The single-crystal X-ray structure of solvent-free (tmtaa)Co-II reveals three different pi-pi intermacrocyclic interactions between tmtaa units (tmtaa = dibenzotetramethyltetraaza[14]annulene). Pairs of inequivalent (tmtaa)Co-II units in the unit cell link into a one-dimensional pi-pi stacked array in the solid state. Magnetic susceptibility (chi) studies from 300 to 2 K reveal the effects of intermolecular interactions between (tmtaa)Co-II units in the solid state. The effective magnetic moment per Co-II center is constant at 2.83 mu(B) from 300 to 100 K and begins to significantly decrease at lower temperatures. The magnetic data are fit to a singlet (S = 0) ground state with a triplet (S = 1) excited state that is 13 cm(-1) higher in energy (-2J = 13 cm(-1)). Toluene solutions of (tmtaa)Co-II have nuclear magnetic resonance (NMR) paramagnetic shifts, a solution-phase magnetic moment mu(eff) (295 K) of 2.1 mu(B), and toluene glass electron paramagnetic resonance spectra that are most consistent with a low-spin (S = 1/2) Coll with the unpaired electron located in the d(yz) orbital. Pyridine interacts with (tmtaa)Co-II to form a five-coordinate monopyridine complex in which the unpaired electron is in the d(z)(2) orbital. The five-coordinate complex has been structurally characterized by single-crystal X-ray diffraction, and the equilibrium constant for pyridine binding at 295 K has been evaluated by both electronic and H-1 NMR spectra. Density functional theory computation using the UB3LYP hybrid functional places the unpaired electron for (tmtaa)Co-II in the d(yz) orbital and that for the monopyridine complex in the d(z)(2) orbital, consistent with spectroscopic observations.