Singlet-to-triplet energy transfer within M(RG)(n) van der Waals clusters has been examined, where M Mg, Zn; RG = Ar, Kr, and/or Xe; and n = 1, 2, 3. For n = 1, the Breckenridge-Malmin mechanism for such energy-transfer processes, whereby the potential curve for a repulsive M . RG((3) Sigma(1)(+)) state crosses the potential curve for an attractive M . RG((1) Pi(1)) state, has been shown to be consistent with both full-collision and half-collision experimental results as well as ab initio calculations of the M . RG((3) Sigma(+)) potential curves. Only for RG = Xe and M = Zn in the n = 1 case is singlet-to-triplet energy transfer efficient, due to the more attractive character of the (1) Pi(1) states as the polarizability of the RG atom increases from Ar to Kr to Xe, and the more repulsive character of the (3) Sigma(1)(+) states as the M(p sigma)-RG(p sigma) repulsion of the RG atom increases in the same ordering. By using pairwise atom-atom potentials obtained from spectroscopic measurements, ab initio calculations, and reasonable estimates, we have examined the possibilities of singlet-to-triplet energy transfer within excited states of the analogous M(RG)(2) and M(RG)(3) clusters. Detailed considerations lead to the conclusion that B-1(1)/B-3(2) surface crossings should cause predissociation to triplet products in C-2v MgXe2, ZnKr2, and ZnXe2. Singlet-to-triplet predissociation is also shown to be likely in the MgXe3, ZnKr3, and ZnXe3 clusters, not by surface crossings in C-3 upsilon symmetry, but via B-1(1)/B-3(2) crossings in a T-shaped C-2 upsilon geometry in which one RG-RG bond is broken and a near-linear RG-M-RG linkage is created.