Energy Decomposition Analysis Coupled with Natural Orbitals for Chemical Valence and Nucleus-Independent Chemical Shift Analysis of Bonding, Stability, and Aromaticity of Functionalized Fulvenes: A Bonding Insight

[Image: see text] The Donor base ligand-stabilized cyclopentadienyl-carbene compounds L–C(5)H(4) (L = H(2)C, aAAC; (CO(2)Me)(2)C, Py; aNHC, NHC, PPh(3); SNHC; aAAC = acyclic alkyl(amino) carbene, aNHC = acyclic N-hetero cyclic carbene, NHC = cyclic N-hetero cyclic carbene, SNHC = saturated N-hetero cyclic carbene, Py = pyridine) (1a-1d, 2a-2c, 3) have been theoretically investigated by energy decomposition analysis coupled with natural orbitals for chemical valence calculation. Among all these compounds, aNHC=C(5)H(4) (2a) and Ph(3)P=C(5)H(4) (2c) had been reported five decades ago. The bonding analysis of compounds with the general formula L=C(5)H(4) (1a-1d) [L = (H(2)C, aAAC, (CO(2)Me)(2)C, Py] showed that they possess one electron-sharing σ bond and electron-sharing π bond between L and C(5)H(4) neutral fragments in their triplet states as expected. Interestingly, the bonding scenarios have completely changed for L = aNHC, NHC, PPh(3), SNHC. The aNHC analogue (2a) prefers to form one electron-sharing σ bond (C(L)–C(C5H4)) and dative π bond (C(L) ← C(C5H4)) between cationic (aNHC)(+) and anionic C(5)H(4)(–) fragments in their doublet states. Similar bonding scenarios have been observed for NHC (2b) and PPh(3) (2c) (P(L)–C(C5H4), P(L) ← C(C5H4)) analogues. In contrast, the SNHC and C(5)H(4) neutral fragments of SNHC=C(5)H(4) (3) prefer to form a dative σ bond (C(SNHC) → C(C5H4)) and a dative π bond (C(SNHC) ← C(C5H4)) in their singlet states. The pyridine analogue 1d is quite different from 2c from the bonding and aromaticity point of view. The nucleus-independent chemical shifts of all the abovementioned species (1–3) corresponding to aromaticity have been computed using the gauge-independent atomic orbital approach.