BONDING FORCES IN SHORT HYDROGEN-BONDS - CRYSTAL-STRUCTURE OF PD-II(DPGH)(2)

An analysis of the structures of 16 square-planar metal(II)-diglyoxime compounds was undertaken to determine the bonding forces which might alter the O...O distance observed for strong, low-barrier hydrogen bon ds. This series of structures includes the X-ray diffraction structure of Pd-II(dpgH)(2) (dpg=diphenylglyoxime), with data collected at 158 K. This compound crystallizes in the space group Ibam with a=15.324(2) Angstrom, b=22.634(3) Angstrom, c=6.8811(7) Angstrom, V=2386.6(5) Ang strom(3), Z=4, and R=4.89% for the 1105 reflections with \F-0\>4 sigma (F) and 7.91% for all 1490 reflections. The observed Pd-N bond distanc es are 1.958(6) and 1.959(6) Angstrom. The distance between the two ox ygen atoms that participate in the short hydrogen bond is 2.550(10) An gstrom; this distance is the shortest thus far reported for a palladiu m-diglyoxime compound. The hydrogen atom appears to be nearly centered , with refined O-H distances of 1.32(22) and 1.39(23) Angstrom in the O..H..O system; this interpretation of the hydrogen position is suppor ted by nearly equivalent N-O distances in the strong-hydrogen-bond-con taining moiety. Additionally, a search of the Cambridge Structure Data base yielded 15 examples of square-planar metal-diglyoxime compounds o f Ni(II), Pd(II), or Pt(II). Analysis of bonding forces in this series of compounds indicates that reduction of electron-electron repulsion between the oxygen atoms, either by delocalization of charge or by ele ctron-withdrawing substituent groups, allows a closer approach of the oxygen atoms and a shorter hydrogen bend.