Relativistic Effects in Gold
Chemistry
Jan Stanek
Jagiellonian University
Marian Smoluchowski Institute of Physics
30-059 Krakow, Poland
Many properties of
gold, including its
“magic” shine are
consequences of the
relativistic effects,
which can be defined
as the difference
between the exact
and nonrelativistic
eigenvalues of the
Hamiltonian of the
system.
The experimentally determined exact electronic level scheme should
be compared with the results of the relativistic numerical calculations,
which are still restricted to the rather simple cases. The most
relativistic effects important for the chemistry are:
The contractions of s and p electron shells
1. The expansion of d and f shells
2. The spin-orbit splitting
The scheme of the relativistic
modification of the energy of Au
electronic levels.
Left: non-relativistic case,
right: the relativistic case.
The relativistic splitting of the 5d
and 6p orbital is due to spin-orbit
coupling, not discussed her.
The 197Au Mössbauer spectroscopy is quite laborious. The high
energy of the recoilless M1/ E2 transition of 77.3 keV requires
measurements near the temperature of liquid helium. The
source of 197Pt may be repeatedly activated by 196Pt (n,g) 197Pt
reaction. The half-life time of 197Pt is 18 h, which causes some
difficulties in complicated experiments as for example
measurements at high pressure or in high external magnetic
field.
The activation of 30 mg of 196Pt
in the neutron flux of
8*1013 neutrons/s·cm2 results in
the 180 mCi activity of 197Pt
Au-1 state in CsAu
The cubic (CsCl-type) ion compound Cs+Au- shows a single line
Mössbauer spectrum with isomer shift of 7 mm/s. In case of the
interaction of Au with highly electropositive elements as alkali metals the
low lying 6s orbital of gold is adequate for the localization of an extra
electron and the [Xe]4f145d106s2 configuration is a consequence of the
relativistic effect.
The ionic type of bonding has been proved by measurements of the melt
conductivity. The spectroscopic method showed the energy gap in the
electronic band structure in solid state. However, the ionic or
semiconducting properties of the “alloy” of these two metals could not be
reproduced by the non-relativistic calculations of the band structure:
neglecting the lowering of the 6s level leads to the overlap of the valence
and conduction band.
Finally, the relativistic approach reproduced the energy gap in the CsAu band
structure and predicted the instability of the CsCl-type structure of AuCs at high
pressure.
This has been confirmed
by Mössbauer experiment.
At 2.7 GPa (27 kbar) the
local symmetry of Au
atoms shows a distortion
as seen by the quadrupole
splitting. The high pressure
structural transformation is
correlated with the
dramatic decrease of the
mean-square displacement
of Au atoms in CsAu
[J. Stanek, S.S. Hafner, F. Hensel.
Phys. Rev. B32 (1985)3129]
1 bar
197Au
Mössbauer
spectra of CsAu
measured at 4.2 K
at ambient pressure
27 kbar
and at high pressures.
40 kbar
The left absorption line
comes from metallic Au
foil. Note the change in
the Au/CsAu line
intensity ratio after
applying the pressure.
Bonding in Au+1 compounds
In all Au+1 compounds gold exhibits the linear coordination.
There were two competitive models of bonding:
“covalent” one based on sp hybridization and
“ionic” based on 5d-6s mixing.
The covalent model implies an increase in electron density
distribution towards ligands which should produce a negative
field gradient (Vzz) at 197Au nucleus while the ionic model,
due to the pure electrostatic interaction, leads to the
depletion of the 5d shell by charge transfer to the spherical
6s shell, which should produce the positive field gradient.
This mechanism is especially effective for gold due to the
relativistic approaching of the 5d and 6s orbitals.
The negative sign of Vzz was experimentally determined in KAu(CN)2
from single crystal measurements and then the negative Vzz was
assumed for all (AuI)2 complexes and used as a proof of the covalent
character of Au+1 compounds. The negative sign of Vzz has been
confirmed, indeed, for AuCN by Mössbauer experiment in external
magnetic field of 9.5 T [J. Stanek, S.S. Hafner, B. Miczko. Phys. Rew. B, 57 (1998)
6219-6223]
The 197Au Mössbauer spectra of AuCN at zero field (left) and at 9.5 T with
the fit assuming negative Vzz (right)
Similar measurements with AuI in
external magnetic field excluded the
negative Vzz at 197Au nuclei in this
compound. The perfect fit was
obtained only for non–axial electric
field gradient with positive Vzz.
[J. Stanek, S.S. Hafner, B. Miczko. Phys. Rew.
B, 57 (1998) 6219-6223]
The 197Au Mössbauer spectra of
AuI at zero field (a) and 9.5 T fitted
with negative Vzz and h=0, (b)
positive Vzz and h=0 (c) and
positive Vzz and h=0.8 (d).
The high asymmetry of the electric field gradient (h=0 ) in linear (AuI)2 clusters in
AuI can be explained by structural consideration when two coordination spheres of
Au are considered. It turns out that the axis of the (AuI)2 cluster is not perpendicular
to the square formed by 4 Au ions as next-nearest neighbors, which obviously
breaks the axial symmetry. The possitive Vzz indicates the ionic bonding in Au+ state,
leading to the depletion of the 5d shell due to the electrostatic interaction. The high
value of h proves that, surprisingly, the Au-Au bonding is important.
Crystal structure of AuI, left, and the coordination of Au (red) by two nearest
neighbors of I (blue) and four next nearest neighbors of Au, right.