First true square-planar Hg(II) compound: synthesis and full characterization of trans-[Hg{Ph2PNP(O)Ph2-P,O}2]

The reaction of Hg(O3SCF3)2 with bis(diphenylphosphino)amine, Ph2PNHPPh2 (dppam), produces the novel, rare face-to-face complex [Hg2(O3SCF3)4(dppam)2] (1). Treatment of 1 with Na2N2O3 leads to trans-[Hg{Ph2PNP(O)Ph2-P,O}2] (2) via regioselective oxidation and simultaneous deprotonation of dppam. 2 is the first true square-planar Hg(II) compound. In 2 the coordination plane and the five-membered rings of the HgNOP2 moieties are completely coplanar indicating strong π-bonding interactions. Both 1 and 2 have been fully characterized by X-ray structure analyses, NMR spectroscopy (199Hg{1H},31P{1H},13C{1H},1H), ESI and FAB mass spectrometry, IR spectroscopy, elemental analyses, and melting points. Since in Hg(II) compounds relativistic effects favour linear coordination and Hg(II) donor atom preferences play a significant role, the Hg–P bond length of 2.4042(7) Å in 2 is short, whereas the Hg–O bond length of 2.7138(15) Å is long. In view of the X-ray structures of 1 and 2, it is shown that the delocalized charge in [Ph2PNP(O)Ph2]− is responsible for the achievement of the square-planar coordination in a Hg(II) compound. A similar π-bonding effect has been observed in several square-planar complexes of Pt(II), Pd(II), and Ni(II) containing cis-1,2-bis(diphenylphosphino)ethene (cis-dppen).