Efforts to obtain large trimetallic Au-Pd-Ni carbonyl clusters have given rise to the first reported high-nuclearity trimetallic carbonyl cluster [Au6Pd6(Pd6-xNix)Ni-20(CO)(44)](6-) (1). The centrosymmetric architecture of its 38-atom core ideally consists of the hcp stacking of two inner Au3M3Ni6 and two outer Pd-3 layers along with two Pds-capped and six AuNi2-capped Ni atoms. The resulting octahedral-like Aug kernel is antiprismatically capped on opposite triangular faces by the two Pd3 triangles. The microscopic nature of its nonstoichiometric composition was unequivocally established from complete X-ray diffraction analyses via a SMART CCD system of seven crystals of its [PPh4](+) Salt from different samples. A substitutional Pd/Ni crystal disorder was found at only six specific nonadjacent atomic M sites (three crystallographically independent); for the composite six-site crystal disorder of the (6-x) Pd/x Ni atoms, x was determined for the seven crystals to range from 2.1 (65% Pd, 35% Ni) to 5.5 (8% Pd, 92% Ni). The overall geometry of 1 including the 44 CO ligands (in the crystal-averaged unit cell) ideally has trigonal D-3d((3) over bar 2/m) symmetry. A structure/bonding analysis as to why this particular Pd/Ni substitutional crystal disorder is found in 1 provides a striking illustration that the occurrence of a bimetallic substitutional crystal disorder at only certain crystallographic sites (coloring problem) in a heterometallic carbonyl cluster is critically dependent upon the extent of dissimilarity in the composite relative bond-energy effects of metal-metal/metal-CO interactions. 1 was obtained as a major product (35-40% yields) from reactions of [Ni-6(CO)(12)](2-) With Pd(OAc)(2)/Au(PPh3)Cl mixtures in DMSO. Our desire to obtain the hypothetical isostructural [Au6Ni32(CO)(44)](6-) (2), in which all Pd atoms are replaced with Ni ones, led to the designed synthesis and structural determination of 2, which in turn provided an "operational test" of our "coloring-problem" analysis of 1. The maximum metal-core diameters in 1 and 2 are ca. 1.1 nm parallel and 0.8 nm perpendicular to the principal 3-fold axis.