We present a study of magnesium and calcium precursor molecules in order to predict which of them would be more successful in atomic layer deposition (ALD) of metal oxides. Precursor chemistry plays a key role in ALD, since precursors must be volatile, thermally stable, chemisorb on the surface, and react rapidly with existing surface groups. We investigate one aspect of this, surface reactivity between ligands and hydroxyl groups, via a gas-phase model with energetics computed at the level of density functional theory (DFT). The precursors with higher reactivity towards hydrolysis (and thus most potentially useful for ALD) are M(bae), M(Ph-nacnac)2, and M(tmtate), which is rationalized as due to strain, particularly in the cyclic ligands bae and tmtate. Calculated trends for Mg and Ca follow each other closely, reflecting the similar chemistry among group 2 metals.