Reactivity of the Re-NO centre: Proton induced oxidation of Re(NO)2+ to Re(NO)3+. Synthesis and characterisation of some Re(II) thiocyanato-halogenonitrosyl complexes

The Re(NO)²⁺ moiety as Re(NO)(NCS)₃H₂O]⁻ or Re(NO)(NCS)₂(L-L)H₂O]· L-L = phen (1,10-phenanthroline) or bipy (2,2′-bipyridine)] undergoes proton-induced oxidation reaction with HX (X = Cl, Br) to produce a Re(NO)³⁺ moiety. The spectral and physico-chemical data suggest that the anionic complex is 5 coordinate and the neutral one is 6 coordinate with axial NO group and two NCS ligands intrans-equatorial positions. The complex, Re(NO)(NCS)₂(phen)Br]·H₂O shows complicated magnetic behaviour which is discussed in the paper. The ESR spectrum of this compound shows typical rhenium hyperfines and <g>-tensor anisotropy compatible with the loss of axial symmetry. However, the spectrum of Re(NO)(NCS)₂Br₂]⁻ quite reasonably shows axial symmetry, other features being grossly comparable to the L-L compounds. The anionic species and the neutral L-L complex show irreversible one-electron oxidation waves at different voltages. This may correspond to a conversion of Re(NO)³⁺ to Re(NO)⁴⁺ in both the cases. Interestingly enough, only the neutral complexes exhibit an irreversible reduction wave due probably to a conversion of Re(NO)³⁺ to Re(NO)²⁺.