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Mixed and partial oxidation states. Photoelectron spectroscopic evidence
CHEMICAL PHYSICS LETTERS
Volume 18, iiiimber 1
1 January 1973
MIXED AND PARTIAL OXIDATION STATES. PHOTOELECT’RON SPE@TROSCOPIC EVIDENCE David CAHEN and Joseph E. LESTER Department
of Chemistry and Materials Research Center, Northwestem
University,
Evanston, Illirtois GOZOI. USA
Received? September 1972
X-ray photoelectron spectra of the iingle valence platinum complexes Ii2 [Pt(CN)4].3HzO(l), K2 [Pt(CN)J] Cioaj-n&0(2) and Kz [Pt(CN)G] Ci2.3H20(3) and the mixed valence compollnd [PtlI(CZHsNHz)4] C14-[PtfV(CzH~NHz)4C12] -4FJz0(4) have been measured. It is found that one can distinguish between mixed and single valence compounds by electron spectroscopy. The Pt spectrum of (4) is a superposition of a PtlI and PtlV spectrum. The chemical shift between (1) and (3) is normal, however (2) shows an anomalouslow binding energy for the Pt 4f electrons. The importance of using reliable reference peaks for obtaining absoiute binding energies is emphasized.
cleariy
Platinum is one of several transition metals that can form compounds with the metal eitnzr in different oxidation states or in a formal partial oxidation state [ I] . Direct evidence for the occurrence of mixed oxidation states (locaiized electrom, class 2 [ 1j ) or partial oxidation states (delocalized eiectrons, class
I!1 b [I’j) has been obtained only from complete structure determination or Mtissbauer spectroscopy. The latter technique is impractical in the case of Pt and the first one is time consuming, expensive and requires singIe.crystds. Here we present X-ray pIlotoeIectron spectroscopic data that show the potential of this relatively simple and fast technique applied tc this problem. We have determined the Pt 4f electron binding energy (B.E.) in the compounds KzfPt(CN)41*3H2~(1); K2 [Pt(CN)41C10.3 .nH20 (2
longer Pt-Pt distance. The structure of 3 is unknown but no anomalies are observed in its physical and chemical pro;i?rties [7]. X-ray diffraction proved 4 to be a mixed valence compound [S] . All these compounds are hydrates and therefore were studied at --I OO°C [9]. No decomposition was observed for 1 or 2; 2 has been shown to retain its basic structure upon dehydration [lo]. Prolonged exposure of 4 to vacuum resulted in some decomposition, but the process was slow enough to enable us to obtain good spectra. 4 dehydrates rapidly and the resulting yellow material has the same X-ray powder diffraction pattern as the hydrate except for a much increased background, pointing to a decrease in crystallinity. No shifts in either diffraction or ESCA peak positions were observed, only changes in intensity. TabIe I summarizes the experimental binding energies of the Pt 4f electrons. In the series of cyanide complexes (1, 2, and 3) a shift of more than 2 e?f is observed between Pt” and PtIV(3). This compares well with other reported values [I 11. The small shift in K 2P,,2 B.E. indicates tid difference
between
only a small Madelung
poten-
1 and 3. A Madelung correc-
tion, though important, would no\ influence the conclusion, t%at the Ptn and Ptw 4f binding energies are well separated.
Volume 18, number 1
1 Januarg’ 1973
CHEMICAL PHYSICS LETTERS Table 1
Compound
a)
pt foil b) K~[FtW041~3H~O(l) Kz[Pt(CWl& 3.rrHzW) Kz [PI(CNL1C12:3H20(3)
Pt 4fs,2
.
[P!Jl(C2HsNH2)4]C14~ [PtTV(C,HsNHz)4C12].4Ht0 “) ----_I_---___
fwhm
Pt 4fT2
fwhm
K 2P3/2
c Is 284.8(l)
74.5(l) c)
1.75
71.2(l)
1.65
77.6(l)
1.7
74.2(l)
1.85
293.3(2)
284.8(l)
76.3(l)
2.1
73.0(l)
2.0
292.3(2)
285.00)
79.90)
1.75
76.6(l)
1.9
292.9(2)
284.W)
::“,
74.1(l) 76.4(!)
