Context <p>Cation-π and cation-lone pair interactions between 3d-metal (II) ions [Fe(II), Co(II), Ni(II) and Cu(II)] and furan are explored in the formation of 1:1 and 1:2 type complexes. Both cation-π (IE_gas= −192.27 to −312.65 kcal mol⁻¹) and cation-lone pair (IE_gas= −163.13 to −271.76 kcal mol⁻¹) interactions are reasonably strong and lead to the formation of stable 1:1 and 1:2 type complexes in gas phase. The complexes are also stable in solvent phase, but their stability is reduced significantly in presence of solvent dielectrics, especially in ethanol, DMSO and water. Formation of the complexes is thermodynamically favourable (exothermic and spontaneous). Charge transfer (Δq = 0.62 to 1.92 e⁻), Laplacian of electron density (∇²ρ = 0.1435 to 0.6628 au) and total electron energy density (H(r) = −0.0019 to −0.0436 au) analysis have argued in favour of partial ionic and partial covalent character of the interactions.</p> Methods <p>Density functional theory (DFT) is exclusively used for the study. Polarizable continuum model (PCM) is used to perform solvent phase study. Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses are performed for understanding other aspects of complex formation.</p>