Stereochemically active lone pair

The reaction of Te(N3)4 with ionic azides generates the [Te(N3)6] anion (Eq. 5.8). The stmeture of this anion is strongly distorted from octahedral by the stereochemically active lone pair on the tellurium atom, which gives rise to substantial differences in the Te-N bond lengths. Eour of these bonds are in the range 2.09-2.24 A, cf. [Te(N3)5] ,... 

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

Heavier main-group elements like T1(I), in complex compounds, generally exhibit a stereochemically active lone pair when the donor atoms are nitrogen, oxygen or fluorine [23]. 

Because of the comparatively large space requirements of a lone pair, low coordination numbers favor the expression of a stereochemically active lone pair [25]. Specific ligand design can trigger the stereochemical activity of a lone pair in complex compounds. 

Lighter elements show a stronger tendency to develop a stereochemically active lone pair than their heavier homologues. For instance, for antimony(III) more distorted structures are known than for bismuth(III) ]29]. 

When dioxane is added to [Au(C6C1s)2T1]00 (Equation (108)),120 a simple dimer results in which the Au-Tl groups are connected via a dioxane bridge between the two thallium atoms 132.120 The thallium atom is coordinated in a 7r-fashion to the toluene which served as the solvent of the reaction. The Tl(i) ion does not exhibit a stereochemically active lone pair, and the blue luminescence was attributed to an Au Tl charge transfer transition. 

The reaction of the diaminocarbene 49 with the diaminoplumbylene 50 furnishes dark red crystals of the zwitterionic adduct 51 in 71% yield. Compound 51 reveals a very long Pb-C bond length of 258.6(7) pm. While the C atom of the Pb-C bond is in an almost trigonal-planar environment (sum of angles 350.3°), the lead atom has a pyramidal environment with a sum of angles of 274.5°. This can be attributed to the presence of a stereochemically active lone pair at the lead atom (Equation (18)).74... 

Two factors combine to lend a greater diversity in the stereochemistries exhibited by bivalent germanium, tin and lead compounds, the increased radius of Mn compared with that of Mw and the presence of a non-bonding pair of electrons. When the non-bonding pair of electrons occupies the isotropic valence level s orbital, as in, for example, the complex cations Pb[SC(NH2)2]6+ and Pb[antipyrine]6+, or when they are donated to conductance band levels, as in the binary tin and lead selenides or tellurides or the perovskite ternary phases CsMX3 (M = Sn, Pb X = Cl, Br, I), then the metal coordination is regular. However, in the majority of compounds an apparent vacancy in the coordination sphere of the metal is observed, which is usually ascribed to the presence of the non-bonding pair of electrons in a hybrid orbital and cited as evidence for a stereochemically active lone pair . 

Tetraphenylbismuthonium diphenylbis(trifluoroacetato)bismuthate (equation 51)182 contains a distorted tetrahedral cation and an anion (69), the structure of which has been interpreted in terms of a stereochemically active lone pair. This appears to be the only four-coordinate trigonal bipyramidal bismuth structure established to date in antimony chemistry these types... 

If in addition to the bonding pairs there are stereochemically active lone pairs, the symmetry will be lowered (BF3 is Dv, NF3 is C3v). Furthermore, the hybridization of the lone pair(s) will be different from that of the bonding pairs (see below). 

Complexes of arseniqlll). anrimony(III), (electron configuration = ( - I lead(II). and bismulh(lll) I(n - 21/14 (n - l)r/ 0/ 2] with polydentate ligands occupying six coordination sites have been found 10 have a stereochemically active lone pair. However, the dichotomy of behavior of the heavier elements that have a lone pair is reflected m Ihe crystal chemistry of Br - When forced into sites of high symmetry, the Bi,+ ion responds by assuming a spherical shape in crystals of lower symmetry the lone pair asserts itself and becomes stereochemically active. 

The octalluoroxenaies are the most stable xenon compounds known they can be heated to 400 °C without decomposition. The anions have square antiprismatic geometry. They, too, present a problem to VSEPR theory analogous to that of XeF6 since they should also have a stereochemically active lone pair of electrons that should lower the symmetry of the anion. If the steric crowding theory is correct, however, the presence of eight ligand atom/, could force the lone pair into a stereochemically inert s Orhital. 

The synthesis of the X2M(Et2Dtc) complexes (X = Cl, Br, I M = As, Sb, Bi) is accomplished by the reaction of the M(Et2Dtc)3 complexes with MX3 in a 1 2 molar ratio in CHC13 (170). The structure of the Br2 As(Et2 Dtc) complex, which is monomeric and a nonelectrolyte in solution, in the solid state shows a loosely held centrosymmetric dimer with bridging bromide ions. The coordination geometry of the five-coordinate As(III) is intermediate between a square pyramid and a trigonal bipyramid, with a stereochemically active lone pair occupying a site of a distorted octahedral coordination sphere (Fig. 9). 

The structure of a 2 1 complex between antimony trichloride and phenanthrene consists of alternating double layers of SbCl3 and arene molecules (276). Unlike complex Cl and SbCl3 itself, Sb—Cl contacts between neighboring SbCl3 units are observed in this complex. In both structures mentioned here, vacant coordination sites are observed, which might be occupied by a stereochemically active lone pair at the antimony center. 

The preference for the +2 over +4 oxidation state increases down the group, the change being due to relativistic effects that make an important contribution to the inert pair. The inert pair concept holds only for the lead ion Pb2+(aq), which could have a 6s2 configuration. In more covalent Pb11 compounds and most Sn11 compounds there are stereochemically active lone pairs. In some MX2 (M = Ge or Sn) compounds Ge and Sn can act as donor ligands. 

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