1.6 1.75
:;.::::
286.W) -
a) Compounds 1, 2 and 3 were kindly supplied by Dr. Doris Lin from the laboratories of Professors Collman and Little at Stanford University and compound 4 by Jayarxnan Rajaram from our laboratories. b) From refs. [II, 121. c) All binding energies (in eV) are with respect to the 4f peaks of gold deposited in situ on the cooled sample (-100°C). The Au 4fs12 and 4f,,2 B.E.‘s were assigned to be 87.7 and 84.0 eV based on their displacement from the Pt metal 4f p&&s [IL, L2I Spectra were taken on a computer controlled AEI ES1008 spectrometer using Al Ka! X-radiation. Tic samples were sprczd on lead plates.
Wolfram’s red salt (4) showsa triplet spectrum (fig. 2) that can be resolved into two doublets. These doublets are clearly separated and compare very well with Ptrl and Ptl” B.E.‘s. and although strictly 4 can only be compared with itself (hecause of its special structure) the logical explanation for the two doublets is to assign them to PtIr and Ptrv. It is not clear why the peak ratio should be 2: 1 (Ptrl:Ptrv); however if the spectrum is taken immediately after sample insertion, the ratio is closer to 1: 1 and then rapidly goes to 2: 1. This suggests initial partial surface decomposition until a steady state is reached. The spectrum of 2 (fig. 1) does not fit in with those of 1 and 3, the corresponding Pt” and Pt” salts. Although its structure is similar to that of 1, the K 2P3,2 B.E. in 2 is lower by about 1 eV. This shift is larger than any shift observed between potassium platinates and platinites by Moddeman et al. [ 11J or Nefedov et al. [ 121. A shift of this order of magnitude in the K peak position could be the result of a change in the K site potential on introduction of Ci- ions into the lattice. It has been observed that the C--T\1 bond length in 2 is longer [3] than in KNa[Pt(CN)4j .3H,O [G] . We feel that this bond lengthening is due to a transfer of electron density from the cyanide to the platinum, which is consistent with our B.E. observations. As the C 1s peak contains contributions from impurities as well as from the cyanide, we could not detect any change in the peak that could be ascribed to charge
transfer. The signal to noise ratio for the N Is peak was not high enough to obtain the peak positions to the accuracy required to observe the expected shift. An alternative explanation for the shift of the Pt 4f peak is suggested by the NMR experiments of Rupp [13]_ He found a lgsPt Knight shift in K, [Pt(CW, I Br0.3 .jzHTO, _ while no sub shift was observed in K, [Pt(CN),] *3H,O. This shift implies delocalized conduction electron density at the nucleus. One could then postulate an additional screening of the 4f electrons and thus a lowering of their B.E.‘s. One should also note that the fwhm of the Pt peak in 2 is larger than in either 1 or 3. This is expected 3s a result of the random introduction of Cl- ions on 64% of the available sites [3]. All B.E.‘s have been assigned, using the Au 4f peaks from vacuum deposited Au. If the Au 4f peaks from a gold sample base were used as the B.E. reference, shifts up to 1.1 eV in the peak positions were obtained. These shifts show the importance of accurate B.E. reference peaks when measuring absolute binding energies. Thus we strongly urge the use of v3cu~1m deposited gold as a standard 114) . Regardless of tie problem of assigning absolute peak positions, we feel that these data iliustrate that one can use ES&L to distinguish easily between a mixed valence compound and a single valence partially oxidized material. (For elements which have energy levels with small enough halfwidth and large enough ionization cross section.) Experiments to probe differences in the vaience 109
CFIEMICAL PHYSICS LETlFRS
Volume 18, number 1
r?XKIlNG EPIERSY
band structur6
of these materials
are
in
f I ] M. Robin and P, Day, Advan; inorg. C&m. R3~och~m. fD c.15’67) 247. 121 H.P. Geserich, H.D. Hausen, K. Krogmmn and P. Stmlpfl, Phys. stat. Sol. 9a (1972) 187: P. Wiirfel, H.D: Hauscn, K. Krogman anal P. Stamps!,
tie pf&um.
,yo ;,_
:’ 5 ‘,‘:.. .-. ,’ ,,_, .” ..,, . . : ,., 1..
.; :
‘;., .,
.j;;.
,,: :
@.‘)
References
preparation.
