Substituent Effects on Aluminyl Anions and Derived Systems: A High-Level Theory

Aluminyl anions are low-valent aluminum species bearing a lone pair of electrons and a negative charge. These systems have drawn recent synthetic interest for their nucleophilic nature, allowing for the activation of σ-bonds, and have been proposed as a pathway to hydrogen energy storage. In this research, we provide high-level ab initio geometries and energies for both the simplest aluminyl anion (AlH₂^-) and several substituted derivatives. Geometries are reported using the gold-standard CCSD(T)/aug-cc-pV(T+d)Z level of theory. Energies were extrapolated to the complete basis set limit through the focal point approach, utilizing coupled-cluster methods through perturbative quadruples and basis sets up to five-ζ quality. Geometries were rationalized using electrostatic, steric, and orbital donation effects. The donation from substituents to Al is accompanied by back-donation effects, a property traditionally thought of in transition-metal systems. Stereoelectronic effects through the secondary orbital interaction play a fundamental role in stabilizing these low-valent aluminum compounds and would likely also affect the feasibility of their use within several industrial applications. The energetic analysis of the formation of each substituted anion is rationalized as the result of three energetic schemes. The effectiveness of these schemes for determining the relative formation energies is discussed.