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

The reaction of HgðO3SCF3Þ2 with bis(diphenylphosphino)amine, Ph2PNHPPh2 (dppam), produces the novel, rare face-to-face complex ½Hg2ðO3SCF3Þ4ðdppamÞ2 (1). Treatment of 1 with Na2N2O3 leads to trans-½HgfPh2PNPðOÞPh2-P;Og2 (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 p-bonding interactions. Both 1 and 2 have been fully characterized by X-ray structure analyses, NMRspectroscopy ð199Hgf1Hg; 31Pf1Hg; 13Cf1Hg; 1HÞ, ESI and FAB mass spectrometry, IRspectroscopy, 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) AA in 2 is short, whereas the Hg-O bond length of 2.7138(15) AA is long. In view of the X-ray structures of 1 and 2, it is shown that the delocalized charge in ½Ph2PNPðOÞPh2 is responsible for the achievement of the squareplanar coordination in a Hg(II) compound. A similar p-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).