VSEPR model trigonal bipyramidal

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

For AX molecules with no lone pairs in the valence shell of A, both the VSEPR model and the LCP model predict the same geometries, namely AX2 linear, AX3 equilateral triangular, AX4 tetrahedral, AX5 trigonal bipyramidal, and AX octahedral. Indeed Bent s tangent sphere model can be used equally as a model of the packing of spherical electron pair domains and as a model of the close packing of spherical ligands around the core of the central atom. 

According to the VSEPR model the T-shaped CIF3 molecule has a trigonal bipyramidal arrangement of three bonding electron pairs and two nonbonding electron pairs in the valence shell of the central Cl atom. 

The final cu-bonded formulas (3.213), (3.214), and (3.219)-(3.221) bear an obvious resemblance to the usual VSEPR representations of these hypervalent species. Indeed, each cu-bonded structure has the same number of formal bond pairs (bp) and lone pairs (lp) as the VSEPR representation. Furthermore, the predicted angular geometries of the two models are essentially identical, with the linear (or near-linear) cu-bonded ligands occupying axial positions in the SN2-like trigonal bipyramidal motif. 

This is why SF4 adopts a trigonal bipyramidal as opposed to a tetrahedral equilibrium geometry. Electrons, like atoms, take up space, which is the basis of such models as VSEPR theory. 

Fig. 5. Geometries detected by TAMREAC according to the Angular Overlap and VSEPR models for four- and five-coordinate complexes. Abbreviations SPL = square planar, Td = tetrahedral, SPY = square pyramidal, TBP = trigonal bipyramidal, Is = low spin, hs = high spin, d" = number of electrons in the d configuration of the metal.

The amended VSEPR model predicts two forms of five-coordination, and experimental chemistry has clearly identified many examples of both forms. These limiting structures are square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure 4.13). The classical square-based pyramidal shape is formed simply by cleaving off one bond from an octahedral shape, which leaves the metal in the same plane as the four square-based ligands. In reality, almost no complexes exhibit this shape, but rather adopt a distorted... 

The VSEPR model we discussed in Chapter 10 accounts for molecular shapes by assuming that electron groups minimize their repulsions, and thus occupy as much space as possible around a central atom. But it does not explain how the shapes arise from interactions of atomic orbitals. After all, the orbitals we examined in Chapter 7 aren t oriented toward the comers of a tetrahedron or a trigonal bipyramid, to mention just two of the common molecular shapes. Moreover, knowing the shape doesn t help us explain the magnetic and spectral properties of molecules only an understanding of their orbitals and energy levels can do that. 

All the Group 5A elements except nitrogen can form molecules with five covalent bonds (of general formula MX5). Nitrogen cannot form such molecules because of its small size. The MX5 molecules have a trigonal bipyramidal shape (see Fig. 20.9) as predicted by the VSEPR model, and the central atom can be described as dsp hybridized. 

The structure of SF4, 16.29, is derived from a trigonal bipyramid and can be rationalized in terms of the VSEPR model. The S-F x and S-F q bond distances are quite different (Table 16.5). Oxidation by O2 in the absence of a catalyst to form SOF4 is slow. The structure of SOF4,... 

Within the VSEPR model, a trigonal bipyramidal coordination environment is expected. 

First, draw the expected structure of [Ph3SiH2]. The question states that the hydride ligands are treats, and a trigonal bipyramidal structure is consistent with the VSEPR model ... 

The A1 atom is surrounded by three covalent bond electron pairs and two dative bond electron pairs. According to the VSEPR model the most stable arrangement of five electron pairs in the valence shell of the central atom is trigonal bipyramidal, and this is indeed the structure observed. The VSEPR model may also be used to rationalize the observation that the donor atoms occupy axial positions an axial bond electron pair is repelled by three (equatorial) bond pairs, while an equatorial bond electron is repelled by two axial bond pairs across the same angle. Since a covalent bond electron pair requires more space at the aluminum atom than a dative bond electron pair, the covalently bonded atoms occupy the equatorial positions. 

A tetravalent Group 16 atom is presumably surrounded by five electron pairs, viz. four bond pairs and one non-bonding electron pair. According to the VSEPR model the five electron pairs should arrange themselves in a trigonal bipyramidal manner. Since the nonbonding electron pair requires more space around the central atom than the bond pairs, it is expected to occupy an equatorial position, and deform the molecule in such a way that the bond electron pairs are pushed away. This is indeed the kind of distortion that is observed. The difference between axial and equatorial bond distances may be explained in the same manner as for PF5 and the other compounds of hypervalent Group 15 elements. 

The structure of H3NICI is consistent with the formation of a weak dative bond N I. This description implies that the iodine atom in the complex is surrounded by one covalent bond pair, one dative bond pair, and three lone pairs. According to the VSEPR model the five electron pairs will be directed towards the corners of a trigonal bipyramid, and since the three non-bonding electron pairs require more space, they will occupy equatorial positions. 

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