VSEPR model linear structures

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... 

A molecule with only two atoms attached to the central atom is BeCl2. The Lewis structure is CI — Be — CE, and there are no lone pairs on the central atom. To be as far apart as possible, the two bonding pairs lie on opposite sides of the Be atom, and so the electron arrangement is linear. Because a Cl atom is attached by each bonding pair, the VSEPR model predicts a linear shape for the BeCL molecule, with a bond angle of 180° (4). That shape is confirmed by experiment. 

The VSEPR model for predicting structure does not work for complex ions. However, we can safely assume that a complex ion with a coordination number of 6 has an octahedral arrangement of ligands, and that complexes with two ligands are linear. On the other hand, complex ions with a coordination number of 4 can be either tetrahedral or square planar there is no reliable way to predict which will occur in a particular case. 

Several of these compounds and ions have interesting structures which have provided tests for models of bonding. For example, structures of the xenon fluorides have been interpreted on the basis of the VSEPR model (Figure 8-31). Xep2 and Xep4 have structures entirely in accord with their VSEPR descriptions Xep2 is linear (three lone pairs on Xe) and Xep4 is planar (two lone pairs). 

When structures are determined by diffraction methods, atom positions are located. Thus, in terms of a structural descriptor XeF2 is linear and [XeF5] is pentagonal planar. In the diagrams above we show two representations of each species, one with the lone pairs to emphasize the origin of the prediction from the VSEPR model. 

Thus CO2 has two "effective pairs" that lead to its linear structure, whereas O3 has three "effective pairs" that lead to its bent structure with a 120° bond angle. Therefore, to use the VSEPR model for molecules (or ions) that have double bonds, we use the same tools as those given on p. 430, but we count any double bond the same as a single electron pair. Although we have not shown it here, triple bonds also count as one repulsive unit in applying the VSEPR model. 

Finally we turn our attention to the suboxide. Experiments show that the atoms are connected in the sequence OCCCO. The Lewis structure of is presumably 0 C C C 0 . The observed C-C and C-O bond distances are indeed consistent with a double bond description 0=C=C=C=0. The VSEPR model then leads to the prediction that the molecule is linear. This is not strictly true the molecule is bent at the central atom with ZCCC = 156° The potential energy maximum at ZCCC = 180° (see Fig. 9.1) is, however, less than 0.25 kJ mol above the minimum, and the molecule is therefore often described as pseudo linear [1]. Why the small energy hump at ZCCC = 180° is there, nobody seems to know. 

Is the VSEPR model consistent with a linear or bent structure for [NOi] ... 

N is in group 15 and has five valence electrons. Allow the positive charge to be localized on the nitrogen centre an N centre has four valence electrons. O is in group 16 and has six valence electrons an atom of O requires two electrons to complete its octet. All four electrons in the valence shell of the N" " centre are involved in bonding, forming two double bonds in [NO]. Since there are no lone pairs on the N atom, the VSEPR model is consistent with a linear structure ... 

Thus CO2 has two "effective pairs" that lead to its linear structure, whereas O3 has three "effective pairs" that lead to its bent structure with a 120° bond angle. Therefore, to use the VSEPR model for molecules (or ions)... 

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. 

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