Bonded interactions in Fe and Cu Sulfides; Do electroneutrality requirements hold in the classical sense for sulfides? G.V. Gibbs 1, David F. Cox 2, Kevin M. Rosso 3, Nancy L. Ross 1 and Robert T. Downs 4 1 Department of Geosciences, Virginia Tech, Blacksburg, VA Department of Chemical Engineering, Virginia Tech, Blacksburg, VA William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratories, Richland, Washington Department of Geosciences, University of Arizona, Tucson, AZ,

Chemical Formula Ni 3 S 2 A representative block of the heazlerwoodite structure: Three NiS 4 tetrahedra sharing a common edge Ni S Ni-Ni bond path 2.498Å Bulk Ni metal R(Ni-Ni) 2.492Å Gibbs, Cox, Ross, Rosso, Downs and Prewitt (2005) J. Phys. Chem., What are the oxidation states of Ni and S?

Ni-Ni Ni-S S-S Comparison of experimental and theoretical bond critical points properties for the Ni-S, Ni-Ni and S-S bonded interactions for heazlewoodite, Ni 3 S 2 Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (2007) Spackman, Gibbs and Downs (Experimental, In prep.)

pyrite, FeS 2 marcasite, FeS 2 troilite, FeS greigite, Fe 3+ Fe 2+ Fe 3+ S 4 Fe2+, Fe3+ Fe 3+ Fe 2+ hs ls S S 2 dimer connected by bond path with a bond critical point

Smythite Fe 3 S 4

cubanite, CuFe 2 S 3 chalcopyrite, CuFeS 2 Cu Fe Cu Fe

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 2+ ? - S IV Fe hs 3+ - S VI Fe ls 2+ - S

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 2+? - S IV Fe hs 3+ - S VI Fe ls 2+ - S

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 2+? - S IV Fe hs 3+ - S VI Fe ls 2- - S

X-ray photoemission and L 2,3 - edge X-ray absorption spectra recently determined by Pearse et al. (2006) show that the Fe atom in chalcopyrite is unequivocally Fe 3+ ‘Given that Fe is trivalent in chalcopyrite, ‘What are the oxidation states of the Fe atoms comprising greigite and cubanite?’

Hoggins and Steinfink (1976) predicted the oxidation state for the Fe atom in greigite to be Fe and that for chalcopyrite and cubanite to be Fe and Fe 2.77+, respectively.

Bob Shannon (1981) observed that a Fe-S bond valence-bond length connection predicts a IV Fe 4+ -S bond length of Å compared with that observed 2.147Å for greigite. He concluded that the oxidation state of Fe atom in greigite is Fe 4+

Fe 4+ is known but it is rare and an unlikely state! But there exist materials like FeS 2 and Ba 3 FeS 5 which must contain Fe 4+ if electrical neutrality requirements hold and (2) only S -2 anions are only present.

Neutron diffraction and Mössbauer studies show that the two edge sharing tetrahedra in cubanite are inversion center equivalent. Mössbauer spectrum indicates that the Fe atom in cubanite has an oxidation state of Fe 2.5+ with a chemical formula CuFe 2.5+ Fe 2.5+ S 3. (McCammon, 1995)

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 3+ - S IV Fe hs 4+? - S Fe ls 2+ - S IV Fe hs 2.5? - S

Structure of the molecule (S 4 point group) geometry optimized at the B3LYP/ G(2d,p) level. R(Fe-S) (opt) = Å R(H-S) = Å <S –Fe –S = o 2x <S –Fe –S = o 4x <Fe –S –H = o R(Fe-S) (greigite) = 2.147Å S Fe 4+ H H 4 Fe 4+ S 4

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 3+ - S IV Fe hs 4+? - S IV Fe hs 2.5+? - S VI Fe ls 2+ - S

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 4+? - S IV Fe hs 2.5+? - S VI Fe ls 2+ - S IV Fe hs 3+ - S

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted) VI Fe hs 2+ - S IV Fe hs 3+ - S IV Fe hs 4+? - S IV Fe hs 2.5+? - S VI Fe ls 2+ - S

The properties of low spin VI Fe-S bonded interactions are distinct from those of high spin VI Fe-S bonded interactions. For the low spin VI Fe-S bonded interactions, (1) Fe-S bond lengths are shorter, (2) ρ(r c ) is larger, (3) the bonded radii are smaller, (4) the Fe and S atomic charges are smaller. Comments

The presence of Fe 4+ in greigite is consistent with (1) the metastability of greigite, (2) the difficulty of its synthesis, (3) the predicted oxidation state of 3.92+, (4) its short observed IV Fe-S bond length and (5) the properties calculated for the H 4 Fe 4+ S 4.

What is the status of the Fe atom in cubanite? CuFe 2.5+ Fe 2.5+ S 3 ? CuFe 2+ Fe 3+ S 3 with Fe 2+,Fe 3+ disorder? None of the above?

The evidence suggests that a simple connection between stoichiometry and oxidation state is not always a virtue of sulfides in the classical sense!

 Congratulations Alex for the many elegant contributions that you have made to our science. 

Shared bonded interaction Intermediate bonded interaction Closed-shell bonded interaction Gibbs, Cox, Ross, Rosso (2006) J. Chemical Physics Vi Fe 2+ -S IV Fe 3+? -S IV Fe 4+? -S

G(rc)G(rc) H(rc)H(rc) Na-O Mg-O Al-O S-O Si-O P-O V(rc)V(rc) Gibbs, Cox, Ross, Rosso (2006) J. Chemical Physics

VI Fe 2+ -S IV Fe 3+ -S IV Fe 4+ -S

Given that a bonded interaction is shared when ¼  2 (r c ) = 2G(r c ) + V(r c )  0 and that a bonded interaction is closed when H(r c ) = G(r c ) + V(r c ) ≥ 0. then 2G(r c ) + V(r c ) = 0  |V(r c )| /G(r c ) ≥ 2 and G(r c ) + V(r c ) = 0,  |V(r c )| /G(r c )  1. A bonded interaction is classified as shared when |V(r c )| /G(r c ) ≥ 2, closed-shell when |V(r c )| /G(r c )  1 and intermediate when 1 < |V(rc)| /G(rc)< 2.

S Fe-S bcp Fe 2+ S-S bcp

Gibbs, Jayatililaka, Spackman, Cox and Rosso (2006) J. Phys. Chem.