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The x-ray photoelectron spectra of inorganic molecules—VII
J. inorg, nucl. Chem., 1973,Vol. 35, pp. 3541-3545. PergamonPress. Printed in Great Britain.
THE X-RAY PHOTOELECTRON SPECTRA OF INORGANIC MOLECULES--VII* MANGANESE COMPLEXES OF NITROGEN AND OXYGEN DONOR MOLECULES J. SHEETS,t D. G. TISLEY and R. A. WALTON:~ Department of Chemistry, Purdue University, Lafayette, Indiana 47907 (First received 2 November 1972; in revised form 28 November 1972) Abstract--The manganese 2p core electron binding energies of several complexes of manganese(II) and (III) have been recorded. These data, which are referenced to a carbon ls binding energy for graphite of 284.0 eV, show little dependence upon the formal oxidation state of the central metal atom. However, for the pair Mn(pic)2,2H20 and Mn(pic)3,H20 ( p i c H = picolinic acid), the binding energy difference fi(Mn 2p3/2,01s) is greater for the manganese(Ill) complex, consistent with its higher oxidation state.
INTRODUCTION
DURING a study of the pyridine-2-carboxylic acid complexes of manganese[l], we became interested in investigating the manganese 2p core binding energies of a series of coordination complexes of manganese(II) and (III) in order to ascertain whether we could use the technique of X-ray photoelectron spectroscopy (ESCA)[2] to provide unambiguous structural information on these species. The results of these studies are now reported and complement our previous work on the X-ray photoelectron spectra of compounds of the heavier transition elements rhenium[3-5], rhodium[6] and silver [7]. EXPERIMENTAL The manganese complexes MnC12,2py, MnCI2,2Quin, Mn(NCS)2,4py, Mn(pic)2,2H20, Mn(pic)3 ,H20, Mn(acac)a ,2H20, and Mn(acac)3 were prepared by standard literature procedures. A sample of the manganese(II) complex of quinolinic acid (pyridine-2,3-dicarboxylic acid, abbreviated quinH2), Mn(quinH) 2 , *Part VI of the series: D. G. Tisley and R. A. Walton, J. molec. Struct. In press. tUndergraduate research participant 1971-72. ~Address correspondence to this author. 1. D. L. Hoof, D. G. Tisley and R.A. Walton, Inorg. nucl. Chem. Lett. 9, 571 (1973). 2. K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J, Hedman, G. Johansson, T. Bergmark, S. W. Karlsson, J. Lindgren and B. Lindberg, ESCA: Atomic, Molecular and Solid-State Structure Studied by Means of Electron Spectroscopy. Almquist and Wiksells, Uppsala (1967). 3. D. G. Tisley and R. A. Walton, J. inorg, nucl. Chem. 35, 1905 (1973). 4. D. G. Tisley and R, A. Walton, J. chem. Soc., Dalton, In press. 5. D. G. Tisley and R. A. Walton, To be published. 6. A. D. Hamer, D. G. Tisley and R. A. Walton, J. chem. Soc., Dalton 116 (1973). 7. D. P. Murtha and R. A. Walton, Inorg. Chem. 12, 368 (1973). 3541
J.I.N.C. V~,l ;5 No. 10- E
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J. SHEETS, D. G. TISLEY and R. A. WALTON
2H20, was kindly provided by D. L. Hoof. Its purity was checked by elemental analysis (Found: C, 39-83; H, 2"95; N, 6'51. Calc. for Mn(CTH4NO4)2,2H20:C, 39.70; H, 2-84; N, 6-61). The X-rayphotoelectron spectra wererecorded usinga Hewlett-Packard Model 5950AESCA spectrometer. Monochromatic aluminum K~1.2radiation (1486'6eV) was used as the X-ray excitation source and the powdered sampleswere dispersed on a gold-plated copper surface. Sampleswere diluted with graphite in the usual way[4,7], to eliminate or reduce to a minimum troublesome charging effects. Core binding energies so measured, were referenced to a carbon ls binding energy for graphite of 284.0 eV[7]. RESULTS Manganese 2pl/2 and 2p3/2 binding energies are given in Table 1 and in all instances refer to the spectra