Wz thank Matthey-3~~op for a generous loan of This work was partiatly supported by the Advanced Research Projects Agency through the Nqrthwestern’University Materials Research Center and by NSF through a grant for the purchase of the; spectiometer.; ,;
I January 1973
Fhys. stat. Sol. 1aa (1972) 537.
: .’
.’
:..
‘. ..’
“ ‘:,
,,,
; _,
;
,:j
: ...” ?. :, ,:: ;j., ,..:;
Volume 18, number 1
CHEMICAL PHYSICS LETTERS
BINDING
ENERGY
I Ianuary
1973
(eV)
Fig. 2. Pt 4f region of XPES spectrum of [ Pt(C~HSNH7)4]C14. [Pt(C21~5NH~)4Clz ] .4H20(4), Wolfram’s red salt. The time . averaged, smoothed spectrum was deconvoluted on n DuPont 310 curve resolver.
131 K. Krogmann and H.D. Hausen, Z. Anorg. Allg. Chem. 358 (1968) 67. (41 Joint Committee on Powder Diffraction Standards, Powder Diffraction file I-0084. [5] S. Jerome Lerutte, Structure Bonding 10 (1972) 153. !6] h1.L. Sloreau-Colin, Structure Bonding 10 (1972) 167. [7] Gmelins Handbuch der anorganischcn Chemie, Vol. 68, part C (Vcrlag Chcmie, Weilheim, 1940) p. 206. (81 B.M. Craven and D. Hall, Acta Cryst 14 (1961) 475. [9] C.I(. Jdrgensen, Chimia 2.5 (1971) 221.
IlO] D. Cahen, Research Report. ARPA contrsct number N@0014-67-A-0112-0056, Stanford University (i972). IllI W.E. hfoddcman, J.R. Blackburn, G. Kumar, K.A. Alorgan, R.G. Albridge and hl.lf. Jones, Inorg. Chem. ii (1972) 1715. II21 V.I. Nefcdov, MA. Porni-Koshits, 1-A. Zakharova, LS. Kolomnikov and N.N. Kuzmina, Izv. Akad. Nauk SSSR Ser. Fiz. 36 (1972) 381. H.H. Rupp, Z. Naturforsch. 263 (1971) 1937. [14] W. Dianis and J.L. Lester, Anal. Chcm., submitted for publication.
Volume 18, iiiimber 1
1 January 1973
MIXED AND PARTIAL OXIDATION STATES. PHOTOELECT’RON SPE@TROSCOPIC EVIDENCE David CAHEN and Joseph E. LESTER Department
of Chemistry and Materials Research Center, Northwestem
University,
Evanston, Illirtois GOZOI. USA
Received? September 1972
X-ray photoelectron spectra of the iingle valence platinum complexes Ii2 [Pt(CN)4].3HzO(l), K2 [Pt(CN)J] Cioaj-n&0(2) and Kz [Pt(CN)G] Ci2.3H20(3) and the mixed valence compollnd [PtlI(CZHsNHz)4] C14-[PtfV(CzH~NHz)4C12] -4FJz0(4) have been measured. It is found that one can distinguish between mixed and single valence compounds by electron spectroscopy. The Pt spectrum of (4) is a superposition of a PtlI and PtlV spectrum. The chemical shift between (1) and (3) is normal, however (2) shows an anomalouslow binding energy for the Pt 4f electrons. The importance of using reliable reference peaks for obtaining absoiute binding energies is emphasized.
