Photodisproportionation of Hg22+

Article abstract of DOI:10.1016/S0009-2614(01)01493-2

The photolysis of Hg²⁺₂ in tetrahydrofuran induced by metal-metal σσ* excitation leads to the generation of Hg⁰ and Hg²⁺. The quantum yield of this photodisproportionation is ? = 0.03 at λ_irr = 254 nm

Full text of DOI:10.1016/S0009-2614(01)01493-2

6 February 2002
Chemical Physics Letters 352 (2002) 375–377
www.elsevier.com/locate/cplett
Photodisproportionation of Hg22þ
*
Horst Kunkely, Arnd Vogler
Institut f€ur Anorganische Chemie, Universit€at Regensburg, D-93040 Regensburg, Germany
Received 8 October 2001; in final form 19 November 2001
Abstract
The photolysis of Hg22þ in tetrahydrofuran induced by metal–metal rrà excitation leads to the generation of Hg0 and
Hg2þ. The quantum yield of this photodisproportionation is / ¼ 0:0 3 atkirr ¼ 254 nm. Ó 2002 Elsevier Science B.V.
All rights reserved.
Å
ÅMðCOÞn þ L ! MðCOÞnL
ð2Þ
1. Introduction
ÅMðCOÞnL þ ÅMðCOÞn ! MðCOÞnLþ þ MðCOÞÀn
The disproportionation of metal ions is a fun-
damental process. In very simple systems two
identical metal ions disproportionate to one unit
higher and one unit lower than the original oxi-
dation state. In principle, such a reaction should
also occur photochemically. Attractive candidates
are binuclear complexes which contain a covalent
metal–metal bond. However, such M–M bonds are
generally split homolytically by rrà excitation.
This occurs, for example, in binuclear carbonyl
complexes [1–3]:
ð3Þ
This observation raises the question if a genuine
disproportionation of a completely symmetric M–
M bond can also occur as a primary photoprocess:
hm
M–M!MþMÀ. While this possibility has been
recognized [2,3] it has apparently not yet been
confirmed experimentally. In this context we wish
to report our present observation of the heterolytic
photocleavage of Hg22þ. This binuclear cation is an
intriguing analog of H2 since its metal–metal bond
is essentially based on the overlap of Hg 6s orbitals
[4,5].
hm
ðOCÞnM–MðCOÞn !2ÅMðCOÞn
ð1Þ
In this case subsequent addition reactions at the
photogenerated metal carbonyl radicals can induce
a thermal disproportionation
2. Results
It has been previously shown that rrà excitation
of aqueous Hg22þ leads to the homolysis of the Hg–
Hg bond yielding two Hgþ ions [5,6]. These radicals
can be intercepted and oxidized by oxygen. In al-
* Corresponding author. Fax: +49-941-943-4488.
E-mailaddress: arnd.vogler@chemie.uni-regensburg.de (A.
Vogler).
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(01)01493-2

376
H. Kunkely, A. Vogler / Chemical Physics Letters 352 (2002) 375–377
Fig. 1. Spectral changes during the photolysis of 8:87 Â 10À5 M Hg2ðClO4Þ2 in THF at room temperature after (a) 0s, (b) 15 s, (c) 30
s, (d) 60s, and (e) 120s irradiation times with white light (Osram HBO 200W/2 lamp), 1 cm cell.
coholic solution they are reduced to elemental
mercury [5,7]. In tetrahydrofuran (THF) as the
solvent the photolysis takes a different course. Light
absorption by the spin-allowed rrà band [4,5]
roughly one half of the photolyzed Hg22þ is recov-
ered as HgðdithizonateÞ2. It follows that the pho-
tolysis proceeds according to the equation:
hm
Hg22þ !Hg0 þ Hg2þ
ð4Þ
ðkmax ¼ 257 nm; e ¼ 24100 dm3 MÀ1 cmÀ1
Þ
of
Hg22þ is associated with spectral changes shown in
Fig. 1. These variations are essentially based on the
conversion of Hg22þ to elemental mercury which fi-
nally separates as shiny droplets. Owing to the re-
sidual solubility of atomic mercury in almost all
solvents ðc ꢀ 10À7Þ the absorption of mercury at-
oms ðkmax ¼ 256 nmÞ [5,7] can be recognized in the
photolyzed solution (Fig. 1). Moreover, mercury
atoms add also to Hg22þ. The formation of small
amounts of Hg23þ are indicated by the appearance
of a weak absorption at kmax ¼ 315 nm (Fig. 1) and
a concomitant emission at kmax ¼ 645 nm [5,8].
After the photolysis of Hg22þ in THF was continued
to completion the solution is separated from ele-
mental mercury. This solution contains Hg2þ which
absorbs only slightly in the UV and does not show
any characteristic band. However, upon addition of
dithizone, Hg2þ is converted to the well known
The progress of the photoreaction is monitored by
measuring the decrease of absorption at
k ¼ 257 nm. The quantum yield for the disap-
pearance of Hg22þ is / ¼ 0:0 3 atkirr ¼ 254 nm.
3. Discussion
The photodisproportionation of Hg22þ seems to
be the first genuine case of a heterolytic photoc-
leavage of a metal–metal bond. The existence of
zwitterionic excited states of the type MþMÀ can
be derived from the hydrogen molecule. Ionic ex-
cited states of H2 had been discussed more than 60
years ago [11–13]. Quite recently, spectroscopic
evidence for
a zwitterionic excited state of
Mo2Cl4ðPMe3Þ has been reported [14]. Generally,
4
the zwitterionic states occur at rather high energy
Å
Å
but approach the singlet biradical states MM if
the corresponding orbitals are only weakly cou-
pled. In the case of the hydrogen molecule or a
metal–metal r-bond this requirement is met when
complex HgðdithizonateÞ with kmax ¼ 485 nm
2
3
ðe ¼ 70500 dm MÀ1 cmÀ1Þ [9,10] which is used
for the spectrophotometric identification and de-
termination of Hg2þ. The analysis shows that

