The octet rule and isoelectronic species

Worked example 2.1 Molecular orbital theory properties of diatomics 

The bond dissociation enthalpies for the nitrogen-nitrogen bond in N2 and [N2I are 945 and 765 kJ mol respectively. Account for this difference in terms of MO theory, and state whether [N2l is expected to be diamagnetic or paramagnetic. 

The lower bond order of [N2] compared with N2 is consistent with a lower bond dissociation enthalpy. 

Using MO theory, rationalize why the N N bond distance in IN2I is greater (112 pm) than in N2 (109 pm). 

Classify the changes from (a) N2 to IN2I, (b) from IN2P to N2 and (c) from N2l to (N2I as one- or two-electron, oxidation or reduction steps. 

Diatomic Bond distance / pm Bond dissociation enthalpy / kJmoF Bond order Magnetic properties 

Each N atom has the ground state configuration of [He]2shp  

An MO diagram for N2, assuming only 2s-2s and 2p-2p orbital interactions, can be constmcted, the result being as shown in Fig. 2.10. From this diagram, the bond order in N2 is 3.0. 

The change from N2 to [N2] is a one-electron reduction and, assuming that Fig. 2.10 is still applicable, an electron is added to a -Kg 2p) orbital. The calculated bond order in [N2l is therefore 2.5. 

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There are several important chemical species that consist of four atoms and have a total of 24 valence-shell electrons. Some of the most common isoelectronic species of this type are C032-, N03 , S03, and P() j (known as the metaphosphate ion). Because four atoms would require a total of 32 electrons for each to have an octet, we conclude that eight electrons must be shared in four bonds. With four bonds to the central atom, there can be no unshared pairs on that atom if the octet rule is to be obeyed. Therefore, we can draw the structure for CO, 2 showing one double C=0 bond and two single C-O bonds as... 

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