of samples diluted with graphite in which the binding energies are referenced to a value of 284.0 eV for the C ls binding energy of graphite. The energies are quoted with a precision of ___0.2 eV, and the most intense 2p spinorbit component (2p3/2) has a typical F.W.H.M. value of 3.0 to 3.5 eV for these complexes. In contrast, the carbon ls line of graphite had a F.W.H.M. value of 1-0 + 0-1 eV under our experimental conditions, during the period when these manganese complexes were investigated. Table 1. Manganese 2p binding energies for complexesof manganese(II)and (liD*
Complext MnCl2,2py MnC12 ,2Quin Mn(NCS)2,4py Mn(quinH)2,2H20
2pl/2
2p3:2
653.3 653.7 653.1 653.4
641.7 641.5 641.1 641.5
Complex'[ Mn(pic)2,2H20 Mn(pic)3,H20 Mn(acac)2,2H20 Mn{acac)3
2Pl/2
2p3/2
653.5 641.8 653-3 642-0 652.7 641-3 653.5 641.5
* Binding energies referenced to a value of 284.0eV for the carbon ls binding energy of graphite. t Ligand abbreviations are as follows: Quin = quinoline; quinH2 = quinolinic acid (pyridine-2,3-dicarboxylic acid); picH= picolinic acid (pyridine-2-carboxylic acid). The binding energies given in Table 1 compare favorably with the 2p3/2 values recently reported[8] for several binary oxide, sulfide, nitride and halide phases of manganese. In this latter study[8], the energy calibration was based upon an assumed carbon ls value of 285.0 eV for a hydrocarbon contaminant always present on the samples. If allowance is made for the difference of 1 eV between the calibrant used in the present work and that chosen by Carver et a/.[8], then the literature value[8], for the 2p3/2 level of MnCI2 (642.1 __+ 0.3 eV) and that of "-'641.5 eV for MnC12,2py and MnCl2,2Quin, are in satisfactory agreement and support our use of a graphite standard. Our decision to also use graphite as a diluent to eliminate charging effects on the sample surfaces was based upon our previous experience with certain rhenium[3-5], rhodium[6] and silver[7] complexes. This is further demonstrated in the present work by measurements on undiluted samples of the acetylacetonate and p y r i d i n e - 2 carboxylate complexes, Mn(acac)2,2H20, Mn(acac) 3, Mn(pic)2,2H20 and Mn(pic)a, H 2 0 . For all four complexes the measured binding energies were lower than those of the diluted samples by up to 7 eV: 8. J. c. Carver, G. K. Schweitzer and T. A. Carlson, J. chem. Phys. 57, 973 (1972).
The X-ray photoelectron spectra of inorganic molecules Mn(acacJ2,2H20 Mn(acac)3 Mn(pic)2,2H20 Mn(pic)3 ,H20
3543
2pl/2,645-7 ; 2p3/2, 633.7 2pt/2,651.0; 2p3/2,639-1 2pl/2,650.0; 2p3/2,638.1 2pl/2,652.2:2p3/2,640.9
With the exception of Mn(acac)2,2H20, this shift was not accompanied by any appreciable broadening of the manganese 2p peaks (F.W.H.M. ~ 3-5 eV), whereas with the former complex, the peaks became very broad (F.W.H.M. "-~ 5 eV) and illdefined. The latter effects are presumably due to different degrees of charging throughout the surface of the sample of Mn(acac)2,2H20. It is clear that the elimination of charging effects, or at least reducing them to a minimum, is essential for a meaningful comparison of binding energies in this type of system, since different degrees of charging within a series of compounds can lead to differences in peak width and peak shape, which make such comparisons questionable since it becomes impossible to clearly define the true binding energy. As expected, the oxygen ls binding energies of these same four complexes parallel the manganese 2p binding energies with values of 528.4, 527.8 and 529.5 eV for Mn(acac)3, Mn(pic)2,2H20 and Mn(pic)3 ,H20, respectively. A value for Mn(acac)2, 2H20 is not reported since the oxygen ls binding energy was observed to decrease