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)Sigma(+) 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. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.