A CNDO/2 localised orbital study of dipole moments of sila- and germa-ethylenes

CNDO/2 calculations have been performed to predict dipole moments for silaethylenes, with either Slater of Burns atomic orbitals, and for germaethylenes with Burns orbitals only. Trends on replacement of two hydrogen atoms at either end of the double bond by fluorine are similar in both sets for the silaethylenes and in accordance with expectation, but an anomalous trend is observed for fluorination of the carbon in germaethylenes. Localised orbitals have been examined to seek an explanation for this anomaly. In all cases the trends are in the unexpected sense for the p orbitals but in the expected sense for the others taken together and for most of these individually. The anomaly with the germaethylenes thus appears to be associated with the relative magnitudes of these opposing trends, where donation of electrons via the p bond to the germanium atom outweighs the accumulation of charge around the fluorine atoms attached to the carbon. © 1977.