G.V. Gibbs 1 , David F. Cox 2 , Kevin M. Rosso 3 , Nancy L. Ross 1 and Robert T. Downs 4

Chemical E ngineeri ng UHV Sur face Scie nce Laborat ory

Bonded interactions in Fe and Cu Sulfides;

Do electroneutrality requirements hold in the classical sense for

sulfides?G.V. Gibbs1, David F. Cox2, Kevin M. Rosso3, Nancy L. Ross1

and Robert T. Downs4

1Department of Geosciences, Virginia Tech, Blacksburg, VA 240612Department of Chemical Engineering, Virginia Tech, Blacksburg, VA

24061

3William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratories, Richland, Washington 99352

4Department of Geosciences, University of Arizona, Tucson, AZ, 85721.


Chemical Formula Ni3S2

A representative block of the heazlerwoodite structure: Three NiS4 tetrahedra sharing a common

edge

Ni

S

Ni-Ni bond path

2.498Å

Bulk Ni metalR(Ni-Ni) 2.492Å

Gibbs, Cox, Ross, Rosso, Downs and Prewitt (2005) J. Phys. Chem.,

What are the oxidation states of Ni and S?



0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

0.6

(r c)

e/Å

3 (cal

cula

ted)

(rc) e/Å3 (experimental)

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, Ni3S2

Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (2007)

Spackman, Gibbs and Downs (Experimental, In prep.)


pyrite, FeS2 marcasite, FeS2

troilite, FeSgreigite, Fe3+Fe2+Fe3+S4

Fe2+, Fe3+

Fe3+

Fe2+ Fe2+

Fe2+

hs

lsls

S

S2 dimer connected by bond path with a bond critical point


Smythite Fe3S4


cubanite, CuFe2S3chalcopyrite, CuFeS2

CuFeCuFe


Gibbs, Cox, Ross, Rosso and Downs, J. Phys. Chem. (accepted)

VIFehs2+- S

IVFehs2+?- S

IVFehs3+- S

VIFels2+- S



VIFehs2+- S

IVFehs2+?- S

IVFehs3+- S

VIFels2+- S



VIFehs2+- S

IVFehs2+?- S

IVFehs3+- S

VIFels2- - S


X-ray photoemission and L2,3 - edge X-ray

absorption spectra recently determined by Pearse et al.

(2006) show that the Fe atom in chalcopyrite is

unequivocally Fe3+‘Given that Fe is trivalent in

chalcopyrite, ‘What are the oxidation states of

the Fe atomscomprising greigite and cubanite?’


Hoggins and Steinfink (1976) predicted the oxidation state for

the Fe atom in greigite to be Fe3.92+ and that for chalcopyrite

and cubanite to be Fe2.79+ and Fe2.77+, respectively.


Bob Shannon (1981) observed that a Fe-S bond valence-bond length

connection predicts a IVFe4+-S bond length of 2.144 Å compared

with that observed 2.147Å for greigite.

He concluded that the oxidation state of Fe atom in greigite is Fe4+


Fe4+ is known but it is rare and an unlikely state!

But there exist materials like FeS2 and Ba3FeS5 which must contain

Fe4+ 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

Fe2.5+ with a chemical formula CuFe2.5+Fe2.5+S3. (McCammon,

1995)



VIFehs2+- S

IVFehs3+- S

IVFehs4+?- S

Fels2+ - S

IVFehs2.5?- S


Structure of the molecule (S4 point group) geometry optimized at the

B3LYP/6-311++G(2d,p) level.

R(Fe-S) (opt) = 2.130 Å

R(H-S) = 1.349 Å<S –Fe –S = 108.30o 2x<S –Fe –S = 110.06o 4x<Fe –S –H = 99.70o

R(Fe-S) (greigite) = 2.147Å

S

Fe4+

H

H4Fe4+S4



VIFehs2+- S

IVFehs3+- S

IVFehs4+?- S

IVFehs2.5+?- S

VIFels2+- S



VIFehs2+- S

IVFehs4+?- S

IVFehs2.5+?- S

VIFels2+- S

IVFehs3+ - S



VIFehs2+- S

IVFehs3+- S

IVFehs4+?- S

IVFehs2.5+?- S

VIFels2+- S


The properties of low spin VIFe-S bonded interactions are

distinct from those of high spin VIFe-S bonded interactions.

For the low spin VIFe-S bonded interactions, (1) Fe-S bond lengths are shorter,

(2) ρ(rc ) is larger,

(3) the bonded radii are smaller, (4) the Fe and S atomic charges are smaller.

Comments


The presence of Fe4+ 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 IVFe-S bond length and (5) the properties calculated for the H4Fe4+S4.


What is the status of the Fe atom in cubanite?

CuFe2.5+Fe2.5+S3?

CuFe2+Fe3+S3 with Fe2+,Fe3+ disorder?

None of the above?


The evidence suggests that a simple connectionbetween stoichiometry and oxidation state is notalways 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

ViFe2+-SIVFe3+?-S

IVFe4+?-S


0.0 0.2 0.4 0.6 0.8 1.0

2.0

2.5

3.0

3.5

4.0 native sulfur sulfide crystals

R(S

-S) Å

(rc) e/Å3


G(rc)

H(rc)

Na-OMg-O

Al-O

S-O

Si-O

P-O

V(rc)




VIFe2+-SIVFe3+-SIVFe4+-S




Given that a bonded interaction is shared when

¼2(rc) = 2G(rc) + V(rc) 0 and that a bonded interaction is closed when

H(rc) = G(rc) + V(rc) ≥ 0.then

2G(rc) + V(rc) = 0 |V(rc)| /G(rc) ≥ 2 and

G(rc) + V(rc) = 0, |V(rc)| /G(rc) 1. A bonded interaction is classified as

shared when |V(rc)| /G(rc) ≥ 2,

closed-shell when |V(rc)| /G(rc) 1 and intermediate when 1 < |V(rc)| /G(rc)< 2.


S

Fe-S bcp

Fe2+

S-S bcp


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

Documents

G.V. Gibbs 1 , David F. Cox 2 , Kevin M. Rosso 3 , Nancy L. Ross 1 and Robert T. Downs 4