidize an element to a low oxidation state than to a higher oxidation state. This energy has to be supplied by ionic or covalent bonds, so if bonding to a particular element is weak, the high oxidation state may be inaccessible. Further work involving relativistic effects confirms this. In view of this it has been suggested that the term inert pair effect should be viewed as a description rather than as an explanation.
which is bent in accordance with VSEPR. Some examples where the lone pair appears to be inactive are bismuth(III) iodide
, BiI3, and the BiI63− anion. In both of these the central Bi atom is octahedrally coordinated with little or no distortion, in contravention to VSEPR theory.
The steric activity of the lone pair has long been assumed to be due to the orbital having some p character, i.e. the orbital is not spherically symmetric. More recent theoretical work shows that this is not always necessarily the case. For example the litharge
structure of PbO
contrasts to the more symmetric and simpler rock salt structure of PbS
and this has been explained in terms of PbII− anion interactions in PbO leading to an asymmetry in electron density. Similar interactions do not occur in PbS. Another example are some thallium(I) salts where the asymmetry has been ascribed to s electrons on Tl interacting with antibonding orbitals.
factors, namely, that the Ga2+, having an unpaired electron, behaves as a free radical and is rapidly destroyed by reaction with another free radical. Drago was right, these compounds are thermodynamically stable
by virtue of the formation of a covalent bond between the gallium ions.
Post-transition metal
In chemistry, the term post-transition metal is used to describe the category of metallic elements to the right of the transition elements on the periodic table...
s. The term inert pair effect is often used in relation to the increasing stability of oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s that are 2 less than the group valency for the heavier elements of groups 13, 14, 15 and 16. The term "inert pair" was first proposed by Nevil Sidgwick in 1927.
Description
As an example in group 13 the +1 oxidation state of Tl is the most stable and TlIII compounds are comparatively rare. The stability of the +1 oxidation state increases in the following sequence:- AlI < GaI < InI < TlI.
The same trend in stability is noted in groups 14, 15 and 16. As such the heaviest members of the groups, e.g. lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
and polonium
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
are comparatively stable in oxidation states +2, +3, and +4 respectively.
The lower oxidation state in each of the elements in question has 2 valence electrons in s orbitals. On the face of it a simple explanation could be that the valence electrons in an s orbital are more tightly bound are of higher energy than electrons in p orbitals and therefore less likely to be involved in bonding. Unfortunately this explanation does not stand up. If the total ionization potential
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
s (see below) of the 2 electrons in s orbitals (the 2d + 3d ionization potential
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
s), are examined it can be seen that they increase in the sequence:
- In < Al < Tl < Ga.
| IP | Boron Boron Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the... | Aluminium Aluminium Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances.... | Gallium Gallium Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies... | Indium Indium Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two... | Thallium Thallium Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy... |
|---|---|---|---|---|---|
| 1st | 800.6 | 577.5 | 578.8 | 558.3 | 589.4 |
| 2nd | 2427.1 | 1816.7 | 1979.3 | 1820.6 | 1971 |
| 3rd | 3659.7 | 2744.8 | 2963 | 2704 | 2878 |
| (2nd + 3rd) | 6086.8 | 4561.5 | 4942.3 | 4524.6 | 4849 |
D-block contraction
d-block contraction is a term used in chemistry to describe the effect of having full d orbitals on the period 4 elements. The elements in question are the Ga, Ge, As, Se and Br. Their electronic configurations include completely filled d orbitals...
, and the higher IP(2nd + 3rd) of thallium relative to indium, has been explained by relativistic effects
Relativistic quantum chemistry
Relativistic quantum chemistry invokes quantum chemical and relativistic mechanical arguments to explain elemental properties and structure, especially for heavy elements of the periodic table....
.
An important consideration is that the compounds in the lower oxidation state are ionic, whereas in the higher oxidation state they tend to be covalent. Therefore covalency effects must also be taken into account. In fact an alternative explanation of the inert pair effect by Drago in 1958 attributed the effect to low M-X bond enthalpies for the heavy p-block elements and the fact that it requires less energy to oxidize an element to a low oxidation state than to a higher oxidation state. This energy has to be supplied by ionic or covalent bonds, so if bonding to a particular element is weak, the high oxidation state may be inaccessible. Further work involving relativistic effects confirms this. In view of this it has been suggested that the term inert pair effect should be viewed as a description rather than as an explanation.
Steric activity of the lone pair
The chemical inertness of the s electrons in the lower oxidation state is not always married to steric inertness, (where steric inertness means that the presence of the s electron lone pair has little or no influence on the geometry of molecule or crystal). A simple example of steric activity is that of SnCl2Tin(II) chloride
Tin chloride is a white crystalline solid with the formula 2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent , and in electrolytic baths for tin-plating...
which is bent in accordance with VSEPR. Some examples where the lone pair appears to be inactive are bismuth(III) iodide
Bismuth(III) iodide
Bismuth iodide is the inorganic compound with the formula BiI3. This gray-black solid is the product of the reaction of bismuth and iodide, which once was of interest in qualitative inorganic analysis....
, BiI3, and the BiI63
The steric activity of the lone pair has long been assumed to be due to the orbital having some p character, i.e. the orbital is not spherically symmetric. More recent theoretical work shows that this is not always necessarily the case. For example the litharge
Litharge
Litharge is one of the natural mineral forms of lead oxide, PbO. Litharge is a secondary mineral which forms from the oxidation of galena ores. It forms as coatings and encrustations with internal tetragonal crystal structure. It is dimorphous with the orthorhombic form massicot...
structure of PbO
Lead(II) oxide
Lead oxide is the inorganic compound with the formula PbO. Lead oxide occurs in two polymorphs, red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure...
contrasts to the more symmetric and simpler rock salt structure of PbS
Lead(II) sulfide
Lead sulfide is an inorganic compound with the formula Pb. It finds limited use in electronic devices. PbS, also known as galena, is the principal ore and most important compound of lead....
and this has been explained in terms of PbII
Failure of the theory
Part of the rationale for describing this as an effect was the fact that, at the time when it was proposed, there were no known compounds of Group 13 elements with the intermediate, +2, oxidation state. This is no longer true since the discovery of complexes of Ga(II) and In(II), such as the halo complexes [M2 X6]2-. These complex ions are stabilized by the formation of a covalent M–M bond. It follows that the instability of simple complexes of ions such as Ga2+ is due to kineticChemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
factors, namely, that the Ga2+, having an unpaired electron, behaves as a free radical and is rapidly destroyed by reaction with another free radical. Drago was right, these compounds are thermodynamically stable
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
by virtue of the formation of a covalent bond between the gallium ions.
External links
- Chemistry guide An explanation of the inert pair effect.