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Metal−Thiometalate Transport of Biologically Active Trace Elements in Sulfidic Environments. 2. Theoretical Evidence for Copper Thioarsenite Complexing

2000; American Chemical Society; Volume: 34; Issue: 8 Linguagem: Inglês

10.1021/es9901359

1520-5851

J. A. Tossell,

Radioactive element chemistry and processing

To obtain an atomistic understanding of the structure and stability of the copper−thioarsenite complex characterized experimentally only by its molecular formula and formation constant in the previous paper (hereafter paper 1) we have carried out quantum mechanical calculations on several isomers of the CuH2AsOS2 complex, a number of related complexes and the corresponding uncomplexed ligands. Using a first principles molecular orbital theory approach which includes electron correlation (second-order Moller−Plesset perturbation theory) with a polarized double-ζ valence orbital basis and effective core potentials we calculate the lowest energy isomer to be CuAsS(SH)(OH) (with bonds from Cu to S, SH and As and bonds from As to Cu, S and O), which has a calculated enthalpy of formation from Cu(OH2)2+(aq) and AsS(SH)(OH)-(aq) of about −100 kJ/mol, suggesting a high stability for this complex. Our method also reproduces the experimental trend in log K values for the formation of the simpler complexes CuCl2-, Cu(SH)2- ,and Cu(CN)2- and enables us to understand why the CuAsS(SH)(OH) complex is so stable. We find that the complexes of Cu+ with AsS(SH)2-, AsS(SH)(NH2)-, and AsS(SH)(CH3)- are also very stable, while the complexes with related As(V), P(III), and P(V) S-containing ligands are considerably less stable. The Cu+ complex formed from the purely oxidic ligand AsO(OH)2- is calculated to be unstable. The determining chemical characteristic of the strongly complexing ligands is the presence of two coordinating S atoms (one −S and one −SH) and an electron-rich As center. When the OH group on As is replaced by the strongly electron withdrawing F, the Cu−As bond is broken, and the stability of the complex is greatly reduced. We have also calculated deprotonation enthalpies and estimated the acid dissociation constants for the weak acids As(OH)3, As(SH)(OH)2, As(SH)2(OH), and AsO(OH)3. We calculate AsO(OH)3, As(SH)(OH)2, and As(SH)2(OH) to all be much more acidic than As(OH)3. Thus, As(SH)2(OH) will be strongly deprotonated near neutral pH and its anion will complex very strongly with Cu+(aq), while As(OH)3 will exist mainly as the neutral molecule, and neither it nor its anion will complex significantly with Cu+.