Structural and energetic trends in Group-I and II hydrohelide cations

The ground states of HMHe+ (M = Li, Na, K) and HMHe2+ (M = Be, Mg, Ca) have been investigated using relativistically-corrected CCSD(T) and IC-MRCI. The HLiHe+, HBeHe2+, HNaHe+ and HMgHe2+ ions were found to be linear in the ground state. Conversely, HKHe+ and HCaHe2+ were quasi-linear, with 2 A ′ - 2 Σ + barrier heights of ca. 2 and 5 cm−1, respectively. Structural and energetic trends are consistent with H and He sharing covalent and electrostatic bonds with the metal ion, respectively. These trends are also seen to be strictly dependent on the polarisability of the central metal ion.