Reactions of perchlorate salts of Hg(II)/Cd(II) with [CpFe(CO)(2)P(Se)(O(i)Pr)(2)] (denoted as L) produced dicationic clusters [HgL(2)](ClO(4))(2), 1; [HgL(3)](ClO(4))(2), 2; and [CdL(3)(H(2)O)](ClO(4))(2), 11. However, the reactions of L with Hg(II)/Cd(II) halides yielded neutral complexes. For instance, HgI(2) produced [Hg(3)I(6)L(2)], 3, or [HgI(2)L(2)], 4, depending on the metal-to-ligand ratio used. Reaction of L with any of the Hg(II)/Cd(II) halides in a 1:1 ratio produced neutral clusters [HgX(mu-X)L](2) (M = Hg; X = Cl, 5; Br, 6; I, 7; M = Cd; X = Cl, 8; Br, 9; I, 10). Thus, variation of the M-to-L ratio made it possible for the formation of compounds with different metal/ligand stoichiometries only when ClO(4)(-) and I(-) salts of Hg(II) were used. Single-crystal X-ray crystallography revealed that two and three L units were connected to a Hg(II) via its Se atom in 1 and 2, respectively, whereas Hg(II) in 4 was connected to two "I" and two "L" units. An L (through its Se) and a terminal halogen were attached to each of the Hg(II)'s of the Hg(2)X(2) parallelogram core in 5 and 6. The cadmium complex 8, with a Cd(2)Cl(2) parallelogram core, was isostructural with the mercury complex 5. Although bonding and connectivity in 8 were similar to those in 9 and 10, the conformation of the bulky L ligands was unusually syn in 9 and 10, unlike the anti orientation in 5, 6, and 8. The syn conformation of L was also observed in compounds 1 and 2. Secondary interactions, namely, Se...Se interaction, the X...H-C type of H-bonding (X = halogen or Se), and pi-stacking, revealed in the structures played a role in determining the ligand conformation. The [P(Se)(O(i)Pr)(2)](-), the conjugate base of secondary phosphine selenide [HP(Se)(O(i)Pr)(2)], acted as a bridge with P bonded to Fe and Se bonded to Hg (or Cd) in these heterometallic clusters. Compound 5 further demonstrated a longer P-Se bond in the secondary phosphine selenide compared to that in isostructural, tertiary phosphine selenide metal complexes.