THE ROLE OF TRANSITION-METAL ATOMS AS HYDROGEN-BOND ACCEPTORS - A NEUTRON-DIFFRACTION STUDY OF [NPRN4]2[PTCL4].CIS-[PTCL2(NH2ME)2] AT 20-K

Single-crystal neutron diffraction has allowed accurate characterisation of N-H...Pt and N-H...Cl interactions linking the two square-planar platinum(II) units which comprise the dianion of [NPr(n)4]2[PtCl4].cis-[PtCl2(NH2Me)2] 1: Pt...H 2.262(11) angstrom, N-H...Pt 167.1(9)-degrees; Cl...H 2.318(12) angstrom, N-H...Cl 151.0(12)-degrees. An extensive network of C-H...Cl hydrogen bonding between the cation and anion species is also revealed, and provides valuable information on what is one of the least well characterised types of hydrogen bond. A weak C-H...Pt interaction between one cation unit and the anion is also observed. In the light of this and particularly the stronger intra-anion N-H...Pt interaction, a new type of three-centre four-electron hydrogen bridge, involving a transition-metal atom, is proposed. Further supporting evidence is drawn from a re-evaluation of spectroscopic data for similar interactions previously reported. This type of interaction is contrasted with the more well known three-centre two-electron M-H half-arrow-pointing-right M, C-H half-arrow-pointing-right M, Si-H half-arrow-pointing-right M and B-H half-arrow-pointing-right M interactions and compared with conventional four-electron hydrogen bonds. Compound 1 is triclinic, space group P1BAR (no.2), Z = 2, with a = 10.680(4), b = 11.926(2), c = 15.350(15) angstrom, alpha = 93.88(4), beta = 100.57(5) and gamma 96.58(4)-degrees at 20 K. Final R(F2) = 0.124, S = 1.136 for 5244 F2 values (F0(2) > 0.0).