- Lone pair
s equal the total number of valence electrons from a compound.
A single lone pair can be found with atoms in the
nitrogen groupsuch as nitrogen in ammonia, two lone pairs can be found with atoms in the chalcogengroup such as oxygen in water and the halogens can carry three lone pairs such as in hydrochloric acid.
The pairs often exhibit a negative polar character with their high charge density and are located closer to the
atomic nucleuson average compared to the bonding pair of electrons. The presence of a lone pair decreases the bond angle between the bonding pair of electrons, due to their high electric charge which causes great repulsion between the electrons. They are also used in the formation of a dative bond. For example, the creation of the hydronium(H3O+) ion occurs when acids are dissolved in water and is due to the oxygenatom donating a lone pair to the hydrogenion.
This can be seen more clearly when looked at in two more common
molecules. For example, in carbon dioxide(CO2), The oxygen atoms are on opposite sides of the carbon, whereas in water(H2O) there is an angle between the hydrogen atoms of 104.5 o. Due to the repulsive force of the oxygen atom's lone pairs, the hydrogens are pushed further away, to a point where the forces of all electrons on the hydrogen atom are in equilibrium. This is an illustration of the VSEPR theory.
Unusual lone pairs
A stereochemically active lone pair is also expected for divalent
leadand tinions due to their formal electronic configuration of n"s"2. In the solid state this results in the distorted metal coordination observed in the litharge structure adopted by both PbO and SnO. The formation of these heavy metal n"s"2 lone pairs which was previously attributed to intra-atomic hybridization of the metal s and p states ["Stereochemistry of Ionic Solids" J.D.Dunitz and L.E.Orgel, Advan. Inorg. and Radiochem. 1960, 2, 1-60] has recently been shown to have a strong anion dependence ["Electronic origins of structural distortions in post-transition metal oxides: experimental and theoretical evidence for a revision of the lone pair model" D.J.Payne, R.G.Egdell, A.Walsh, G.W.Watson, J.Guo, P.-A.Glans, T.Learmonth and K.E.Smith, Phys. Rev. Lett.2006, 96, 157403 DOI|10.1103/PhysRevLett.96.157403] . This dependence on the electronic states of the anion can explain why some divalent lead and tin materials such as PbS and SnTe show no stereochemical evidence of the lone pair and adopt the symmetric rocksalt crystal structure ["The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS" A.Walsh and G.W.Watson, J. Sol. Stat. Chem. 2005, 178, 5 DOI|10.1016/j.jssc.2005.01.030] , ["Influence of the Anion on Lone Pair Formation in Sn(II) Monochalcogenides: A DFT Study" A.Walsh and G.W.Watson, J. Phys. Chem. B2005, 109, 18868 DOI|10.1021/jp051822r] .
In molecular systems the lone pair can also result in a distortion in the coordination of ligands around the metal ion. The lead lone pair effect can be observed in supramolecular complexes of
Lead(II) nitrateand in 2007 a study ["Is an Electronic Shield at the Molecular Origin of Lead Poisoning?A Computational Modeling Experiment" C.Gourlaouen and O.Parisel Angew. Chem. Int. Ed.2007, 46, 553 –556 DOI|10.1002/anie.200603037] linked the lone pair to lead poisoning. Lead ions in the human metabolism replac e native metallic ions in several key proteins, for example: zinc cations in the ALAD protein, which is also known as Porphobilinogen synthase. This seems to be the molecular basis of "lead poisoning", or "saturnism" ("plumbism"). Computational experiments reveal that although the coordination numberdoes not change upon substitution in calcium-binding proteins, the introduction of lead distorts the way the ligands organize themselves to accommodate such an emerging lone pair: consequently, these proteins are perturbed. This lone-pair effect becomes dramatic for zinc-binding proteins, such as the above-mentionned porphobilinogen synthase, as the natural substrate cannot bind anymore: in those cases the protein is inhibited. Group 14lone pairs manifest themselves in triple bonds as well. The familiar alkynes have bond order3 with 180° bond angles (A) but going down the row germaniumto germanium formal triple bonds have an effective bond order 2 with one lone pair (B) and trans-bent geometries. In lead the bond order is even 1 with lone pairs for each lead atom (C). In the organogermanium compoundD, the bond order is also 1 with complexation of the acidic isonitrilegroups based on interaction with germaniums empty 4p orbital ["Lewis base induced tuning of the Ge–Ge bond order in a digermyne" G.H.Spikes and P.P.Power Chem. Commun., 2007, 85 - 87, DOI|10.1039/b612202g]
*Highest occupied molecular orbital
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