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d-Electron Density in Formal d0 Systems from Multiplet Splitting in the Photoelectron Spectrum of Permanganates.

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(1) On irradiation with visible light, >but not indigo
(2)-is transformed into its cis f0rm;[5334-361
the latter, in
turn, transforms back into the trans isomer in the dark. In
comparison to planar photoisomerizable indigoids, such as
t h i ~ i n d i g o , [351~ ~both
.
processes are appreciably facilitated
in 3 by the sterically caused torsion of the molecule. Since, in
the photoisomerization of 3, in contrast to that of other
indigo dyes, no short-lived triplet intermediate has been observed, the trans-cis rearrangement presumably occurs via a
381
singlet mechanism in the case of 3.r37%
(2) The high electron density at the central C atoms of 3
makes it readily oxidizable in solution.t391Apparently, this
oxidation occurs via a [2+ 21 cycloaddition of singlet oxygen
to form a dioxetane, since we have found that N-methylisatin
(4) is ultimately formed by cleavage of the C=C bond of 3.
(3) Because of the pyramidalization of the N atoms, N,Ndimethylindigo (3) is more basic than planar indigo (2). This
explains why 3, unlike 2, reacts with mineral acids to form
soluble salts.t5.331
(4) The twisted structure of 3, together with its inability to
form hydrogen bridges, explains why 3 possesses a much
lower melting point (182°C) and a lower density
(1.31 g ~ m - thanindigo
~)
(2: 392°C and 1.518 g ~ m - ~Fur).
thermore, it is responsible for the fact that 3, in contrast to
the planar indigo dyes, is soluble even in nonpolar solvents
and that 3, in contrast to planar thioindigo, has no affinity
for cotton fibers.
[22] V. Kupcik, M. Wendschuh-Josties, A. Wolf, R. Wulf, Nucl. Instrum. Methods Phys. Res. Sect. A 246 (1986) 624.
[23] Taken from: International Tables for X-ray Crystallography, Kynoch
Press, Birmingham 1969.
[24] G. H. Goldschmidt, F. J. Llewellyn, Acta Crystallogr. 3 (1950) 291.
[25] A. Almenningen, 0. Bastiansen, L. Fernholt, B. N. Cyvin, S . J. Cyvin, S .
Samdal, J. Mol. Struct. 128 (1985) 59.
[26] G. P. Charbonneau, Y. Delugeard, Acta Crysrallogr. B 33 (1977) 1586.
[27] The CI calculations take into account the 45 lowest-energy singly excited
configurations. The deviations of the molecule from planarity have been
accounted for by the angle dependence of the B integrals of the respective
bonds according to the equation B(8) = ~ ( O ) C O S ~ .
[28] 0. Ermer: Aspekte von Krafrfeldrechnungen, W.-Baur-Verlag, Munchen
1981, p. 62ff.
[29] W. L. Mock, Tefrahedron Lett. 1972,475.
[30] L. Radom, J. A. Pople, W. L. Mock, Tetrahedron Lett. 1972, 479.
[31] K. N. Houk in W. H. Watson (Ed.): Stereochemistry and Reactivity of
Systems Containing n-Electrons, Verlag Chemie International, Deerfield
Beach, FL, USA 1983, p. 1 ff.
[32] N. G. Rondan, M. N. Paddon-Row, P. Caramella, K. N. Houk, J. Am.
Chem. Soc. 103 (1981) 2436.
[33] R. C. Haddon, Ace. Chem. Res. 21 (1988) 243.
(341 R. Pummerer, G. Marondel, Justus Liebigs Ann. Chem. 602 (1957) 228;
Chem. Ber. 93 (1960) 2384.
[35] G. M. Wyman, A. Zenhausern, Ber. Bunsenges. 72 Phys. Chem. (1968) 326.
[36] C. R. Giuliano, L. D. Hess, J. D. Margerum, J. Am. Chem. SOC.90 (1968)
587.
