Abinitiominimal basis set STO-6G calculations show axial-SiF4•NH3andtrans-SiF4•2NH3to be energetically more stable with respect to SiF4and ammonia. All other geometrical isomers are 1.0–2.0 eV higher.Our calculation results rule out the steric hindrance between ligands as the reason SiF4•2NH3is, according to experimental evidences,ciswhile complexes of bulkier ligands aretrans.Since thecis-SiF4•2NH3has a large electric dipole moment and a suitable charge arrangement it is conceivably the favoured form in the solid phase. Thecis → transrearrangement may take place under conditions where the advantages of thecisare 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.