H. Kunkely, A. Vogler / Chemical Physics Letters 352 (2002) 375–377
377
the H–H or M–M bond is sufficiently stretched.
Even then a real disproportionation would not
occur since both zwitterionic states Mþa MÀb
5þ
5
½ðNH3Þ OsIII–CN–OsIIIðNH3Þ ꢁ
5
hm
5þ
5
!½ðNH3Þ OsIV–NC–OsIIðNH3Þ ꢁ
ð5Þ
$
5
MÀa Mþb are degenerate in a symmetric system.
However, when the environment of the M–M
bond becomes asymmetric a real photodispropor-
tionation may indeed take place. In organic mol-
ecules such a spontaneous formation of zwitter
ions has been described as ‘sudden polarization’
[15,16]. In a simple molecule or ion of the type M–
M an asymmetric environment for a sudden po-
larization could be induced by the solvent or
counter ions. While it apparently occurs in the case
of Hg22þ in THF the reason for this sudden po-
larization is yet unknown. The different behavior
of Hg22þ in aqueous solution and THF could be
related to the strength of the Hg–Hg bond in both
solvents. According to the shift of the rrà ab-
sorption from 235 nm in water [6] to 257 nm in
THF the metal–metal bond seems to be distinctly
weaker in THF. The concomitant extension of the
Hg–Hg bond might favor the photodispropor-
tionation (see above). Although our observations
are consistent with a dismutation as primary
photoreaction a definitive proof for this mecha-
nism is yet missing. However, our observations
may stimulate further studies, in particular calcu-
lations and time-resolved spectroscopy, to solve
this problem. Quite recently, trinuclear complexes
of the type Os3ðCOÞ10(a-diimine) have been shown
to undergo photochemical zwitterion formation
[17]. In this case, the diimine ligand which is co-
ordinated only to one metal center certainly facil-
itates the generation of the zwitterion.
However, the binuclear complex is not light sen-
sitive since a rapid back electron transfer appar-
ently restores the original complex. Some other
complexes which contain the FeIII–O–FeIII moiety
undergo indeed a photodisproportionation to
FeIV@O2þ and Fe2þ [19–21], but the nature of the
photoactive excited state is not quite clear [21].
Acknowledgements
Support of this research by the Fonds der
Chemischen Industrie is gratefully acknowledged.
References
[1] G.L. Geoffroy, M.S. Wrighton, Organometallic Photo-
chemistry, Academic Press, New York, 1979.
[2] A.E. Stiegman, D.R. Tyler, Acc. Chem. Res. 17 (1984) 61.
[3] A.E. Stiegman, D.R. Tyler, Coord. Chem. Rev. 63 (1985)
217.
[4] W.R. Mason, Inorg. Chem. 22 (1983) 147.
[5] H. Kunkely, O. Horvath, A. Vogler, Coord. Chem. Rev.
159 (1997) 85.
[6] A. Vogler, H. Kunkely, Inorg. Chim. Acta 162 (1989) 169.
[7] O. Horvath, P.C. Ford, A. Vogler, Inorg. Chem. 32 (1993)
2614.
[8] H. Kunkely, A. Vogler, Chem. Phys. Lett. 240(1995) 31.
[9] L.S. Meriwether, E.C. Breitner, N.B. Colthup, J. Am.
Chem. Soc. 87 (1965) 4448.
[10] B. Borderie, D. Lavabre, G. Levy, J.C. Micheau, J. Chem.
Educ. 67 (1990) 459, and references cited therein.
[11] R.S. Mulliken, Phys. Rev. 50(1936) 1017.
[12] R.S. Mulliken, Phys. Rev. 50(1936) 1028.
[13] C.A. Coulson, I. Fischer, Philos. Mag. 40(1949) 386.
[14] D.S. Engebretson, J.M. Zaleski, G.E. Leroi, D.N. Nocera,
Science 265 (1994) 759.
In the context of the present study it should be
pointed out that a photodisproportionation of
metal compounds can also take place by a different
mechanism. In binuclear complexes both metals
may be not connected only by a direct metal–metal
bond but also by a suitable ligand which mediates
the electronic interaction of both metals in the
same oxidation state. If the metal is simulta-
neously reducing and oxidizing a metal-to-metal
charge transfer (MMCT) absorption can be ob-
[15] L. Salem, Acc. Chem. Res. 12 (1979) 87.
[16] T.A. Albright, J.K. Burdett, M.-H. Whangbo, Orbital
Interactions in Chemistry, Wiley, New York, p. 159.
[17] F.W. Vergeer, C.J. Kleverlaan, D.J. Stufkens, Inorg. Chim.
Acta, in press.
[18] H. Kunkely, V. Pawlowski, A. Vogler, Inorg. Chim. Acta
238 (1995) 1.
[19] M.W. Peterson, D.S. Rivers, R.M. Richman, J. Am. Chem.
Soc. 107 (1987) 2907.
served. 5þThe complex ½ðNH3Þ OsIII–CN–OsIII
5
[20] L. Weber, R. Hommel, J. Behling, G. Haufe, H. Hennig, J.
Am. Chem. Soc. 116 (1994) 2400.
ðNH3Þ5ꢁ is an appropriate example [18]. MMCT
excitation leads then to a disproportionation by
definition
[21] H. Kunkely, A. Vogler, J. Chem. Soc., Chem. Commun.
(1994) 2671.