As E is varied by descending group 15, there is a clear shift in the weights of the resonance structures towards the phosphaketene analogue . This reflects the decrease in effective orbital overlap between E and C which in turn disfavours multiple bond formation. This increasing tendency to form double and not triple E-C bonds is also reflected in calculated E-C bond lengths . The data from Table 1 is evidence of E-C bond elongation which correlates with the change from triple to double bond.
In addition, NBO analysis highlights that the greatest electron delocalisation within the anions stems from the donation of an oxygen lone pair into the E−C π antibonding orbital. The energy value associated with this donation is seen to increase down the group . This explains the increasing resonance weight towards the ketene like isomer as populating antibonding orbitals usually suggests the breaking of a bond.
The shift towards the ketene isomer will also cause an increase in charge density on the elemental E atom; this makes the elemental atom an increasing source of nucleophilicity .