T. Ramasami

Synthesis and characterization of the µ-disulphido complexes CrS2Cr4+ and CrS2HFe4+

Novel chromium(III) complexes with µ-di-sulphido(S–S) bridging ligands have been prepared by I2 and FeIII oxidation of CrSH2+.

Kinetic and mechanistic studies on the reaction of dioxygen with hexa-aquamolybdenum(III)

The reaction of hexa-aquamolybdenum(III) with O2in p-toluenesulphonic acid (Hpts) solutions, I= 2.0 M [Li(pts)], gives the di-µ-oxo-molybdenum(V) ion [Mo2O4(H2O)6]2+ as product: 2Mo3++ O2+ 2H2O → Mo2O42+ 4H+. Use of isotopically labelled O2followed by i.r. spectroscopy on the isolated Mo2O42+ has demonstrated that at least some 18O is taken up in the µ-oxo-positions. With Mo3+ in large excess an intense yellow intermediate MoO2Mo6+(or a related form) with a peak at 395 nm (Iµ= 2 300 M–1 cm–1) is observed. At 25 °C stopped-flow studies on the decay of this intermediate give rate constants k3(obs.) which can be expressed as in equation (i). The various constants are as defined in equations (ii)–(iv), where values indicated are for 2.0 M Hpts. k3(obs.)=K1K2k3[Mo3+]2//1 +K1[Mo3+]+K1K2[Mo3+]2(i), Mo3++ O2⇌ MoO23+(K1 360 M–1)(ii), MoO23++ Mo3+⇌ MoO2Mo6+(K2 535 M–1)(iii), MoO2Mo6++ 2H2O → Mo2O42++ 4H+(k3 0.23 s–1)(iv) Estimates of rate constants for reactions (ii) and (iii) were obtained from studies on the formation of MoO2Mo6+. All parameters were further refined by an iterative fitting procedure using the program KINSIM for multi-step reactions. The values of k1(180 M–1 s–1) and k2(42 M–1 s–1) are in excess of previous substitution rate constants for Mo3+. With O2 in excess of Mo3+(0.72 × 10–4 M) there was no evidence for MoO2Mo6+, and an alternative route for conversion of MoO23+ to Mo2O42+, which is first order in MoO23+(k4= 0.037 s–1) was identified. In the refinement this step was included alongside (ii)–(iv). The reactions of MoO2Mo6+(k3) and MoO23+(k4) exhibit [H+]–1 dependences consistent with an increase in hydrolysis of the metal ion.

Active site chemistry of hemerythrin

Details of the stepwise reduction of the FeIII,III active stie of methemerythrin to the FeII,II deoxy form by one-equivalent reductants [Co(nane)2]2+, [Co(sep)]2+, [Cr(bipy)3]2+ and [Co(sarCl2)]2+ are reported.

Aquation of formato-, NN-dimethylformamide-, and fluoro-penta-amminechromium(III) complexes. Identification of reaction paths and kinetic studies

The formato- and NN-dimethylformamide (DMF) complexes [Cr(NH3)5(O2CH)]2+ and [Cr(NH3)5(OCHNMe2)]3+ have been prepared and characterized. Ion-exchange separation of the aquation products in the case of the formato-complex indicates that ammonia loss features prominantly. With the DMF complex a single process corresponding to aquation of DMF is observed. Kinetic studies give k1= 1.79 × 10–4 s–1 at 50 °C. I = 1.00M(LiClO4),with activation parameters ΔH‡= 20.2 ± 0.5 kcal mol–1 and ΔS‡=–13.5 ± 1.7 cal K–1 mol–1. Aquation of [Cr(NH3)5F]2+ yields [Cr(NH3)5(H2O)]3+(k2) and [Cr(NH3)4(H2O)F]2+(k3) with the latter predominating over the conditions [H+]= 0.01–1.00M investigated. From product analyses and kinetic studies at different [H+] rate constants have been obtained at 50 °C. I = 1.00M(LiClO4). Aquation of fluoride is [H+]-dependent,k2=a+b[H+] with a= 2.1 × 10–6 s–1 and b= 2.9 × 10–6 l mol–1 s–1, while k3 for ammonia loss is independent of [H+], k3= 7.7 × 10–6 s–1.

The effect of ligand environment on the mechanism of substitution at chromium(III)

Kinetic and thermodynamic data for the formation and aquation of acidopenta-amminechromium(III) complexes provide strong evidence for a dissociative mechanism.