with increase in time of irradiation, a clear indication of progressive charging. The oxygen ls binding energies of the graphite diluted samples were not recorded due to the presence of "adsorbed" oxygen on the graphite, whose oxygen ls binding energy overlapped that due to the complexes. Our observation of realistic, reproducible binding energies for these same complexes when diluted with graphite, is support for our contention that generally it is a satisfactory medium for reducing charging problems. DISCUSSION The most striking feature of the data in Table 1 is the absence of a clear correlation between the oxidation states of the central metal atoms in the complexes and the manganese 2p binding energies. In fact, these binding energies span the rather narrow ranges 653.7-652.7 and 642.0-641.1 eV, so that it is not possible to unambiguously distinguish manganese(II) and manganese(III) species of this type. These results are clearly consistent with those of Carver et al.[8] who observed relatively little variation in the manganese 2p3/2 binding energies for species as diverse as KMnO 4 (642.6 eV), MnC12 (642"1 eV) and KaMn(CN) 6 (641.3 eV). The comparison of binding energies for complex molecules of the type discussed in the present report is most meaningful when the molecules involve a similar environment about the central metal atom. In the present work it is obvious that we are unable to compare compounds of manganese(II) and (III) complexes in identical environments, so that the pairs Mn(pic)2,2H20/Mn(pic) 3 , n 2 0 and Mn(acac)2,2H20 / Mn(acac)3 constitute the best approximation to the ideal choice. At first sight, an obvious e~planation of the similaritY of these metal core binding energies, is that an increase in covalency in the metal-ligand bonds in the manganese(III) complexes results in an enhanced build-up of elecfronic charge at the metal center which counteracts the formal charge difference between Mn +a and Mn + 2 and therefore leads to
3544
J, SHEETS, D. G. TISLEY and R. A. WALTON
similar effective nuclear charges and hence closely similar binding energy values. The result is certainly intuitively reasonable, and it is interesting to see that within the pairs MnF2/MnF 3 and Mn(CN)64-/Mn(CN)6a-, the manganese 2p3/2 binding energies are likewise very similarE8]. The absence of shifts between manganese(II) and (III) seems to be fairly characteristic of these species. These results are rather disappointing and once again emphasize the caution needed in correlating chemical shift data with oxidation state changes for the transition metal ions. It has been our previous observation that the direct comparison of metal core binding energies does not always reflect differences in metal oxidation states. However, if these energies are first referenced to some common internal standard, such as the oxygen ls or chlorine 2p binding energies of an appropriate ligand molecule, then themetal binding energies may nowshow the expected correlation with oxidation state. We have successfully used this approach to interpret the spectra of several systems containing rhenium-oxygen and rhenium-halogen bonds [4, 5]. Accordingly, if we consider the binding energy differences 6(Mn 2p3/2 , 01s) for the pyridine-2carboxylate complexes Mn(pic)E,2H20 and Mn(pic)a,H20 , using the data obtained on the undiluted samples (see Results Section), then we obtain values of 110.3 and 111.4 eV, respectively, for this difference. In other words, if we first reference to a standard oxygen ls value, then the "corrected" 2p bi~ading energies are greater for the manganese(III) complex. This result is encouraging and it further suggests that without an internal