Analysis and modeling of X-ray and neutron Bragg and total diffraction data show that the compounds referred to in the literature as "Pd(CN)(2)" and "Pt(CN)(2)" are nanocrystalline materials containing small sheets of vertex-sharing square-planar M(CN)(4) units, layered in a disordered manner with an intersheet separation of similar to 3.44 angstrom at 300 K. The small size of the crystallites means That the sheets' edges form a significant fraction of each material. The Pd(CN)(2) nanocrystallites studied using total neutron diffraction are terminated by water and the Pt(CN)(2) nanocrystallites by ammonia, in place of half of the terminal cyanide groups, thus: maintaining charge neutrality. The neutron samples contain sheets of approximate dimensions 30 A x 30 A. For sheets of the size we describe, our structural models predict compositions of Pd(CN)(2).xH(2)O and Pt(CN)(2).yNH(3) (x approximate to y approximate to 0.29). These values are in good agreement with those obtained from total neutron diffraction and thermal analysis, and are also supported by infrared and Raman spectroscopy measurements. It is also possible to prepare related compounds Pd(CN)(2)center dot pNH(3) and Pt(CN)(2)center dot qH(2)O, in which the terminating groups are exchanged. Additional samples showing sheet sizes in the range similar to 10 angstrom x 10 angstrom (y similar to 0.67) to similar to 80 angstrom x 80 angstrom (p = q similar to 0.12), as determined by X-ray diffraction, have been prepared. The related mixed-metal phase, Pd1/2Pt1/2(CN)(2)center dot qH(2)O (q similar to 0.50), is also nanocrystalline (sheet size similar to 15 angstrom x 15 angstrom). In all cases, the interiors of the sheets are isostructural with those found in Ni(CN)(2). Removal of the final traces of water or ammonia by heating results in decomposition of the compounds to Pd and Pt metal, or in the case of the mixed-metal cyanide, the alloy, Pd1/2Pt1/2, making it impossible to prepare the simple cyanides, Pd(CN)(2), Pt(CN)(2), or Pd1/2Pt1/2(CN)(2), by this method.