Tetrahedral VSEPR structure

Although we have described the structures of several molecules in terms of hybrid orbitals and VSEPR, not all structures are this simple. The structures of H20 (bond angle 104.4°) and NH3 (bond angles 107.1°) were described in terms of sp3 hybridization of orbitals on the central atom and comparatively small deviations from the ideal bond angle of 109° 28 caused by the effects of unshared pairs of electrons. If we consider the structures of H2S and PH3 in those terms, we have a problem. The reason is that the bond angle for H2S is 92.3°, and the bond angles in PH3 are 93.7°. Clearly, there is more than a minor deviation from the expected tetrahedral bond angle of 109° 28 caused by the effect of unshared pairs of electrons ... 

Thus, according to this alternate approach, C1F4+ belongs to the VSEPR class AX. Molecules of this type adopt a tetrahedral structure. 

Once the Lewis diagram is complete, we can then use the VSEPR method to establish the geometry for the electron pairs on the central atom. The Lewis structure has two bonding electron pairs and two lone pairs of electrons around the central chlorine atom. These four pairs of electrons assume a tetrahedral geometry to minimize electron-electron repulsions. 

D—SiC, is tetrahedral. BrF4 is square planar. C2H2 is linear. TeF6 is octahedral. N03 is trigonal planar. If you are uncertain about any of these, Lewis structures and VSEPR are needed. 

SOLUTION The Lewis structure of nitrogen trifluoride is shown in (22) we see that the central N atom has four electron pairs. According to the VSEPR model, these four electron-rich regions adopt a tetrahedral arrangement. Because one of the pairs is a lone pair, the molecule is expected to be trigonal pyramidal (23). Spectroscopic measurements confirm this prediction. 

In all the three molecules, the central atom has four shared pairs of electrons like AB4 discussed in (c) above. The favourable geometrical arrangement according to VSEPR theory, which keeps the shared pairs of electrons as far as possible, should be tetrahedral like CH4, CC14, etc. But the actual geometry is different which can be easily followed when we look at their Lewis structures given above. 

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. 

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... 

Finally, water, H2O, has steric number 4 with two shared pairs and two unshared pairs on the oxygen atom. VSEPR theory predicts a bent structure, as a subcase of tetrahedral structure, with angles significantly less than the tetrahedral value of 109° due to repulsion between the two unshared pairs and the bonding pairs. Experimentally determined bond angles of 104° verify this prediction. 

E14.3 SiCl F, SiCLFi, and SiClF3 have tetrahedral structures as shown below. Each Si has an octet and each molecule conforms with VSEPR theory ... 

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