Structures with multiple bonds

If the classical structure of a molecule involves one double bond to the central atom, the others being single bonds, then the hybrids involve only the s, and Py orbitals the p orbital is left as is. Thus we can form the trigonal hybrids, tr  

The requirements that the three hybrid orbitals have maximum extension and be equivalent to each other yield three orbitals that are directed to the apices of an equilateral triangle in the xy-plane. The set of sp hybrids is illustrated in Fig. 23.10. The unhybridized Pz orbital has its charge density above and below the plane of the hybrid orbitals. 

The electron configurations of the various atoms in the first period are 

The simplest example is formaldehyde, H2CO, which has the classical structure 

The stability gained by the molecule through the overlapping of the orbitals in the n bond locks the molecule in a planar configuration. If the plane of one CH2 group were at 90° to the plane of the other, the orbitals would not overlap the molecule would be much less stable in such a configuration. This accounts for the absence of rotation about the double bond and makes possible the existence of geometric isomers, the cis and trans forms of disubstituted ethylene. 

---

Drawing Lewis Structures with Multiple Bonds... 

The heavier elements favour ring structures rather linear structures with multiple bonds. 

EXAMPLE 3.6 Accounting for the structure of a molecule with multiple bonds... 

Multiple bonds can be treated as ring structures with bent bonds. The distortions dealt with in the preceding paragraph must be taken into account. For example, in ethylene every C atom is surrounded tetrahedrally by four electron pairs two pairs mediate the double bond between the C atoms via two bent bonds. The tension in the bent bonds reduces the angle between them and decreases their repulsion toward the C-H bonds, and the HCH bond angle is therefore bigger than 109.5°. 

There is some confusion as to a suitable representation for the actual structure of 7 shown in Fig. 1. The metal-metal bond is 2.462 A long and seems too short for a V—V single bond. Certainly, the reactivity of 7 is consistent with multiple bonding. What is at question is its nature and the role of the semi-bridging carbonyls. 

Molecules with multiple bonds between metal atoms often have structures with beautiful and highly symmetrical polyhedral shapes. The square prismatic [Re2Cl8]2 ion, shown in Figure 3-29, played an important role in the history of the discovery of metal-metal... 

The existence of multiple bonds between metal atoms in discrete complexes was first demonstrated by Cotton and coworkers in the early 1960 s (1 ). Since that time, several hundred papers have appeared dealing with the synthesis and structural and spectroscopic (2.) characterization of complexes with multiple bonds between metal atoms. However, there have been relatively few systematic investigations of the reactivity of the metal-metal multiple bonds. 

A major distinction for nucleophilic reactions with ambident anions is whether they proceed with kinetic or thermodynamic control.80 N-Substituted saccharins (10) should be thermodynamically more stable because of amide character than the isomeric pseudosaccharin (3) of imidate structure. In fact 3 may be rearranged thermally to 10 in an irreversible reaction.96 The threshold for thermodynamic control appears to be lowered for electrophiles with multiple bonds, e.g., formaldehyde, reactive derivatives of carboxylic acids, but also quaternary salts of N-heterocyclic compounds.80 It will be seen that in those cases substitution indeed occurs at the nitrogen, not necessarily through thermodynamic control. 

So far in our treatment of the VSEPR model, we have not considered any molecules with multiple bonds. To see how these molecules are handled by this model, let s consider the NO3- ion, which requires three resonance structures to describe its electronic structure ... 

---