5-COORDINATE DIPHOSPHINE COMPLEXES OF THE (CONO)8 GROUP, AND THEIR DISPROPORTIONATION REACTIONS TO COBALT(III) AND (CO(NO)2)10 DERIVATIVES

The syntheses and characterization of some new five-coordinate, mononitrosyl {CoNO}8 complexes of the type [Co(NO)(P-P)2](BF4)2 (P-P=alkyl- or phenyl-substituted ditertiary phosphine) are described, and their stereochemistry and dynamic behaviour studied by 31P{1H} NMR. The mononitrosyl complexes, which contain linearly coordinated CoNO groups, disproportionate in solution in the presence of anionic ligands (X-=halide or pseudohalide), giving dinitrosyl derivatives [Co(NO)2(P-P)]+ and cobalt(III) complexes trans-[CoX2(P-P)2]+, which were isolated in the solid state as the tetraphenyl- or tetrafluoroborate salts. The reaction provides a useful synthetic route, especially for pseudohalide cobalt(III) complexes. The IR, electronic and 31P NMR spectroscopic properties of the cobalt(III) complexes are consistent with a trans stereochemistry, while those of the {Co(NO)2}10 derivatives suggest a pseudotetrahedral structure with linearly-coordinated NO groups. The stoichiometry of the dismutation reactions was examined in dichloromethane solution by NMR and IR techniques. A possible mechanism is presented, which involves the formation of a six-coordinate trans-[CoX(NO)(P-P)2]+ intermediate containing a strongly bent CoNO moiety. © 1990.