The bonding situation in mercury-alkali diatomics HgA ((2)Sigma(+)) (A = Li, Na, K, Rb) has been investigated employing the relativistic all-electron method Normalized Elimination of the Small Component (NESC), CCSD(T), and augmented VTZ basis sets. Although Hg, A interactions are typical of van der Waals complexes, trends in calculated De values can be explained on the basis of a 3-electron 2-orbital model utilizing calculated ionization potentials and the De values of HgA(+)((1)Sigma(+)) diatomics. HgA molecules are identified as orbital-driven van der Waals complexes. The relevance of results for the understanding of the properties of liquid alkali metal amalgams is discussed.