The anionic dinitrile ligand dicyanonitrosomethanide (dcnm), C(CN)(2)(NO)(-), and the anion resulting from its addition product with water, carbamoylcyanonitrosomethanide (ccnm), C(CN)(CONH2)(NO)(-), have been incorporated into lanthanoid complexes and display unusual eta(2)(N,O) nitroso coordination modes. (Et4N)(3)[Ln(ccnm)(6)] (1Ln; 1Ln = 1La, 1Ce, 1Pr, 1Nd, 1Sm) and (Me4N)(3)[Ln(ccnm)(6)] (2Ln; 2Ln = 2La, 2Ce, 2Pr, 2Nd) are systems containing 12-coordinate homoleptic trianionic lanthanoidate complexes. The nitroso groups of the ccnm ligands form three-membered ring chelates with the lanthanoid metal centre, with the asymmetry of the nitroso eta(2) interactions dependent upon the intramolecular N-H center dot center dot center dot O=N hydrogen bonding. Additional intermolecular hydrogen bonding interactions exist between adjacent amide and nitrile groups giving rise to 3D alpha-Po and 6,8-connected (4(12).6(3))(4(20).6(8)) networks in 1Ln and 2Ln, respectively. The compounds (Me4N)(3)[Ln(dcnm)(6)] (3Ln; 3Ln = 3La, 3Ce, 3Nd, 3Sm) also contain a 12-coordinate trianionic lanthanoidate complex with the nitroso group exhibiting a highly symmetrical eta(2) interaction.