A density functional theory investigation on 1H-4-germapyridine-4-ylidene & the unsaturated heterocyclic substituted ones

•Investigation of the stability, aromativity, charge and reactivity of twenty singlet and triplet NHGes (3s–12t).•The order of stabilizing effect for fused aromatic rings is: furan > thiophene > pyrrole > benzene > phosphole.•The higher ρ(r) and ∇2ρ(r) leads to the shorter the bond length as well as the stronger bond between germylenic center and fused rings in 5s and 9s.•The "W"−substituted pyrrole triplet NHGe is considered as the strongest nucleophilic and the weakest electrophilic species.•The order of stabilization energy (E(2) in kcal/mol) is 9s (5.88) > 5s (4.91) > 11s (4.14) > 7s (3.69). In this computational survey, substituent effects of two fused benzene and two five-membered rings including pyrrole, phosphole, furan and thiophene are inspected through stability, aromaticity, charge distribution, global reactivity indexes of singlet (s) and triplet (t) germapyridines, at density functional theory (DFT). Results based on B3LYP/AUG-cc-pVTZ, M06-2X/6-311++G** and B3PW91/6-311++G** levels of theory, reveal: (1) The highest thermodynamic and kinetic stability is considered by germylene situated among two substituted furan rings, in a "zigzag" arrangement with the divalent center; (2) In spite of "zigzag and/or chair" arranged, the order of thermodynamically stabilization for fused rings is furan > thiophene > pyrrole > phosphole; (3) The substituted germylenes with two fused furan and thiophene rings, in zigzag or chair arrangement, exhibit more stability than unsubstituted germapyridine; (4) While two benzene rings destabilize the resulted germapyridine, two five-membered rings stabilize their corresponding N-heterocyclic germylenes (NHGes); (5) In all scrutinized structures, singlets appear as ground state, revealing more stability than their corresponding triplet congeners; (6) The most aromaticity character is considered for the substituted furan in a "zigzag" orientation towards the germapyridinicʼs nitrogen atom; (7) Atoms in molecules (AIM) analysis exhibits the highest electron density (ρ(r) = 0.129) at bond critical point (BCP) of pyrrole and furan−fused NHGes in a "zigzag" arrangement. (8) Every NHGe reveals lower nucleophilicity and higher electrophilicity than the corresponding triplet congener; (9) The natural bond orbital (NBO) analysis exhibits the highest value of E(2) for "W"−substituted furan singlet NHGe which is in consistent with the strongest inter-molecular bonding (IB). In this computational survey, we have probed substituent