. Can. J. Chem. 62, 27 (1984). Ab initio minimal basis set STO-6G calculations show axial-SiF4. NH3 and trans-SiF4. 2NH3 to be energetically more stable with respect to SiF, and ammonia. All other geometrical isomers are 1 .O-2.0 eV higher.Our calculation results rule out the steric hindrance between ligands as the reason S F 4 . 2NH3 is, according to experimental evidences, cis while complexes of bulkier ligands are trans.Since the cis-SiF,. 2NH, has a large electric dipole moment and a suitable charge arrangement it is conceivably the favoured form in the solid phase. The cis + trans rearrangement may take place under conditions where the advantages of the cis are not operative.Although there is some degree of covalency in the Si-N bond it remains largely electrostatic, with the lone pair of ammonia attracted to the positively charged Si atom. The lone pair does not appear to be extensively altered by the complex formation.