538
J. Phys. Chem. 1980, 8 4 , 538-542
Proton NMR Studies of the Lewis Acid-Base Reactions between Pyridinium Chlorides and the Acids ZnCI, and AICI, C. A. Angell” and J. W. Shuppert Department of Chemisiw, Purdue University, West Lafayette, Indiana 47907 (Received July 5, 1979) Publlcatlon costs assisted by the National Science Foundation
Pyridinium chloride (PyHCl), which is the salt formed from the Lowry-Bronsted acid-base reaction between pyridine and HC1, is utilized as the base in a study of chloride transfer equilibria in the Lewis acid-base systems PyHCl + AlC13, PyHCl + ZnCl,, and cu-MePyHCl+ ZnCl,. Upfield shifts in the PyHCl proton resonance occur in each case as the Lewis acid is added, providing a probe of the electrical field adjacent to transferred chloride ions, and hence a measure of the effectiveness of the transfer process. In the presence of excess A1C13 in the PyHCl + AlC13system, the proton becomes localized on the pyridine nitrogen to the extent that splitting due to interaction with the three spin states of the nitrogen can be observed, as in amines. PyH+ may be considered a “free” cation under these conditions. On the basic side of the equivalence point in this system the complex hydrogen-bonded chlorodipyridinium cation (PyH-.Cl-HPy)+ is stabilized, with an approximate free energy of formation from the isolated ions of 50 kJ mol-’. At the maximum Lewis acidity reached in this study the chemical shifts for the nitrogenic proton, and for two of the three types of ring proton, come into agreement with values calculated for the “free” pyridinium cation. An implication is that in molten PyHCl itself the proton exchanges freely between Py and C1- entities and probably contributes much of the high electrical conductivity of this liquid.
Introduction In a previous paper a proton magnetic resonance (lH NMR) investigation of the system pyridinium chloride (PyHC1) + HC1 was rep0rted.l The results of that study showed that there exists a strong hydrogen bonding interaction between the PyH+ cation and C1- anion in molten PyHCl. Furthermore, when 1 mol of HC1 is added to PyHC1, the compound PyH-C1-HC1 is formed, and ‘H NMR data indicate there is still a high degree of association among the ions present. With increasing HC1 concentration, polymerized or solvated H,C1,+l- anions are believed to form, and the PyHf-C1- interaction loses importance. In the present study the consequences of replacing HCl in the PyHCl + HC1 part of the above system by ZnClz or AlCl, are examined, with the following two objectives: (1)elimination of the association between PyH+ and C1by complexation of the C1- in order that the consequences of that association in the pure molten PyHCl would become more obvious, hence more susceptible to analysis; (2) use of the proton spectra to provide information on the chloride transfer acid-base reactions which dominate the behavior of these systems. The proton shifts will be seen to serve as a probe of the electric field near the chlorides of the reaction complexes ZnClt- and A1C14-, hence as a useful indicator of the effectiveness of the chloride transfer process. Some information on the system PyHCl + ZnC12 is already available from the study of Easteal and Angell.2 These authors determined the phase equilibrium diagram and the electrical conductance, density, and viscosity of melts in this system over the whole composition range. They also measured the glass transition temperatures [T ) in the glass-forming region above 33 mol % ZnC1,. A!though the electrical conductance exhibits general decreases across this system, it is only in the temperature dependence of conductance that one sees any evidence that an acid-base equivalence point has been passed. The information obtained from such studies thus offers little if any insight into the structural and energetic aspects of 0022-3654/80/2084-0538$0 1.OO/O
the basic chloride transfer process. To a large extent the present study provides this information; to a smaller extent it also shows why the conductance measurements are as insensitive to the equivalence point as they are.
