Recent computational work has shown that light metals adsorbed onto the oxygenated diamond (1 0 0) surface have the potential to give diamond a temperature-stable negative electron affinity (NEA). Here, we use density functional theory to study three of these metals, lithium, magnesium and aluminium, on the (1 1 1) surface. We show that all three of these metals adsorbed onto the ketone O-terminated diamond surface can possess a large NEA and adsorption energies above that of H-termination at monolayer (ML) or sub-ML coverages. Adsorption onto the ether O-terminated surface gives similarly large NEAs but lower adsorption energies. These results are promising for the development of novel NEA surfaces such as those required for thermionic devices.