Palladium interatomic distance

Other Covalent Radii.—Bipositive nickel, palladium, and platinum and tripositive gold form four coplanar dsp bonds, directed to the comers of a square, with attached atoms. Examination of the observed values of interatomic distances reveals that square dsp radii of atoms have the same values as the corresponding octahedral d sp radii, as given in Table 7-15. This is shown by the comparisons on the following page. 

The Pd—Pd bond length lies between 2.61 A (for 84) and 2.72 A (for 57), varying surprisingly little for the different types of bridging ligands. This relatively short distance, comparable with the interatomic distance in metallic palladium (2.74 A), points to the existence of a direct metal-metal bond. 

PdF, is isostruotural with MnF, FeF, CoF, NiF, and ZnF,. Each palladium atom has six fluorine atoms forming an almost regular octahedron around it. The PdF, coordination octahedra are joined by sharing comers. The interatomic distances are given in Table 2. 

Third, the doublet and, especially, sextet models require very precise superimposing of the molecule on the catalyst lattice. We have found that the cyclohexane derivatives, in accordance with the sextet model, smoothly dehydrogenate only on the following metals nickel, cobalt, iridium, palladium, platinum, ruthenium, osmium, and rhenium, all of which crystallize in Al, A3 lattices with certain interatomic distances. These results extend to the alloys of these metals. The catalytic activity of rhenium for this reaction was predicted by the multiplet theory as this metal maintains the square of activity this prediction was realized experimentally in the laboratory of the author. Similar correlations take place in the exchange of cyclanes with deuterium. 

SAXS data showed that the molecules of the palladium complex [Pd2(PPh2)H c]n are nearly spherical in shape with a diameter of 15.6 A. EXAFS data showed that both the platinum, [(PPh2)Pt]8 -io, and palladium, [Pd2(PPh2)Hx] , complexes contained only phosphorus and metal atoms in the first coordination sphere. The interatomic distance between Pt(Pd) and the phosphorus atoms is 2.26 A, which is typical of the Pt(Pd)-P distances in platinum and palladium complexes. Comparison with the spectral data of the reference compound, Pd(PPh3)4, showed that each platinum atom is surrounded by four phosphorus atoms (the coordination number, u(P/Pt), is four). The corresponding value for palladium atoms, (P/Pd), is three. 

Reference to the bond angles and interatomic distances for these molecules shows that as far as the carbon atoms are concerned their values are little different from those of the olefin molecule. It would be expected, therefore, that catalysts which were effective for hydrogenation of the latter would also function with the heterocyclic molecules. This is found to be so, in that nickel catalysts are known to give tetrahydrofuran and pyrrolidine by the hydrogenation of furan and pyrrole at 180° (Padoa, 25). Tetrahydrofuran is also formed by the use of platinum (Starr and Hixon, 26), osmium, or palladium (Shuikin, Nikiforov, and Stolyarova, 27) as the catalyst, and pyrrolidine is similarly produced by palladium or rhodium catalysts (Zelinskii and Yurev, 28). In all these metals there are spacings of the atoms very similar to those in metallic nickel, the hexagonal osmium lattice having a equal to 2.71 A. 

Fig. 94. Cutouts of Rhlujc, Pdluj, and Ptinjc (x = 3-9) units in various binary and ternary indides. The rhodium, palladium, and platinum atoms are drawn as blaek eircles, the indium atoms as open eircles. Relevant interatomic distances in pm and atom labels are indicated. The site symmetries are given in parentheses. Note, that the alkaline earth and rare earth metal atoms are omitted for clarity.

The structure type TiNiSi is a superstructure of anti-PbCl2. Johrendt and Mewis (1990) determined the ErPdP structure by a single-crystal method, = 0.046. Phosphorus atoms fill the trigonal prisms, erbium and palladium atoms have a CN of 15 and 12, respectively. The shortest interatomic distances are 5Er-pd = 0-2950 6Er p = 0.2803 5pd pd =0.2981 5pd-p = 0.2522. Isotypic compounds are ScCoP and RPdP where R=Tb, Dy, Ho, Tm, Yb, Lu. 

The form of the potential for the system under study was discussed in many publications [28,202,207,208]. Effective pair potentials are widely used in theoretical estimates and numerical calculations. When a many-particle interatomic potential is taken into account, the quantitative description of experimental data improves. For example, the consideration of three-body interactions along with two-particle interactions made it possible to quantitatively describe the stratification curve for interstitial hydrogen in palladium [209]. Let us describe the pair interaction of all the components (hydrogen and metal atoms in the a. and (j phases) by the Lennard Jones potential cpy(ry) = 4 zi [(ff )12- / )6], where Sy and ai are the parameters of the corresponding potentials. All the distances ry, are considered within c.s. of radius r (1 < r < R), where R is the largest radius of the radii of interaction Ry between atoms / and /). 

---