Theoretical assessment of donor–acceptor complexes [X(PPh3)2 → AlH2]+ (X = C–Pb): structures and bonding

Abstract

Quantum chemical investigations at the BP86/def2-SVP, BP86/def2-TZVPP and BP86/TZ2P+ levels of theory have been done for the series of AlH2+ complexes that carry carbodiphosphorane and analogues called tetrylones [X(PPh3)2–AlH2]+ (Al-XPPh) (X = C–Pb) using charge and partitioning methods. The most stable structures of Al-XPPh have been found for carbone CPPh as a mildly side-on style in carbone complex Al-CPPh, while the heavier tetrylone adducts Al-SiPPh−Al-PbPPh have significantly different side-on fashions SiPPh−PbPPh, which exhibit the more acute bending angles in tilted forms linked to AlH2+ fragment. Bond dissociation energies (BDEs), De (kcal/mol), slightly decrease from the strongest bonded carbone, Al-CPPh, to the weaker bonded heavier homologues. The bulky tetrylone ligands XPPh have significantly influenced to the Al-X bond strength in complexes Al-XPPh when calculating BDEs with dispersion interaction. The NBO analysis revealed that the [X(PPh3)2 → AlH2]+ donation comes mainly from the σ- and π-contributions of the ligands. The EDA–NOCV calculations showed that the bond sturdiness of the Al–X bond results from the decrease in [X(PPh3)2 → AlH2]+ donation and electrostatic attraction. The EDA–NOCV data also indicated that AlH2+–tetrylone complexes exhibit not only (PPh3)2X → AlH2+ strong σ-donors and weak π-donors but also (PPh3)2X ← AlH2+ weak π-back donation as π–π electrons shared in complexes.