[37] D. Schulte-Frohlinde, H. Hermann, G. M. Wyman, Z. Phys. Chem. N . E
101 (1976) 115.
[38] H. Corner, J. Pouliquen, J. Kossanyi, Can. L Chem. 65 (1987) 708.
I
Am. Chem. SOC.91 (1969) 3851.
1391 G. A. Russell, G. Kaupp, .
Received: March 7, 1991 [Z 4477 IE]
German version: Angew. Chem. 103 (1991) 1008
[l] L. Ettinger, P. Friedlander, Ber. Dtsch. Chem. Ges. 45 (1912) 2074.
[2] E. B. Knott, J. SOC.Dyers Colour. 67 (1951) 302.
131 K. J. Braakmann, Dissertation, Universitat Amsterdam 1943.
[4] The UV bands of 3 are also shifted to longer wavelengths to varying
extents in comparison of those of 2.
[5] J. Weinstein, G. M. Wyman, J. Am. Chem. Sor. 78 (1956) 4007.
[6] M. Klessinger, Diplomarbeit, Universitat Freiburg 1959.
[7] W. Luttke, M. Klessinger, Chem. Ber. 97 (1964) 2352.
181 This interpretation is also supported by the largely identical group electronegativities of alkyl groups with different volumes: J. Mullay, L Am.
Chem. Soc. 107 (1985) 7271.
[9] K. J. Brunings, A. H. Corvin, L Am. Chem. Soc. 64 (1942) 593.
[lo] E. Heilbronner, R. Gerdil, Helv. Chim. Acta 39 (1956) 1966.
[I 11 A. Reis, W. Schneider, Z. Kristallogr., Kristallgeom., Kristallphys.,
Kristallchem. 68 (1928) 543.
[I21 H. von Eller, Bull. Soc. Chim. Fr. 106 (1955) 1444.
[I31 E. A. Gribowa, Kristallografiva 1 (1956) 53.
[I41 G. Miehe, V. Kupcik, P. Susse, Z. Kristallogr. 149 (1979) 149.
[15] P. Susse, M. Steins, V. Kupcik, Z. Kristallogr. 184 (1988) 269.
[16] Synthesis of 3: P. Friedlander, L. Ettinger, Ber. Dtsch. Chem. Ges. 45
(1912) 2074.
[I71 Crystal structure determination of 3: space group P2Jc (needlelike crystals along c), a = 13.99, b = 27.54, c =7.35 A, p = 97.75"; V = 2806A3,
e = 1.31 g ~ m - ~The
.
asymmetric unit contains two molecules
C,,H,,NzOz (Z = 8). Additional information on the probable molecular
shape and a stacking vector revealed by the Patterson function afforded
normalized structure factors, which were used successfullywith MULTAN
77 (Mo,,: 2624 unique reflections, 1774 of which were observed
R , = 0.064). See also [21].
[18] H. J. Lindner, Tetrahedron 30 (1974) 1127.
[19] H. Beck, Staatsexamensarbeit, Universitat Gottingen 1985.
[20] Synthesis of 4: G. Heller, Ber. Dtsch. Chem. Ges. 40 (1907) 1291.
[21] Structure determination of 4 : space group P2,/c, a = 8.327(1), b =
10.802(2), c = 17.609(3) A, p = 103.26(1)", V = 1542 A3, Z = 8; crystal
dimensions: 0.2 x 0.3 x 0.7 mm3; 4868 measured dimensions 28,,, =
50" (Mo,), 1690 symmetry-independent reflections with IF1 > 3 4 F )
used in the structure solution (direct methods) and refinement (223
parameters); C, N and 0 atoms were refined anisotropically, H atoms were
located by difference electron density determination and refined with the
riding model R = 0.081 ( R , = 0.068, w - ' = o f f / * + 0.0004 x P ) [17].
Further details on the crystal structure determination are available on
request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur
wissenschaftlich-technischeInformation mbH, W-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-52827, the
names of the authors, and the journal citation.
