New Ni- containing heterometallic cuboidal cluster aqua complex [W-3( NiCl) Se-4(H2O)(9)](3+), the missing link in the family of the M(3)NiQ(4) clusters (M = Mo, W; Q = S, Se), has been prepared by the reaction of [W3Se4(H2O)(9)](4+) with Ni in 2 M HCl. Single crystals of edge-linked double-cuboidal cluster [{W3NiSe4(H2O)(9)}(2)](pts)(8)center dot 18H(2)O (pts = p-toluenesulfonate) were grown from the solution of the aqua complex in 3 M Hpts, and their structures were determined. The Ni site in the clusters [W-3(NiCl)Q(4)(H2O)(9)](3+) selectively coordinates typical pi-acceptor ligands such as CO, olefins, acetylenes, phosphines, arsines, or SnCl3-. This allows stabilization by coordination of such elusive species as HP(OH)(2) and As(OH)(3). The stability constants for coordination of HP(OH)(2), As(OH)(3), and SnCl3- were determined. The Se for S substitution increases the stability by 1-2 orders of magnitude. Supramolecular adducts with cucurbit[6]uril (Cuc), [W-3(Ni(HP(OH)(2)))Q(4)(H2O)(9)]Cl-4 center dot Cuc center dot 11H(2)O and [W-3(NiAs(OH)(3))S-4(H2O)8Cl]Cl-3 center dot Cuc center dot 13H(2)O, were isolated and structurally characterized.