High-nuclearity [Pd13Ni13(CO)(34)](4-) containing a 26-atom Pd13Ni13 core with an unprecedented five-layer close-packed triangular stacking geometry:possible substitutional Pd/Ni crystal disorder at specific intralayer nickel sites

[Pd13Ni10(Ni3-xPdx)(CO)(34)](4-) 1 has been prepared in essentially quantit ative yields (> 90%) from room-temperature reactions of [Ni-6(CO)(12)](2-) with either Pd(OAc)(2) in DMF or Pd(MeCN)(4)(BF4)(2) in DMSO. The crystal s tructure of 1 as the [PPh4](+) salt was determined from low-temperature CCD X-ray crystallographic analyses of three crystals. 1 was also characterize d by elemental analysis, IR, H-1 NMR, and electrochemical (CV) measurements . Its structurally unprecedented 26-atom metal polyhedron consists of a 5-l ayer close-packed triangular stacking arrangement of pseudo-C-3v symmetry w ith an [a(Ni)b(Ni-3)c(Pd-6)a(Pd7Ni3)c(Ni3M3)] sequence. The overall geometr y of the 20-atom nu (3) tetrahedral Pd13Ni7 framework in the top four ccp l ayers is analogous to the pseudo-T-d cubic geometry of the metal-core previ ously found in [Pd16Ni4(CO)(22)(PPh3)(4)](2-) and [Os-20(CO)(40)](2-). Each M in the bottom fifth Ni3M3 layer denotes an inner triangular atomic site where a substitutional Pd/Ni crystal disorder may occur; the composite comp osition of the three M sites corresponds to Ni3-xPdx with x=0.45 for crysta l A and 0 for B and C. We thereby formulate 1 to possess a stoichiometric P d13Ni13 core even though crystallographic evidence from crystal A indicates the existence of a crystal disorder involving a small substitution of Pd f or Ni at two of the three inner atomic M sites within the Ni3M3 layer. The entire geometry of 1 (including the 34 carbonyl ligands) ideally conforms t o C-3v trigonal symmetry. A qualitative structure/bonding analysis was perf ormed in order to correlate the geometry of 1 with its electron count. The maximum metal-core diameter is ca. 0.87 nm parallel and 0.56 nm perpendicul ar to the principal 3-fold axis.