cleariy
Platinum is one of several transition metals that can form compounds with the metal eitnzr in different oxidation states or in a formal partial oxidation state [ I] . Direct evidence for the occurrence of mixed oxidation states (locaiized electrom, class 2 [ 1j ) or partial oxidation states (delocalized eiectrons, class
I!1 b [I’j) has been obtained only from complete structure determination or Mtissbauer spectroscopy. The latter technique is impractical in the case of Pt and the first one is time consuming, expensive and requires singIe.crystds. Here we present X-ray pIlotoeIectron spectroscopic data that show the potential of this relatively simple and fast technique applied tc this problem. We have determined the Pt 4f electron binding energy (B.E.) in the compounds KzfPt(CN)41*3H2~(1); K2 [Pt(CN)41C10.3 .nH20 (2
longer Pt-Pt distance. The structure of 3 is unknown but no anomalies are observed in its physical and chemical pro;i?rties [7]. X-ray diffraction proved 4 to be a mixed valence compound [S] . All these compounds are hydrates and therefore were studied at --I OO°C [9]. No decomposition was observed for 1 or 2; 2 has been shown to retain its basic structure upon dehydration [lo]. Prolonged exposure of 4 to vacuum resulted in some decomposition, but the process was slow enough to enable us to obtain good spectra. 4 dehydrates rapidly and the resulting yellow material has the same X-ray powder diffraction pattern as the hydrate except for a much increased background, pointing to a decrease in crystallinity. No shifts in either diffraction or ESCA peak positions were observed, only changes in intensity. TabIe I summarizes the experimental binding energies of the Pt 4f electrons. In the series of cyanide complexes (1, 2, and 3) a shift of more than 2 e?f is observed between Pt” and PtIV(3). This compares well with other reported values [I 11. The small shift in K 2P,,2 B.E. indicates tid difference
between
only a small Madelung
poten-
1 and 3. A Madelung correc-
tion, though important, would no\ influence the conclusion, t%at the Ptn and Ptw 4f binding energies are well separated.
Volume 18, number 1
1 Januarg’ 1973
CHEMICAL PHYSICS LETTERS Table 1
Compound
a)
pt foil b) K~[FtW041~3H~O(l) Kz[Pt(CWl& 3.rrHzW) Kz [PI(CNL1C12:3H20(3)
Pt 4fs,2
.
[P!Jl(C2HsNH2)4]C14~ [PtTV(C,HsNHz)4C12].4Ht0 “) ----_I_---___
fwhm
Pt 4fT2
fwhm
K 2P3/2
c Is 284.8(l)
74.5(l) c)
1.75
71.2(l)
1.65
77.6(l)
1.7
74.2(l)
1.85
293.3(2)
284.8(l)
76.3(l)
2.1
73.0(l)
2.0
292.3(2)
285.00)
79.90)
1.75
76.6(l)
1.9
292.9(2)
284.W)
::“,
74.1(l) 76.4(!)
1.6 1.75
:;.::::
286.W) -
a) Compounds 1, 2 and 3 were kindly supplied by Dr. Doris Lin from the laboratories of Professors Collman and Little at Stanford University and compound 4 by Jayarxnan Rajaram from our laboratories. b) From refs. [II, 121. c) All binding energies (in eV) are with respect to the 4f peaks of gold deposited in situ on the cooled sample (-100°C). The Au 4fs12 and 4f,,2 B.E.‘s were assigned to be 87.7 and 84.0 eV based on their displacement from the Pt metal 4f p&&s [IL, L2I Spectra were taken on a computer controlled AEI ES1008 spectrometer using Al Ka! X-radiation. Tic samples were sprczd on lead plates.
Wolfram’s red salt (4) showsa triplet spectrum (fig. 2) that can be resolved into two doublets. These doublets are clearly separated and compare very well with Ptrl and Ptl” B.E.‘s. and although strictly 4 can only be compared with itself (hecause of its special structure) the logical explanation for the two doublets is to assign them to PtIr and Ptrv. It is not clear why the peak ratio should be 2: 1 (Ptrl:Ptrv); however if the spectrum is taken immediately after sample insertion, the ratio is closer to 1: 1 and then rapidly goes to 2: 1. This suggests initial partial surface decomposition until a steady state is reached. The spectrum of 2 (fig. 1) does not fit in with those of 1 and 3, the corresponding Pt” and Pt” salts. Although its structure is similar to that of 1, the K 2P3,2 B.E. in 2 is lower by about 1 eV. This shift is larger than any shift observed between potassium platinates and platinites by Moddeman et al. [ 11J or Nefedov et al. [ 121. A shift of this order of magnitude in the K peak position could be the result of a change in the K site potential on introduction of Ci- ions into the lattice. It has been observed that the C--T\1 bond length in 2 is longer [3] than in KNa[Pt(CN)4j .3H,O [G] . We feel that this bond lengthening is due to a transfer of electron density from the cyanide to the platinum, which is consistent with our B.E. observations. As the C 1s peak contains contributions from impurities as well as from the cyanide, we could not detect any change in the peak that could be ascribed to charge