reference, differences in solid-state effects may often render comparisons of metal core binding energies unrewarding. Also, provided charging is not such a serious problem, as it is in Mn(acac) 2,2H20, that peaks become broadened and asymmetric, binding energies need only be compared for undiluted samples since it is the binding energy differences 6 which then become useful. At this point it is worth comparing these manganese 2p binding energy measurements (this work and E8]) with the X-ray emission spectra of several manganese compounds reported by Koster and Mendel[9]. From measurements of the Kfll.3 emission spectra (i.e. 3p ~ ls) of MnO, MnO2, KMnO 4, MnS, K3MnF6, K2MnF 6 and K3Mn(CN)6, the energies of the Mn ls shells were determined relative to the Fermi Level. For the oxide and fluoride phases, these were found to follow a smooth trend with change in oxidation state: MnO2 (6544 eV) < MnO 2 (6542 eV) < MnO (6540 eV) and MnF 2- (6545 eV) < MnF6a- (6544 eV), Since shifts in manganese 2p binding energies should mirror those of the manganese ls, it is possible that the absence of a clear correlation in manganese core binding energies, as measured by X-ray photoelectron spectroscopy, can be overcome, as we have suggested above for Mn(pic)2,2H20 and Mn(pic)3 ,H20, by the use of a convenient internal standard. Complicating any further detailed interpretations of these binding energies are the rather large peak widths which were observed and the asymmetry (on the high binding energy side) of the peaks. The occurrence of multiplet splittings due to the coupling of the hole in the metal atom 2p core with the unfilled metal 3d valence shell[10-12], most probably accounts for the asymmetry and thereby contributes to 9. A. S. Koster and H. Mendel, J. Phys. Chem. Solids 31, 2523 (1970). 10. R. L. Cohen, G. K. Wertheim, A. Rosencwaig and H. J. Guggenheim, Phys. Rev. BS, 1037 (1972). I1. C. S. Fadley and D. A. Shirley, Phys. Rev. A2, 1109(1970). 12. K. S. Kim and R. E. Davis, J. Electron Spectrosc. 1,251 (1972/73).
The X-ray photoelectron spectra of inorganic molecules
3545
the broad nature of the binding energies. Since the widths of the photoelectron lines associated with vacancies in core levels also depend on the chemical environment [ 13], which influences the vacancy lifetime through the relaxation process, it is clear that a simple interpretation of these features is not feasible at the present time. Acknowledgements--This work was supported by the National Science Foundation (Grant No. GP-19422 and MRL Program GH-33574). We also thank the National Science Foundation for providing funds for the purchase of the ESCA spectrometer. We are grateful to Dr. Jon W. Amy and Mr. William E. Baitinger for their invaluable assistance in solving instrumental problems. 13. R. M. Friedman, J. Hudis and M. L. Perlman, Phys. Rev. Lett. 29, 692 (1972).
THE X-RAY PHOTOELECTRON SPECTRA OF INORGANIC MOLECULES--VII* MANGANESE COMPLEXES OF NITROGEN AND OXYGEN DONOR MOLECULES J. SHEETS,t D. G. TISLEY and R. A. WALTON:~ Department of Chemistry, Purdue University, Lafayette, Indiana 47907 (First received 2 November 1972; in revised form 28 November 1972) Abstract--The manganese 2p core electron binding energies of several complexes of manganese(II) and (III) have been recorded. These data, which are referenced to a carbon ls binding energy for graphite of 284.0 eV, show little dependence upon the formal oxidation state of the central metal atom. However, for the pair Mn(pic)2,2H20 and Mn(pic)3,H20 ( p i c H = picolinic acid), the binding energy difference fi(Mn 2p3/2,01s) is greater for the manganese(Ill) complex, consistent with its higher oxidation state.
