Crystallisation from MeCN solutions containing copper(I) tetrafluoroborate, 2,9-dimethyl-1,10-phenanthroline (dmp) and either 2-cyanoguanidine (cnge) or one of its substituted derivatives, 2-cyano-N,N′-dimethylguanidine (dmcnge) and 2-cyanoimino-4,6-pyrimidine (cidmp), by Et₂O vapour diffusion methods yielded [Cu(dmp)(nitrile)][BF₄]· xMeCN (nitrile = cnge, x = 1 2; dmcnge, x = 0; or cidmp, x = 0). In the absence of an added nitrile [Cu(dmp)(NCMe)][BF₄] 3 formed. Crystallisation from CH₂Cl₂ solutions containing copper(I) tetrafluoroborate, dmp and cnge by Et₂O vapour diffusion methods yielded [Cu(dmp)(cnge)][BF₄]·0.5Et₂O 1. Structural studies of 1, 2 and 3 have established that the [Cu(dmp)(nitrile)]⁺ cations are three-co-ordinate trigonal planar (Y-shaped) species with bidentate dmp and monodentate nitrile ligands. The MeCN molecule in 2 is hydrogen bonded to the cnge ligand in a position adjacent to the copper(I) atom. When 1, 2 and 3 are combined with a tetrahedral copper(I) species co-ordinated by a bidentate ligand, cnge and MeCN, they represent stages in a crystallographic sequence depicting associative substitution at trigonal planar copper(I). In solution an equilibrium [K_e = 3.9(6) at 298 K] exists between [Cu(NCMe)₄]⁺, [Cu(dmp)(nitrile)_x]⁺ (x = 1 or 2) and [Cu(dmp)₂]⁺ cations, indicating that the stability of the [Cu(dmp)(nitrile)]⁺ cations in the solid phase must be due to intermolecular packing interactions. For all three structurally characterised complexes, π–π (face-to-face) stacking interactions between co-ordinated dmp molecules generate an efficient parallel packing system thus promoting the trigonal planar copper(I) co-ordination geometry.