Symmetric [4-np…H…4-np]− and unsymmetric [4-np-H…4-np−] dimers of 4-np− have been isolated in s-fac-[Co(dien)2](4-np)3(4-npH)·2H2O (dien = diethylenetriamine) using a one pot synthesis. The crystal structure of the title complex revealed a strong network of H-bonding interactions which lead to the formation of layers of [H(4-np)2]− (symmetric and unsymmetric), s-fac-[Co(dien)2]3+ and water molecules. These layers formed their respective columns and the intermolecular cohesion of 4-npH within the columns of the 4-np− ions is achieved via nitro–π and π-π stacking interactions. Water also played a dominant role in stabilizing the structure, in which the HOH…O (4-np−) interaction is weaker than [4-np…H…4-np]− , but stronger than the NH…O (4-np)− and NH (dien)…OHO interactions. The H-bond present in the symmetric dimer is the strongest one, having an O…O contact distance of 2.433(3) Å. The presence of the s-fac-isomer of the cation is also supported by IR and 1H, 13C and 2D 1H–13C NMR spectroscopy. The XRD pattern of the product (bulk) was also determined and it shows the presence of a single phase. The binding capacity (log β = 27.101 ± 0.06) of [Co(dien)2]3+ with the 4-np− was determined by UV–vis spectroscopic titrations and the ionic behaviour of the complex was confirmed by molar conductance measurements at infinite dilution (λo = 800 S cm2 mol−1).