INTRODUCTION
DURING a study of the pyridine-2-carboxylic acid complexes of manganese[l], we became interested in investigating the manganese 2p core binding energies of a series of coordination complexes of manganese(II) and (III) in order to ascertain whether we could use the technique of X-ray photoelectron spectroscopy (ESCA)[2] to provide unambiguous structural information on these species. The results of these studies are now reported and complement our previous work on the X-ray photoelectron spectra of compounds of the heavier transition elements rhenium[3-5], rhodium[6] and silver [7]. EXPERIMENTAL The manganese complexes MnC12,2py, MnCI2,2Quin, Mn(NCS)2,4py, Mn(pic)2,2H20, Mn(pic)3 ,H20, Mn(acac)a ,2H20, and Mn(acac)3 were prepared by standard literature procedures. A sample of the manganese(II) complex of quinolinic acid (pyridine-2,3-dicarboxylic acid, abbreviated quinH2), Mn(quinH) 2 , *Part VI of the series: D. G. Tisley and R. A. Walton, J. molec. Struct. In press. tUndergraduate research participant 1971-72. ~Address correspondence to this author. 1. D. L. Hoof, D. G. Tisley and R.A. Walton, Inorg. nucl. Chem. Lett. 9, 571 (1973). 2. K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J, Hedman, G. Johansson, T. Bergmark, S. W. Karlsson, J. Lindgren and B. Lindberg, ESCA: Atomic, Molecular and Solid-State Structure Studied by Means of Electron Spectroscopy. Almquist and Wiksells, Uppsala (1967). 3. D. G. Tisley and R. A. Walton, J. inorg, nucl. Chem. 35, 1905 (1973). 4. D. G. Tisley and R, A. Walton, J. chem. Soc., Dalton, In press. 5. D. G. Tisley and R. A. Walton, To be published. 6. A. D. Hamer, D. G. Tisley and R. A. Walton, J. chem. Soc., Dalton 116 (1973). 7. D. P. Murtha and R. A. Walton, Inorg. Chem. 12, 368 (1973). 3541
J.I.N.C. V~,l ;5 No. 10- E
3542
J. SHEETS, D. G. TISLEY and R. A. WALTON
2H20, was kindly provided by D. L. Hoof. Its purity was checked by elemental analysis (Found: C, 39-83; H, 2"95; N, 6'51. Calc. for Mn(CTH4NO4)2,2H20:C, 39.70; H, 2-84; N, 6-61). The X-rayphotoelectron spectra wererecorded usinga Hewlett-Packard Model 5950AESCA spectrometer. Monochromatic aluminum K~1.2radiation (1486'6eV) was used as the X-ray excitation source and the powdered sampleswere dispersed on a gold-plated copper surface. Sampleswere diluted with graphite in the usual way[4,7], to eliminate or reduce to a minimum troublesome charging effects. Core binding energies so measured, were referenced to a carbon ls binding energy for graphite of 284.0 eV[7]. RESULTS Manganese 2pl/2 and 2p3/2 binding energies are given in Table 1 and in all instances refer to the spectra of samples diluted with graphite in which the binding energies are referenced to a value of 284.0 eV for the C ls binding energy of graphite. The energies are quoted with a precision of ___0.2 eV, and the most intense 2p spinorbit component (2p3/2) has a typical F.W.H.M. value of 3.0 to 3.5 eV for these complexes. In contrast, the carbon ls line of graphite had a F.W.H.M. value of 1-0 + 0-1 eV under our experimental conditions, during the period when these manganese complexes were investigated. Table 1. Manganese 2p binding energies for complexesof manganese(II)and (liD*
Complext MnCl2,2py MnC12 ,2Quin Mn(NCS)2,4py Mn(quinH)2,2H20
2pl/2
2p3:2
653.3 653.7 653.1 653.4
641.7 641.5 641.1 641.5
Complex'[ Mn(pic)2,2H20 Mn(pic)3,H20 Mn(acac)2,2H20 Mn{acac)3
2Pl/2
2p3/2
653.5 641.8 653-3 642-0 652.7 641-3 653.5 641.5
* Binding energies referenced to a value of 284.0eV for the carbon ls binding energy of graphite. t Ligand abbreviations are as follows: Quin = quinoline; quinH2 = quinolinic acid (pyridine-2,3-dicarboxylic acid); picH= picolinic acid (pyridine-2-carboxylic acid). The binding energies given in Table 1 compare favorably with the 2p3/2 values recently reported[8] for several binary oxide, sulfide, nitride and halide phases of manganese. In this latter study[8], the energy calibration was based upon an assumed carbon ls value of 285.0 eV for a hydrocarbon contaminant always present on the samples. If allowance is made for the difference of 1 eV between the calibrant used in the present work and that chosen by Carver et a/.[8], then the literature value[8], for the 2p3/2 level of MnCI2 (642.1 __+ 0.3 eV) and that of "-'641.5 eV for MnC12,2py and MnCl2,2Quin, are in satisfactory agreement and support our use of a graphite standard. Our decision to also use graphite as a diluent to eliminate charging effects on the sample surfaces was based upon our previous experience with certain rhenium[3-5], rhodium[6] and silver[7] complexes. This is further demonstrated in the present work by measurements on undiluted samples of the acetylacetonate and p y r i d i n e - 2 carboxylate complexes, Mn(acac)2,2H20, Mn(acac) 3, Mn(pic)2,2H20 and Mn(pic)a, H 2 0 . For all four complexes the measured binding energies were lower than those of the diluted samples by up to 7 eV: 8. J. c. Carver, G. K. Schweitzer and T. A. Carlson, J. chem. Phys. 57, 973 (1972).
