Sterically active lone pair

In a similar manner, the potentially sexadentate macrocycle (21) yields Pb(n) and Cd(n) complexes which have unusual geometries (Drew etal., 1979). With Pb(n), the stereochemistry is hexagonal pyramidal with one axial site occupied by a water molecule and the other filled by a sterically-active lone pair of electrons on the metal ion. The Cd(n) complex is eight-coordinate (with a water molecule and a perchlorate group occupying axial positions) however the cadmium ion is not held centrally in the macrocyclic hole but is displaced to one side, presumably reflecting the... 

The most common structures of arsenic compounds are tetrahedral and pyramidal, which are similar when the sterically active lone pair is counted. Tetrahedral symmetry holds the potential for chirality and indeed many chiral organoarsenic compounds have been prepared. Arsenic may also use d orbitals for (d-d)n bonding and for hybridization with s2 and p3 orbitals, resulting in trigonal bipyramidal or octahedral structures. In the former the more electronegative substituents occupy the apical position. 

Exploration of the potential applications of molecular mechanics to compounds of s-, p- and f-block elements is only just beginning. The difficulties arising from the electrostatic bonding in the s- and f-block elements have been tackled in a number of different ways, in most cases with reasonable success. In general, p-block elements are modeled relatively readily however, the problems of sterically active lone pairs have yet to be tackled. 

The structures of these ions normally conform to those predicted by the VSEPR theory, as shown in Fig. 17.2.2. Since the anion XY has two more electrons than the cation XY+, they have very different shapes. The anion IFj- is planar with lone pairs occupying the axial positions of a pentagonal bipyramid. In [Me4N](IF6), IFg is a distorted octahedron (C3V symmetry) with a sterically active lone pair, whereas both BrFg and ClFg are octahedral. The anion IF " has the expected square antiprismatic structure. 

The P-donor ligands we consider in this Part I are phosphoms(III) compounds. We avoid the classification difficulties of phosphine, PH3, as based on P-oxidation state —HI by referring throughout to tervalent phosphoms. Low-coordination number phosphorus species, such as RP, RCP, R2C=PR, and P , are presented in Part II. The P-donor ligands considered in Part I are covered by the generalized formula PR3 for a P-donor ligand. The PR3 ligands have a pyramidal shape due to their sterically active lone pair of electrons. In terms of a Valence Shell Electron Pair Repulsion Model model, the lone pair occupies the vacant tetrahedral site of the phosphoms center. 

Vibrational spectra of the [SC1 F3 ] ions (n = 0-3) [663] and PH F3 (n = 0-3) [664] have been assigned. The [Xe02F]+ ion takes a pseudo-tetrahedral structure owing to the presence of a sterically active lone-pair electron of the Xe atom [661] ... 

Previously, the Raman spectrum of the [OXeF5] ion was assigned on the basis of an octahedral structure that is distorted to symmetry because of the presence of a sterically active lone-pair on the octahedral faces adjacent to the axial fluorine [1327,1328] ... 

Antimony(III) complexes with bidentate sulfur ligands form an interesting series of structural types. In tris(diethyldithiocarbamate)antimony(IIl) the sulfur atoms fall into two groups. Firstly, there are three fac sulfurs with short d(Sb—S) 2.487-2.631 A bonds, one from each ligand, and then there are three longer bonds d(Sh—S) 2.886-2.965 A (40). There is a large gap in the coordination sphere and this has been ascribed to a sterically active lone pair. There is a weak Sb- -S 3.389 A interaction between molecules within van der Waals radii. Similar results have been obtained for Sb(S2CNPr2) and tris(l-pyrrolidinecarbo-dithioato)antimony(III). Hoskins et used the bidentate EtOCSj to prepare... 

This is NOT the VSEPR theory at all. It is solely a steric or electrostatic scheme that happens to work on this carefully chosen group of coordination compounds. However, where there are no sterically active lone pairs, surely this is precisely what the simplest version of the VSEPR formalism is. What is remarkable is that so simple a model, originally designed solely for covalent bonded compounds of nonmetals, should succeed for so many coordination complexes of transition metals The only requirement for the success of this particular form of the VSEPR approach seems to be that the compounds obey the EAN rule. 

Fio. 18. (a) Pyramidal structure of the SnX3 ion due to the presence of a sterically active lone pair of electrons, (b) The complex ion [X3B.SnCl3] illustrating coordination by the lone pair of electrons on SnCb". (c) Structure of the SnaFj" anion. 

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