Angew. Chem. Int. Ed. Engl. 30 (1991) N o . 8
0 VCH
d-Electron Density in Formal do Systems from
Multiplet Splitting in the Photoelectron Spectrum
of Permanganates**
By Michael Schmalz, Robert Schollhorn,
and Robert Schlogl*
An important aspect of current ideas on the bonding in
high-temperature superconductors of the oxocuprate type is
electron transfer from the 2p band of the oxygen ions to the
copper d band. The resulting holes in the anion band lead to
a mixed-valence state. This can be formulated according to
the ionic model as shown in Equation (a).['. 21
In order to assess how real the differences in charge density are in Equation (a), it is necessary to quantify the difference between formal oxidation state and real charge density.
The magnitude of this difference allows an assessment of the
relevance of the ionic model, which is used throughout structural chemistry, to a discussion of bonding.
A proven method for the spectroscopic analysis of charge
density distribution is photoelectron spectroscopy (PES). It
has been applied, for example, to transition-metal chalcogenides of the type CuCr,X,, which were shown to have
["I Prof. Dr. R. Schlogl
Institut fur Anorganische Chemie der Universitat
Niederurseler Hang, W-6000 Frankfurt am Main 50 (FRG)
Dipl.-Chem. M. Schmalz, Prof. Dr. R. Schollhorn
Institut fur Anorganische und Analytische Chemie
der Technischen Universitiit
Strasse des 17. Juni 135, W-1000 Berlin 12 (FRG)
[**I This work was supported by the Fonds der Chemischen Industrie and by
the Bundesministerium fur Forschung und Technologie. We thank the
Fritz-Haber-Institut, Berlin, fur allowing us to use their Leybold EAll
photoelectron spectrometer.
Verlagsgesellschafi mbH. W-6940 Weinheim, 1991
0570-0833j91jO808-0967 d 3.50+-.25/0
961
holes in the p band.[31This charge transfer was found by
analysis of the Se 3d spectra. An analogous procedure is not
possible for oxidic solids, since the apparently unavoidable
presence of OH - ions makes it very difficult to distinguish
between the 0 1s binding energy of formal oxide ions (02-)
and that of less shielded oxygen species (O'-), which are
possibly also present [Eq. (a)] .[2,4, 51 Nonetheless, permanganates may be used as suitable model substances for an
investigation of the difference between formal oxidation
state and real charge density in oxides, since the color of
these salts cannot be explained in terms of an ionic model
with the formal oxidation states Mn7+ (do system) and
0'- (s2p6). Moreover, the MnO, compounds (AgMnO,,
KMnO,) are solids that, owing to their molecular-ion lattice,
exhibit only small band-structure contributions to the total
electron structure, so that the charge distribution can be
studied locally.
PES is a surface-sensitive method (information depth ca.
1000 pm); for the application discussed here, therefore, the
surface of the sample must be an exact representation of the
bulk. This condition can be fulfilled by cleaving the permanganate single crystals under ultrahigh vacuum. Owing to
its molecular-ion character, moreover, this system tolerates
only small oxygen disorder, in contrast to binary or polynary
copper oxides, so that the uncertainty in the valence of the
transition metal is much reduced. Furthermore, numerous
quantum-mechanical calculations have already been carried
out for the MnO; ion, which has long had the same importance in theoretical inorganic chemistry as benzene has had
in theoretical organic chemistry.[61
PES allows the charge density distribution to be determined on the basis of the chemical shifts. However, the
system discussed here poses difficulties, since, on the one
hand, electrostatic c h a r g i n g a u e to the insulator properties
of the compounds-and, on the other hand, different lattice
energies-due to the differing crystal structures of the compounds required as reference systems-perturb the chemical
shifts.I7I
Therefore, the analysis of the d-electron population was
performed on the basis of a quantum-mechanical effect seldom used in PES, namely, multiplet splitting. Multiplet splitting, which is caused by the exchange interaction between an
unpaired core electron arising through photoionization and
the total spin of the d electrons of the valence shell, leads to
two final states. The very observation of multiplet splitting,
therefore, shows the presence of d electrons in the valence
shell.181Multiplet splitting is unambiguously observable for
photoemission from s states, since intense additional structures appear in the spectra of p and d shells, resulting primarily from a very large number of possible final states. However, a disadvantage is the low intensity of the s bands. In
practice, the interpretation of all photoemission structures is
further hindered by additional characteristic energy losses.[71
The samples were first characterized by X-ray diffraction"] and their composition was established by atomic absorption spectrometry (AAS). The lattice constants of the
compounds AgMnO,, KMnO,, and MnF, were found to be
in good agreement with the reported values.[lo1Older publications reported that the Mn 3s spectrum of KMnO, was not
This may have been due to features of the
instruments used, which resulted in a low intensity, or, alternatively, to sample surfaces that did not consist of MnO,
ions.