The X-ray photoelectron spectra of inorganic molecules Mn(acacJ2,2H20 Mn(acac)3 Mn(pic)2,2H20 Mn(pic)3 ,H20
3543
2pl/2,645-7 ; 2p3/2, 633.7 2pt/2,651.0; 2p3/2,639-1 2pl/2,650.0; 2p3/2,638.1 2pl/2,652.2:2p3/2,640.9
With the exception of Mn(acac)2,2H20, this shift was not accompanied by any appreciable broadening of the manganese 2p peaks (F.W.H.M. ~ 3-5 eV), whereas with the former complex, the peaks became very broad (F.W.H.M. "-~ 5 eV) and illdefined. The latter effects are presumably due to different degrees of charging throughout the surface of the sample of Mn(acac)2,2H20. It is clear that the elimination of charging effects, or at least reducing them to a minimum, is essential for a meaningful comparison of binding energies in this type of system, since different degrees of charging within a series of compounds can lead to differences in peak width and peak shape, which make such comparisons questionable since it becomes impossible to clearly define the true binding energy. As expected, the oxygen ls binding energies of these same four complexes parallel the manganese 2p binding energies with values of 528.4, 527.8 and 529.5 eV for Mn(acac)3, Mn(pic)2,2H20 and Mn(pic)3 ,H20, respectively. A value for Mn(acac)2, 2H20 is not reported since the oxygen ls binding energy was observed to decrease with increase in time of irradiation, a clear indication of progressive charging. The oxygen ls binding energies of the graphite diluted samples were not recorded due to the presence of "adsorbed" oxygen on the graphite, whose oxygen ls binding energy overlapped that due to the complexes. Our observation of realistic, reproducible binding energies for these same complexes when diluted with graphite, is support for our contention that generally it is a satisfactory medium for reducing charging problems. DISCUSSION The most striking feature of the data in Table 1 is the absence of a clear correlation between the oxidation states of the central metal atoms in the complexes and the manganese 2p binding energies. In fact, these binding energies span the rather narrow ranges 653.7-652.7 and 642.0-641.1 eV, so that it is not possible to unambiguously distinguish manganese(II) and manganese(III) species of this type. These results are clearly consistent with those of Carver et al.[8] who observed relatively little variation in the manganese 2p3/2 binding energies for species as diverse as KMnO 4 (642.6 eV), MnC12 (642"1 eV) and KaMn(CN) 6 (641.3 eV). The comparison of binding energies for complex molecules of the type discussed in the present report is most meaningful when the molecules involve a similar environment about the central metal atom. In the present work it is obvious that we are unable to compare compounds of manganese(II) and (III) complexes in identical environments, so that the pairs Mn(pic)2,2H20/Mn(pic) 3 , n 2 0 and Mn(acac)2,2H20 / Mn(acac)3 constitute the best approximation to the ideal choice. At first sight, an obvious e~planation of the similaritY of these metal core binding energies, is that an increase in covalency in the metal-ligand bonds in the manganese(III) complexes results in an enhanced build-up of elecfronic charge at the metal center which counteracts the formal charge difference between Mn +a and Mn + 2 and therefore leads to
3544
J, SHEETS, D. G. TISLEY and R. A. WALTON
similar effective nuclear charges and hence closely similar binding energy values. The result is certainly intuitively reasonable, and it is interesting to see that within the pairs MnF2/MnF 3 and Mn(CN)64-/Mn(CN)6a-, the manganese 2p3/2 binding energies are likewise very similarE8]. The absence of shifts between manganese(II) and (III) seems to be fairly characteristic of these species. These results are rather disappointing and once again emphasize the caution needed in correlating chemical shift data with oxidation state changes for the transition metal ions. It has been our previous