We present a structural investigation of thirteen homoleptic complexes containing {Co(tpy) 2 } 2+ or {Co(tpy) 2 } 3+ cores and tpy, Cltpy, MeOtpy, EtOtpy or PrOtpy ligands (Cltpy ¼ 4 0 -chloro-2,2 0 :6 0 ,2 00terpyridine, MeOtpy ¼ 4 0 -methoxy-2,2 0 :6 0 ,2 00 -terpyridine, EtOtpy ¼ 4 0 -ethoxy-2,2 0 :6 0 ,2 00 -terpyridine, PrOtpy ¼ 4 0 -propoxy-2,2 0 :6 0 ,2 00 -terpyridine). For the twelve hexafluoridophosphate salts, several trends in the ways in which the cations pack in the solid state emerge. In [Co(tpy) 2 ][PF 6 ] 2 $2MeCN and [Co(Cltpy) 2 ][PF 6 ] 2 , two-dimensional grids of cations assemble through tpy embraces. Oxidation of cobalt(II) to cobalt(III) results in a loss of the grid, consistent with the presence of an increased number of anions and the need to minimize cation/cation repulsions. In the solid state, the cobalt(II) complexes containing MeOtpy or EtOtpy ligands exhibit face-to-face ROtpy/tpyOR embraces supported by CH alkoxy /p interactions. These are the dominant packing interactions in [Co(MeOtpy) 2 ][PF 6 ] 2 and [Co(EtOtpy) 2 ][PF 6 ] 2 $2MeCN. The transition from [Co(MeOtpy) 2 ][PF 6 ] 2 to [Co(MeOtpy) 2 ][PF 6 ] 2 $MeCN causes the packing to revert to a two-dimensional grid with tpy embraces; the MeCN solvent molecules produce periodic dislocations in the grid-like array. On going from ethoxy to propoxy derivatives, we observe a reversion to assemblies dependent upon tpy embraces. Onedimensional chains are favoured in [Co(PrOtpy) 2 ][PF 6 ] 2 $MeCN and [Co(PrOtpy) 2 ][PF 6 ] 2 $MeCN$Et 2 O, with the degree of solvation influencing the packing motifs. Oxidation of each of [Co(ROtpy) 2 ][PF 6 ] 2 (R ¼ Me, Et, Pr) results in increased separation of the cations. The crystal packing in [Co(Xtpy) 2 ][PF 6 ] 3 $MeCN for X ¼ Cl and MeO is virtually superimposable, and the [Co(Xtpy) 2 ] 3+ cation only has to undergo a slight twist within its lattice cavity to accommodate the Cl or MeO substituent. On going from [Co(PrOtpy) 2 ][PF 6 ] 2 $MeCN or [Co(PrOtpy) 2 ][PF 6 ] 2 $MeCN$Et 2 O to 2{[Co(PrOtpy) 2 ][PF 6 ] 3 }$5MeCN, oxidation of the metal tips the balance between tpy embraces and CH alkoxy /p interactions, and pairing of cations through the latter interactions is favoured. In [Co(EtOtpy) 2 ]Br 3 $MeCN$H 2 O, the [Co(EtOtpy) 2 ] 3+ cations assemble into one-dimensional chains through CH alkoxy /p interactions.Scheme 1 Ligands and numbering scheme for NMR assignments (tpy ¼ 2,2 0 :6 0 ,2 00 -terpyridine).