Trigonal bipyramidal VSEPR structure

Section 8.13 molecular structure valence shell electron-pair repulsion (VSEPR) model linear structure trigonal planar structure tetrahedral structure trigonal pyramid trigonal bipyramid octahedral structure square planar structure... 

This allocation is the one used in VSEPR theory the repulsions between the five bond pairs of the five P—F bonds give rise to a PF5 molecule with a trigonal-bipyramidal shape (Structure 8.2). Forced to choose between the octet and the two-centre electron-pair bond, we retain the latter and abandon the former. An important consequence of this choice emerges when we consider which phosphorus orbitals must be used to form the five electron-pair bonds. The outer electronic configuration of phosphorus is 3s 3p (Figure 8.6) five unpaired electrons are needed to form the five bonds, and only three are available. 

An interesting application of VSEPR is illustrated by the structure of SF4. The sulfur atom has 10 electrons around it (six valence shell electrons from S and one from each of the four F atoms). We predict that the structure will be based on a trigonal bipyramid, but there are two possible structures ... 

Thus, according to this alternate approach, PCI5 belongs to the VSEPR class AX5. Molecules of this type adopt a trigonal bipyramidal structure. 

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. 

B—The Lewis (electron-dot) structure has five bonding pairs around the central Sb and no lone pairs. VSEPR predicts this number of pairs to give a trigonal bipyramidal structure. 

The heavier atoms of Group 13 appear not to form p -p bonds. Five- and six-coordinate structures (schemes (4) and (5)) are almost entirely restricted to the heavier atoms, although at least one complex containing five-coordinate boron is known. According to VSEPR theory, scheme (4) structures should be trigonal bipyramidal. However, InClf- and TlClf are found to be square pyramidal in crystalline solids. As noted in Section 8.2, this shape (also adopted by MnClf-) is possibly favoured by crystal packing requirements. 

The Lewis structure reveals a VSEPR number of 5 for the central iodine atom, two bonded neighbors and three unshared pairs. To determine which corners of the trigonal bipyramid are occupied by the terminal iodine atoms, find the arrangement which maximizes the angles between the unshared pairs. The preferred arrangement, Fig. 9-45(a), must be the one in which the unshared pairs are all at 120° because any other alternative [Fig. 9-45(b) and (c)] would have two sets of pairs at 90°. Therefore, the two terminal iodine atoms must occupy the axial positions (180° to each other), making the molecule linear. 

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... 

A systematic study of the series (CH3) PFs n [ = 0,1,2,3] has been recently completed by Bartell and co-workers ° The observed variations in the bond distance of these trigonal bipyramidal compounds are well correlated with the number of methyl substituents. In all cases the least electronegative ligands (or CH3) occupy equatorii sites. The sterochemistry and trends in structure parameters are well accounted for by the VSEPR theory. Furthermore, the increase of the axial P-F bond lengths in this series correlates well with the increase in P—F amplitudes of vibration. The methyl groups essentially rotate freely. 

The structure of SF4, 15.26, is derived from a trigonal bipyramid and can be rationalized in terms of VSEPR theory. The S—F and S—F q bond distances are quite different (Table 15.5). Oxidation by O2 in the absence of a... 

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

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