Electronic Equilibrium Between the ⁶A₁ and ²T₂ States in Iron(III) Dithio Chelates


Further results are reported for ferric dithiocarbamate complexes Fe(S2CNR2)3, which lie at the crossover between high-spin (sextet) and low-spin (doublet) states. Magnetic measurements made over a range of temperatures and pressures are used to estimate E, the separation of the zero-point energies of the 2T2 and 6A1 states, and ΔV, the difference between the volumes of the 6A1 and 2T2 states. The values of E extend over a large range, both positive and negative, for various complexes, while ΔV is generally 5-6 cm3/mol; this corresponds to an increase of 0.1 Å of the Fe-S bond lengths in passing from the 2T2 to the 6A1 state. The compounds may be divided into four distinct classes on the basis of their solution magnetic moments (μeff); in order of decreasing μeff values (and hence decreasing population of the 6A1 state), these are: (1) the pyrrolidyl complex where NR2 = pyrrolidyl, μeff = 5.8 BM; (2) the N,N-di-n-alkyl complexes, μeff = 4.3 BM; (3) the N-alkyl, N-aryl complexes, μeff = 3.5 BM; (4) the N,N-di-sec-alkyl complexes, μeff = 2.5 BM. The RNR angle in the Fe(S2CNR2)3 complexes is expected to increase from class 1 through to 4, with a corresponding change in the C-N bond order, which is expected to affect ligand field strength (Δ) and μeff. Comparisons of the electronic spectra of complexes falling into the various classes and the pressure dependence of the spectral positions and band intensities are used in the assignment of electronic transitions. From the tentative spectral assignments are estimated values of the ligand field strengths and the electronic pairing energies (π), and these are shown to obey the necessary inequality for the crossover situation: Δ(high spin) < π < Δ(low spin). The results are extended to include other dithio chelates such as the xanthates Fe(S2COR)3, which are almost purely low spin, and the dithiophosphates Fe(S2P(OR)2)3, which are high spin. Monoalkyldithiocarbamates Fe(S2CNHR)3 are found to be almost pure high spin.



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