Reaction of [Th(I)(NR2)3] (R = SiMe3) (2) with KECPh3 (E = O, S) affords the thorium chalcogenates, [Th(ECPh3)(NR2)3] (3, E = O; 4, E = S), in moderate yields. Reductive deprotection of the trityl group from 3 and 4 by reaction with KC8, in the presence of 18-crown-6, affords a new thorium terminal oxo complex, [K(18-crown-6)][Th(O)(NR2)3] (6), and the first thorium terminal sulphide complex, [K(18-crown-6)][Th(S)(NR2)3] (7), respectively. The Natural bond orbital and quantum theory of atoms in molecules approaches are employed to explore the metal–ligand bonding in 6 and 7 and their uranium analogues, and in particular the relative roles of the actinide 5f and 6d orbitals.