VSEPR hydrides

According to the VSEPR model developed in Chapter 9, an inner atom with a steric number of 4 adopts tetrahedral electron group geometry. This tetrahedral arrangement of four electron groups is very common, the only important exceptions being the hydrides of elements beyond the second row, such as H2 S and PH3. Thus,... 

C3V o shape (Fig. 4.5(a)), ReH5 (Fig. 4.7(b)) resembles the idealized ML5 Cs shape (Fig. 4.4(a)), OsH4 (Fig. 4.7(c)) is the idealized ML4 Td shape (Fig. 4.3(a)), and so forth. Thus, the rather unusual hydride geometries immediately suggest the role of sdM hybridization and covalency, leading to molecular shapes quite unlike those expected from simple ionic or packing ( VSEPR-like 15) forces.16... 

These simplified VSEPR rules may seem a far cry from the more elegant application of symmetry and molecular orbitals to the beryllium hydride molecule and the nitrite ion (Chapter 5). or the BH2 molecule (Problem 6.27). Although the molecular orbital approach can rationalize these structures, the direct application of the VSEPR rules is by far the easier way to approach a new structure. 

POLYATOMIC MOLECULES Specifying the three-dimensional structure of polyatomic molecules requires that we include bond angles and bond lengths. Any successful theory of bonding must explain and predict these structures. Let s test the VB approximation on the second-period hydrides, the structures of which we have already examined in Chapter 3 using VSEPR theory. 

Beryllium hydride, BeH2, has four valence electrons, two from beryllium and one each from the two hydrogen atoms, all of which appear in its Lewis diagram. In VSEPR theory, the steric number is 2, so the molecule is predicted to be linear, and this prediction is verified by experiment. The electron configuration of the central atom is Be (ls) (2s). There are no unpaired electrons to overlap with H(ls) orbitals, so the VB model fails to predict the formation of BeHi. 

Methane, CH4, has steric number 4, and VSEPR predicts a tetrahedral structure, which is confirmed by experimental results. Starting with the electron configuration C (ls) (2s) (2p), the VB model cannot account for the formation of CH4 and predicts that CH2 would be the stable hydride, which is again contrary to the... 

The structures of the hydrogen compounds of the nonmetals are adequately described by VSEPR theory. Incorrect not all structures can be predicted by VSEPR theory, only those of electron-precise and electron-rich hydrides this theory cannot explain BiH. an electron-poor hydride. 

Most molecular hydrides are volatile and have simple structures which comply with VSEPR theory (see Section 1.19). However, BH3, 9.9, although known in the gas phase, dimerizes to give B2H5, 9.10, and GaH3 behaves similarly B2H6 and Ga2Hg are described in Section 12.5. 

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 ... 

Most molecular hydrides are volatile and have simple structures which comply with the VSEPR model (see... 

Here we have used the hybridization model after the molecular structure had been determined experimentally. Could the hybridization model - like the VSEPR model - be used to predict the structure The answer is yes we could have begun by asking if it is possible to combine the s, Px and py orbitals to form three new hybrids which are mutually orthogonal and have the same shape. The derivation is more involved, but it can be shown that the only possible candidates are the sp hydrides that we have just described, and which form angles of 120° relative to one another. There is, however, not much reason to carry out the derivation, since the VSEPR model leads us directly and quickly to the same conclusion. 

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