Experimental Section AlCl, was prepared from Fischer Certified ACS grade aluminum wire., The aluminum wire was placed in a Vycor vessel through which anhydrous gaseous HCl was passed. The aluminum metal was heated to the melting point, a t which temperature the reaction with the HC1 to form AlC13 proceeds a t a convenient rate. AICIBwas allowed to sublime through a quartz wool plug on the downstream side of the molten aluminum. When the reaction terminated, the HC1 flow was stopped and stopcocks a t each end of the Vycor vessel were closed. Generally the aluminum would not completely react due to the presence of a coating, probably the oxide. The generation of HC1 and preparation of ZnCl,, tetramethylammonium chloride (TMA+Cl-),and PyHCl have been previously described.’i2 As an alternative to PyHC1, the methyl-substituted compound a-picolinium chloride (a-PicHC1) was used in a study with ZnC1,. A sample of sublimed salt was obtained from H ~ d g e . ~ All the anhydrous compounds were handled in a drybox, dried by continual pumping of the enclosed atmosphere through columns of molecular sieve. All melts were individually prepared by mixing weighed amounts of the salts in NMR tubes. TMAfC1- was used as an internal standard5 in these studies. In the previously reported HCl + Py system, evidence is cited to show that TMA’ behaves as an inert reference despite quite severe changes in its local envir0nment.l Its continued inertness in the face of the large changes in anion character produced by chloride complexing in the present work is, however, unproven. Any reference shifts which might occur would, however, be very minor in comparison with the dramatic >NH proton shifts on which we focus most attention. 0 1980 American Chemical Society
PyCl and ZnCI,
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980 539
or AICI, Acid-Base Reactions
-14
I
1)
I
l
I
l
( N-tI'
l
1
I
0
l
t
I
-HCI
PyHCl
I TMA+
/
b)
A a.picHCI -ZnCI,
-I2
PPm
I
C)
-13
-I1
-10 8 6 4 2
d)
-9
(Ci
' 20 '
-8l
1 I
40
'
'
60
I
'
80
'
100
EQUIVALENT '10 LEWIS ACID I II
-12
-10 -8
I
-6
I
I
-4
-2
I
0
Figure 2. Composition (equiv %) dependences of the >NH+ proton resonance at 148 "C for presently reported systems compared with that of the PyHCl HCI system studied earlier (ref 1). The dashed line indicates the value calculated' for the isolated PyH' cation.
+
Ftgure 1. 'H NMR spectra at 148 O C of (a) pure pyridinium chloride (PyHCI), (b) PyHCl t 49 mol % AICI,, (c) PyHCl 70 mol % AICl,,(note the resolution of the 14N..coupled signal), and (d) pure a-picolinium chloride. The inset to (a) shows the TMA' internal reference signal and subsidiary weak resonances due to methyl group scrambling.
+
Proton spectra were recorded with a Varian A60 six-turn probe spectrorneter equipped with a temperature control (range 200 to -100 "C). For sharp lines, the measurement uncertainty is f0.02 ppm. Temperatures were measured by insertion of a copper constantan thermocouple into the probe.
Results In Figure 1 representative spectra of various AlCl, + PyHCl melts are shown, plus the spectrum of pure aPicHC1. As can be seen, the >NH+ signal of the AlCl, + PyHCl melt moves upfield with increasing A1C13 concentration, finally broadening and becoming resolved into a well-defined three-band 14N coupled signal (coupling constant 69 Hz,) upon the formation of the A1C14- complex. The changes occur mainly in the region between 30 and 50 mol % AlC:13 (= equivalent % AlC1,) as depicted in Figure 2. More subt,le changes occur in the shifts of the ring proton signals as this complex is formed (Figure 3). A similar but less abrupt upfield movement is also found for the >NH+ shift in each of the systems PyHCl + ZnC1, and a-PicHC1 + ZnC1, corresponding to formation of the ZnC1:- complex. Results are plotted vs. equivalent percent ZnC1, (Figure 2) to assist comparison of the behavior with that of the AlC13-based system. In the case of the ZnC1, systems, the >.NH+ signal was never observed as a wellresolved 14Ncoupled signal as in the AlC13 system (Figure IC) but, rather, as a single rather broad band. The somewhat distinct ring proton behavior in these latter systems is shown in the Figure 4. (The signals of the two ring protons meta to the nitrogen of a-PicHC1 overlapped and were difficult to distinguish and, hence, they are not shown in Figure 4.) Melts above 70 mol 70 ZnC1, had melting points above the 200 "C limit of the temperature control and spectra were therefore unobtainable. The >NH+ :signals in both pure PyHCl and a-PicHC1 were observed as sharp singlets, -13.45 and -13.35 ppm, respectively. This signal for pure PyHCl had no detectable temperature dependence over the limited range 145-200
0
50
100
Mole % AIC13
Flgure 3. Composition dependences of ring proton resonances in the system PyHCi 4- AiCI, (mol % = equiv %). The Inset is a phase diagram for this system showing compounds at 33.3 and 5 0 % AICI, (taken from ref 4).
