New low-spin Co(II) complexes with novel tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane ligand: Electron transfer kinetics and spectroscopic characterization of Co(II)P₆ and Co(II)P₃S₃ ions in aqueous solution

Substitution-inert Co(II) complexes with tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane were produced in aqueous solution by the controlled-potential electrolysis. EPR studies indicated that the Co(II)P₆ and Co(II)P₃S₃ species are in the low-spin t_2g⁶e_g¹ state. The analyses of the absorption spectra of the Co(II) species in solution indicated significant Jahn-Teller distortion in Co(II)P₆ and Co(II)P₃S₃ ions as expected for the low-spin d⁷ electronic configuration, while less distortion was observed for the Co(II)S₆ ion. Kinetic studies of the redox reactions involving these Co(III/II) species confirmed that the electron self-exchange reactions for the Co(III/II)P₆ and Co(III/II)P₃S₃ couples are relatively fast (k_ex ∼ 1 × 10⁴ dm³ mol^-1 s^-1), which is consistent with the results for other low-spin - low-spin Co(III/II) couples. It was concluded that the nephelauxetic effect of the P-donor atom stabilizes the low-spin state in Co(II), and that the rather rapid electron exchange rates for these Co(III/II) couples were explained by the lack of the spin reorganization process required for the ordinary low-spin - high-spin Co(III/II) couples.