That the substances we investigated contained authentic
MnO, was revealed by the composition of the surfaces measured using theoretically determined effective cross-sect i o n ~ I ' ~(Table
l
1). The somewhat larger surface values for
968
0 VCH
Veriagsgeseiischaji mbH, W-6940 Weinheim. 1991
Table 1. Binding energies E,, and mulitplet splitting AEA (determined by PE
spectroscopy) as well as surface and volume composition of the manganese
compounds investigated.
Compound
EB
Mn 2p,,,
[evl
KMnO,
643.1
AgMnO,
642.3
MnO, (pyrolusite) 642.9
MnO, (manganite) 642.9
643.7
MnF,
AEA [evl
Mn 3s
Surface
4.18
4.2
4.69
K , 23Mn04.2,
Ag,2,Mn0,
MnO,,,,
~
6.5
Composition
Volume
K0.99Mn0,.76
~
MnO2.t
MnO, 997
MnO2.3,
MnO4.35
MnF2 19
~
the cations in the MnO; compounds are probably due to
small contamination with KOH, as indicated by the results
of the AAS analysis and the form of the 0 1s peak in the
photoelectron spectrum.['"l The slightly too large content of
oxygen is due to the incorporation of OH- groups into the
crystal; these groups are also revealed by a shoulder at about
531.5 eV in the 0 Is PE spectra. Under our experimental
conditions (surfaces cleaved in situ, ultrahigh vacuum), the
Mn 3s spectra of the permanganates exhibited a splitting that
was smaller than that of the other manganese compounds;
this finding is in agreement with the highest formal oxidation
state Mn". The Mn 3s spectrum of KMnO,, as an example
of the permanganates, is compared with that of MnF, in
Figure 1. The contribution of the splitting, AEA, is given in
Table 1 for all compounds investigated.
I
I
MnF,
6.5
0.0
--E,,,[eVI
Fig. 1. Mn 3s PE spectra of KMnO, and MnF,. The origin of the binding-energy scale in both cases was taken to be the maximum of the lower-energy peak
[8. IS].
Further evidence for the presence of MnO, is provided
by a comparison with the Mn 2p spectra. These spectra display the same line profile for the two compounds (AgMnO,,
KMnO,). The binding energies of the Mn 2p electrons are
higher relative to the oxygen 1s electrons than in the reference compounds; the relatively small difference between
these energies and that in the formally only tetravalent manganese ion in MnO, shows, however, that the permanganate
ion cannot be identified by its chemical shift alone. The exact
OS70-0833/9l/0808-0968$3.50+ ,2510
Angew. Chem. h i . Ed. Engl. 30 (1991) No. 8
position of a photoemission line is not only influenced by the
electron density at the atom under consideration in its
ground state, but also by a number of intra- and interatomic
effects (see above). The difference in the values for the two
permanganates results from the differing polarizability of
the counterion (K’ is harder than Ag’) and confirms the
effect of the spectroscopic ground state (extraatomic electrostatic shielding[’’) on the binding energies; an actual difference in the Mn charge density is ruled out by the identical
geometries of the MnO, tetrahedra.[l6The multiplet splittings for AgMnO, and KMnO, (4.2
and 4.18 eV, respectively) can now be converted into the
corresponding d-electron populations by using theoretical
data adjusted for the binary transition-metal compounds.[1g1
For AEA = 4.2 eV, 3.5 d electrons were calculated for
MnO, . This value is in general agreement with the calculated electron densities, which predict an electron deficit in the
Mn 3d orbitals of 0.1-1.5 charges relative to the free
atom.[201The charge on manganese, which is small compared to the formal oxidation state, indicates that the real
charge on oxygen is not - 2, but rather much smaller, so
that the binding energy usually given for 0 2 -in transitionmetal oxides (- 530.5 eV) in reality always includes a considerable covalent interaction. An attempt[”] to identify occupied Mn 3d levels in KMnO, directly through
valence-band photoemission gave results that were qualitatively in agreement with the data presented here. Since valence-band spectroscopy is not state-selective, however, the
data from Ref. [21] provide only evidence and not proof.
A value of - 0.5 resulted for the charge on the oxygen
atoms in the permanganate ion. Presumably, however, there
is also an interaction between the Mn 4s/4p orbitals and the
0 2p orbital.[201The spectroscopically determined d-electron
population should only be regarded as a lower limit. This
uncertainty results from the fact that PES, as a typical highenergy spectroscopy, does not measure the pure electronic
ground state of a system.
The results obtained here for the molecular species MnO;
cast doubt on an analysis of bonding that assumes formal
oxidation states, at least in oxidic solids. Thus, the high-temperature superconductors that show metal-like conduction
at room temperature, should exhibit considerable orbital interaction between the “cations” and “anions” owing to the
formation of bands (overlap between 0 2p and Cu 3d orbitals). Clearly, therefore, Equation (a) does not reflect the
actual bonding, since neither ionic formula in Equation (a) is
correct. Owing to covalent interactions, oxygen always has
an appreciably smaller net charge than would have been
expected according to the ionic formula. The differentiation
of oxygen ions with differing charge (0’- and O’-), attempted on the basis of PE 0 1s data,‘,’ should be viewed
very critically. The nonequivalence of the oxygen atoms
might be due to differing geometric surroundings and not to
different oxidation states.
Thus, the valence problems in inorganic solids are actually
a problem of describing chemical interactions, which do not
appear in the context of the multicenter orbital interaction
long assumed in the theory of molecules. The description of
solids in terms of ions with integral charges, which is very
common in structural chemistry, is only a very crude model
for the analysis of chemical bonding in extended periodic
solids.
Received: December 17, 1990;
revised: March 8, 1991 [Z 4334 IE]
German version: Angew. Chem. 103 (1991) 983
Angen. Chem. Inr. Ed. Engl. 30 (1991) No. 8
0 VCH
[l] R. J. Cavam Spektrum Wiss. 1990, No. 10, p. 118; Sci. Am. 263 (1990).
No. 8, p. 24.
[2] R. Schlogl, H. Eickenbusch, W. Paulus, R. Schollhorn, Mafer. Res. Bull.
24 (1989) 181.
[3] A. Payer, M. Schmalz, W. Paulus, R. Schollhorn, R. Schlogl. Maler. RPS.
Bull. 25 (1990) 515.
[4] C. N. R. kdo, P. Ganguly, J. Gopalakrishnan, D. D. Sarma, Mafer. Res.
Bull. 22 (1987) 1159.
[5] S. L. Qiu, M. W Ruckman, N. B. Brookes, P. D. Johnson, J. Chen, C. L.
Lin, M. Strongin, B. Sinkovic, J. E. Crow, C . Lee, Phys. Rev. 8 3 7 (1988)
3747.
[6] J. P. Dahl, C. J. Dahlhausen in P. Lowdin (Ed.): Advances in Quantum
Chemistry, Academic Press, New York 1968, pp, 170-226.