"C. a-PicHC1, however, did seem to have a temperature dependence of -0.0016 ppm/ "C (slightly outside error limits) over the temperature range 89-150 "C. After complex formation, melts of 60 mol % ZnC1, with PyHCl or a-PicHC1 gave >NH+ signals with slight temperature dependences of about -0.0015 ppm/"C. By contrast, a 75 mol % ZnC1, with a-PicHC1 melt had a marked temperature dependence of 0.0080 ppm/"C over the range 120-190 "C (well outside the error limits). Ring proton signals showed no temperature dependence in any system.
540
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980
I
I
’
‘ a ’
TABLE I: Theoretical and Observed PyH+ Ring Proton Chemical Shifts ( p p m ) Relative to Corresponding Protons in Pyridine
’
I
Angel1 and Shuppert
CY
theor free cation shifts -1.66 exptl PyH’ shifts -0.83 PyH’ in PyHCl-70 mol % HCl - 0.70 PyH’ in PyHC1-70 mol % ZnCl, -0.73 PyH’in PyHCl-70 mol % AICI, -0.68
ir
Y
-1.92 -1.35 - 1.46 -1.51 -1.70
-2.02 -1.50 - 1.52 -1.72 -1.88
salts were used in the results here reported. The sublimation of the distilled PyHCl product is thought to remove a slight excess of HC1, and thereby decelerate the scrambling mechanism.
a-CH,
-4.50
o 1
I
I
0
I
PyHCl a-picHCI I
50
I
1
1
100
Mole % ZnC12
Figure 4. Composition (mol %) dependences of ring proton resonances for PyHCI ZnCI, and a-PicHCI ZnCI, systems and a-methyl proton resonance for the btter system. No fi ring proton resonance is reported for the latter system (see text).
+
+
Spectra of the AlC13 + PyHCl solutions were all obtained a t the same temperature of 148 “C, and the temperature dependence was not investigated.
Methyl Exchange One of the interesting sidelights of this research is the evidence it provides for methyl exchange about methylammonium ions in melts of the PyHCl + ZnC12 system. The PyHCl giving these results was a distilled but unsublimed product. Two peaks were observed in the region where the reference TMA+ signal was expected, while another faint signal was observed 1 ppm further downfield (see inset to Figure la). The separation between the two peaks varied with composition but above 30 mol % ZnC12 (C1- deficient melts) only one reference signal was observed. The lH NMR signals of various methyl-substituted ammonium ions, (CH3)3NH’, (CH3)2NH2f,and CH3NH3+,in a molten salt mixture of TMA+Cl- and NH4C1have been found to lie within a 0.6-ppm range.6 The two signals found in the TMA+ region did not exceed this separation and are therefore thought to be due to these various methyl-substituted ammonium ions, implying that scrambling is active in these melts at temperatures as low as 200 “C. The faint signal further downfield is probably a product of a similar scrambling leading to the NMePy’ cation, through the acid-catalyzed reaction xPYH++ (CH3)4N+--* x[PYCH~]+ + (CH3)4-xNH,+ According to the study of Newman et a1.7 the NMe+NC signal should lie around 1.1-ppm downfield from TMA’, corresponding to the shift observed for this faint peak. