Cationic complexes with a series of tridentate ligands, L = 4X-substituted 2,6-bis(benzimidazol-2-yl)pyridines, [MLJ[ CIO,],( M = Fe or Ni; X = H, OH or CI), were isolated and characterized, together with the free pyridines, by elemental analysis, Fourier-transform IR, 'H NMR and UV/VlS spectroscopy. The syntheses were performed via condensation of o-phenylenediamine with 4substituted pyridine-2,6-dicarboxylic acids. Ligand-field parameters were estimated for the nickel complexes. The [ FeL,IZ+ species show thermally induced spin-crossover behaviour ('A, ST, , ) which has been investigated in methanol, nitromethane and 20% (v/v) dimethylformamide in MeOH. The behaviour is complicated by two complex dissociation equilibria, for which equilibrium constants have been evaluated. Ligand substitution is reflected in a change of the spin state in solution [pex,,, = 2.50, X = H; 4.19, OH; and 4.49 pB, CI at 295 K, in MeOH) and in the metal-to-ligand chargetransfer band (500-557 nm); when M = Fe and X = H there is a pronounced spin-crossover equilibrium in methanolic solution (pexpt, = 1.31-3.45 pB for 21 3-328 K). A small variation of the magnetic moments when M = Fe and X = OH (paxpt, = 3.77-4.73 pB at 220-332 K) might indicate a temperature-variable population of the 5E, sublevel or variation in hydrogen bonding. The results are compared with quasi-relativistic quantum-mechanical calculations, and the spin-crossover behaviour of the new ligands, L, with substituents X = CHO, NH,, CN, Me, NO, , OH, CONH,, COCI, SH. F, CI, Br or I has been estimated. The differences in the calculated heats of formation between the high-and low-spin forms of [FeLJz+ when plotted against A8 (=lH NMR para increment for substituents X in benzene) show a turning point in the region around X = H and in this region spin-crossover behaviour is observed. Outside this region there is very little or no such behaviour and it is therefore possible to predict the spin-crossover behaviour for other substituents X from the A8 value.Thermochromism due to thermally induced equilibria between low-(1s.) and high-spin (h.s.) states of iron@) complexes can be observed when the potential minima of the 'A, and the 5T2 surfaces are so similar in energy that both can be thermally populated. ' The ligand-field splitting might then be overcome by the spin-pairing energy. Such spin-crossover phenomena have been investigated in the solid state,24 and in ~o l u t i o n , ~-' ~ where, however, ~o l v o l y s i s , ~~~~~ ligand-and anion-exchange reactions,' complex deprotonation ' and hydrogen-bonding