[7] M. Cardona, L. Ley, Top. Appl. Phys. 17 (1978) 60-84.
[8] P. S. Bagus, A. J. Freeman, E Sasaki, Phys. Rev.Lett. 30 (1973) 850.
[9] Measured lattice constants [A] [“I for the compounds investigated:
KMnO,: a = 9.109(6), b = 5.720(3), c =7.426(3); AgMnO,: a =
5.6250(7), b = 8.3378(9),
= 92.49(1); MnO,(pyrolusite): a = b =
4.390(5), c = 2.880(10); MnO,(maganite): a = b = 4.403(2), c = 2.869(4);
MnF,: a = b = 4.8653(6), c = 3.3030(6).
[lo] JCPDS powder diffraction tile 12-716, 24-735 (MnO,), 24-727 A (MnF,),
20-487 (AgMnO,), and 7-23 (KMnO,).
[ I l l J. C. Carver, G. K. Schweitzer, T. A. Carlson, J. Chem. Phys. 57(1972)973.
[12] J. S. Ford, R. B. Jackman, G. C. Allen, Philos. Mag. A 49 (1984) 657.
[13] J. H. Scofield, J. Elecrron Spectosc. Relat. Phenom. 8 (1976) 129.
[14] R. D. W. Kemmit in A. F. Trotman-Dickenson (Eds.): Comprehensive Inorganic Chemislry, Pergamon, Oxford 1973, pp. 798-812.
[15] Owing to electron-correlation effects, the intensity ratio in MnF, deviates
from 1:l and has an idealized value of 7 5 .
[16] E. G. Boonstra, Acta Crystallogr. 24 (1968) 1053.
[17] J. Palenik, Inorg. Chem. 6 (1967) 503.
[18] E M. Chang, J. Jansen, Z. Krisrallogr. 169 (1984) 295.
[19] D. A. Shirley. Phys. Scri. (1975) 177.
[20] A review is given by E. Schleitrer-Sleinkopf in Gmelin: Hundbuch der
Anorganischen Chemie, Mangan Teil C 2 , Springer, Berlin 1975, pp. 28- 30.
1211 R. Prins, T. Novakov, Chem. Phys. Let[. 16 (1972) 86.
The cyclo-P, Ligand as 12-Electron Donor **
By Manfred Scheer.* Eckhard Herrmann,* Joachim Sieler,
and Matthias Oehme
A P, unit can be present in complexes as the tetrahedro-P,
ligand A as in 1 [ 1 1 and 2’’’ and as planar cyclo-P, ligands B
(rectangular) and C (square) as nearly realized in 3I3I and
4,I4I respectively. Complexes in which the four lone pairs of
a cyclo-P, ligand are each bonded to a metal atom were
previously unknown.[51
.
/P\
:P-I-P:
P
A
/,
.
‘P-P‘
II
.P-P..
II
B
.
.
. P-P‘
I01
..P-P,.
C
Reaction of white phosphorus with [M(CO),(thf)], M =
Cr or W [Eq. (a)], has now afforded complexes in which a
square-planar P, unit serves as a 12-electron donor.
[‘I
[‘I
[**I
Dr. M. Scheer, Prof. Dr. E. Herrmann
Institut fur Allgemeine und Anorganische Chemie der Universitat
Weinbergweg 16, 0-4050 Halle (Saale) (FRG)
Doz. Dr. J. Sieler‘+l, Dipl.-Chem. M. Oehme“’
Institut fur Anorganische Chemie der Universitat Leipzig
X-ray structure analysis
This work was supported by the Fonds der Chemischen Industrie
Verlagsgesellschaft mbH, W-6940 Weinheim, 1991
OS70-0833~91/0808-0969$ 3 . 5 0 + . 2 5 / 0
969
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forma, spectrum, permanganate, photoelectrode, electro, system, multiple, splitting, density
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