Ab initio quantum mechanics calculations have been carried out on all possible conformations of the terpyridine ligand and its mono- and di-protonated forms. Results show that the lowest energy form of the ligand is when the N–C–C–N torsion angles are trans but that in the protonated forms, the cis arrangement is prevalent being stabilised by intramolecular N–H· · ·N hydrogen bonds. Results are consistent with the experimental crystal structure data found in the literature and also with two crystal structures reported here in which two different ion pairs formed between protonated terpyridine cations and lanthanate(III) nitrate anions have been prepared and analysed structurally. Compound 1 consists of discrete diprotonated terpyridine cations and hexanitratolanthanate anions, namely 3[H₂terpy]²⁺2[La(NO₃)₆]^3–·3H₂O. Water molecules are present as hydrogen bond acceptors in the diprotonated terpyridyl cavities. Each lanthanum atom is 12-co-ordinate and the La–O bond lengths vary between 2.609(11) and 2.765(10) Å. Compound 2 consists of a diprotonated [H₂terpy]²⁺ cation together with a [Sm(terpy)(NO₃)₄]^– anion, and a NO₃^– anion which is present as a hydrogen bond acceptor in the diprotonated terpyridyl cavity. The samarium atom is 11-co-ordinate, the Sm–O bond lengths vary between 2.494(5) and 2.742(5) Å while the Sm–N bond lengths vary between 2.626(4) and 2.650(5) Å.