transfer. The signal to noise ratio for the N Is peak was not high enough to obtain the peak positions to the accuracy required to observe the expected shift. An alternative explanation for the shift of the Pt 4f peak is suggested by the NMR experiments of Rupp [13]_ He found a lgsPt Knight shift in K, [Pt(CW, I Br0.3 .jzHTO, _ while no sub shift was observed in K, [Pt(CN),] *3H,O. This shift implies delocalized conduction electron density at the nucleus. One could then postulate an additional screening of the 4f electrons and thus a lowering of their B.E.‘s. One should also note that the fwhm of the Pt peak in 2 is larger than in either 1 or 3. This is expected 3s a result of the random introduction of Cl- ions on 64% of the available sites [3]. All B.E.‘s have been assigned, using the Au 4f peaks from vacuum deposited Au. If the Au 4f peaks from a gold sample base were used as the B.E. reference, shifts up to 1.1 eV in the peak positions were obtained. These shifts show the importance of accurate B.E. reference peaks when measuring absolute binding energies. Thus we strongly urge the use of v3cu~1m deposited gold as a standard 114) . Regardless of tie problem of assigning absolute peak positions, we feel that these data iliustrate that one can use ES&L to distinguish easily between a mixed valence compound and a single valence partially oxidized material. (For elements which have energy levels with small enough halfwidth and large enough ionization cross section.) Experiments to probe differences in the vaience 109
CFIEMICAL PHYSICS LETlFRS
Volume 18, number 1
r?XKIlNG EPIERSY
band structur6
of these materials
are
in
f I ] M. Robin and P, Day, Advan; inorg. C&m. R3~och~m. fD c.15’67) 247. 121 H.P. Geserich, H.D. Hausen, K. Krogmmn and P. Stmlpfl, Phys. stat. Sol. 9a (1972) 187: P. Wiirfel, H.D: Hauscn, K. Krogman anal P. Stamps!,
tie pf&um.
,yo ;,_
:’ 5 ‘,‘:.. .-. ,’ ,,_, .” ..,, . . : ,., 1..
.; :
‘;., .,
.j;;.
,,: :
@.‘)
References
preparation.
Wz thank Matthey-3~~op for a generous loan of This work was partiatly supported by the Advanced Research Projects Agency through the Nqrthwestern’University Materials Research Center and by NSF through a grant for the purchase of the; spectiometer.; ,;
I January 1973
Fhys. stat. Sol. 1aa (1972) 537.
: .’
.’
:..
‘. ..’
“ ‘:,
,,,
; _,
;
,:j
: ...” ?. :, ,:: ;j., ,..:;
Volume 18, number 1
CHEMICAL PHYSICS LETTERS
BINDING
ENERGY
I Ianuary
1973
(eV)
Fig. 2. Pt 4f region of XPES spectrum of [ Pt(C~HSNH7)4]C14. [Pt(C21~5NH~)4Clz ] .4H20(4), Wolfram’s red salt. The time . averaged, smoothed spectrum was deconvoluted on n DuPont 310 curve resolver.
131 K. Krogmann and H.D. Hausen, Z. Anorg. Allg. Chem. 358 (1968) 67. (41 Joint Committee on Powder Diffraction Standards, Powder Diffraction file I-0084. [5] S. Jerome Lerutte, Structure Bonding 10 (1972) 153. !6] h1.L. Sloreau-Colin, Structure Bonding 10 (1972) 167. [7] Gmelins Handbuch der anorganischcn Chemie, Vol. 68, part C (Vcrlag Chcmie, Weilheim, 1940) p. 206. (81 B.M. Craven and D. Hall, Acta Cryst 14 (1961) 475. [9] C.I(. Jdrgensen, Chimia 2.5 (1971) 221.
IlO] D. Cahen, Research Report. ARPA contrsct number N@0014-67-A-0112-0056, Stanford University (i972). IllI W.E. hfoddcman, J.R. Blackburn, G. Kumar, K.A. Alorgan, R.G. Albridge and hl.lf. Jones, Inorg. Chem. ii (1972) 1715. II21 V.I. Nefcdov, MA. Porni-Koshits, 1-A. Zakharova, LS. Kolomnikov and N.N. Kuzmina, Izv. Akad. Nauk SSSR Ser. Fiz. 36 (1972) 381. H.H. Rupp, Z. Naturforsch. 263 (1971) 1937. [14] W. Dianis and J.L. Lester, Anal. Chcm., submitted for publication.