observation that the direct comparison of metal core binding energies does not always reflect differences in metal oxidation states. However, if these energies are first referenced to some common internal standard, such as the oxygen ls or chlorine 2p binding energies of an appropriate ligand molecule, then themetal binding energies may nowshow the expected correlation with oxidation state. We have successfully used this approach to interpret the spectra of several systems containing rhenium-oxygen and rhenium-halogen bonds [4, 5]. Accordingly, if we consider the binding energy differences 6(Mn 2p3/2 , 01s) for the pyridine-2carboxylate complexes Mn(pic)E,2H20 and Mn(pic)a,H20 , using the data obtained on the undiluted samples (see Results Section), then we obtain values of 110.3 and 111.4 eV, respectively, for this difference. In other words, if we first reference to a standard oxygen ls value, then the "corrected" 2p bi~ading energies are greater for the manganese(III) complex. This result is encouraging and it further suggests that without an internal reference, differences in solid-state effects may often render comparisons of metal core binding energies unrewarding. Also, provided charging is not such a serious problem, as it is in Mn(acac) 2,2H20, that peaks become broadened and asymmetric, binding energies need only be compared for undiluted samples since it is the binding energy differences 6 which then become useful. At this point it is worth comparing these manganese 2p binding energy measurements (this work and E8]) with the X-ray emission spectra of several manganese compounds reported by Koster and Mendel[9]. From measurements of the Kfll.3 emission spectra (i.e. 3p ~ ls) of MnO, MnO2, KMnO 4, MnS, K3MnF6, K2MnF 6 and K3Mn(CN)6, the energies of the Mn ls shells were determined relative to the Fermi Level. For the oxide and fluoride phases, these were found to follow a smooth trend with change in oxidation state: MnO2 (6544 eV) < MnO 2 (6542 eV) < MnO (6540 eV) and MnF 2- (6545 eV) < MnF6a- (6544 eV), Since shifts in manganese 2p binding energies should mirror those of the manganese ls, it is possible that the absence of a clear correlation in manganese core binding energies, as measured by X-ray photoelectron spectroscopy, can be overcome, as we have suggested above for Mn(pic)2,2H20 and Mn(pic)3 ,H20, by the use of a convenient internal standard. Complicating any further detailed interpretations of these binding energies are the rather large peak widths which were observed and the asymmetry (on the high binding energy side) of the peaks. The occurrence of multiplet splittings due to the coupling of the hole in the metal atom 2p core with the unfilled metal 3d valence shell[10-12], most probably accounts for the asymmetry and thereby contributes to 9. A. S. Koster and H. Mendel, J. Phys. Chem. Solids 31, 2523 (1970). 10. R. L. Cohen, G. K. Wertheim, A. Rosencwaig and H. J. Guggenheim, Phys. Rev. BS, 1037 (1972). I1. C. S. Fadley and D. A. Shirley, Phys. Rev. A2, 1109(1970). 12. K. S. Kim and R. E. Davis, J. Electron Spectrosc. 1,251 (1972/73).
The X-ray photoelectron spectra of inorganic molecules
3545
the broad nature of the binding energies. Since the widths of the photoelectron lines associated with vacancies in core levels also depend on the chemical environment [ 13], which influences the vacancy lifetime through the relaxation process, it is clear that a simple interpretation of these features is not feasible at the present time. Acknowledgements--This work was supported by the National Science Foundation (Grant No. GP-19422 and MRL Program GH-33574). We also thank the National Science Foundation for providing funds for the purchase of the ESCA spectrometer. We are grateful to Dr. Jon W. Amy and Mr. William E. Baitinger for their invaluable assistance in solving instrumental problems. 13. R. M. Friedman, J. Hudis and M. L. Perlman, Phys. Rev. Lett. 29, 692 (1972).