Kinetics and mechanism of the equilibration reaction between (2,2′,2″-nitrilotriethoxy)nitrosylvanadate(1–) and cyanide. Crystal structures of sodium (2,2′,2″-nitrilotriethoxy)nitrosylvanadate(I)–sodium perchlorate tetrahydrate and of barium cyano(2,2′,2″-nitrilotriethoxy)nitrosylvanadate(I) pentahydrate

Abstract

The reaction of triethanolamine, ammonium vanadate(V), and hydroxylamine in aqueous solution (pH 10) affords quantitatively a red nitrosyl complex (reductive nitrosylation), Na[V(NO){N-(C₂H₄O)₃}]·NaClO₄·4H₂O, (1), which upon addition of cyanide forms a blue complex, Ba[V(NO)-(CN){N(C₂H₄O)₃}]·5H₂O, (2). The structures of these two compounds have been determined by single-crystal X-ray studies: (1), monoclinic, space group P2₁/a, with a= 12.234(3), b= 11.405(3), c= 12.478(4)Å, β= 93.56(3)°, and Z= 4, R= 0.089 from 2 104 reflections; (2), orthorhombic, space group P2₁ab, with a= 10.918(1), b= 11.604(3), c= 12.327(3)Å, and Z= 4, R= 0.047 for 3 285 reflections. The complex anion of (1) exhibits distorted trigonal-bipyramidal co-ordination (O₃N₂) about the vanadium atoms with a linear {V–N–O}⁴ moiety. In contrast, the anion in (2) has distorted octahedral co-ordination (O₃N₂C) about the vanadium, involving the quadridentate ligand, a linear V–N–O group, and a cyanide. The rather long N–O distances in complexes (1) and (2)(average 1.253 Å) are in accord with low ν(N–O) stretching frequencies at 1 490 and 1 450 cm^–1. The {V–NO}⁴ moiety of (1) is susceptible to electrophilic attack by protons, generating most probably a hydroxylamidovanadium(V) species. This process is reversible; in alkaline solutions (1) is regenerated. The kinetics of the equilibration reaction of complex (1) with cyanide have been measured by stopped-flow spectrophotometry at pH 11 [I= 2.0 mol dm^–3(NaClO₄)]. A rate constant, k_f= 1.12 dm³ mol^–1 s^–1, for the forward step and one, k_r= 0.61 s^–1, for the reverse have been determined at 20 °C.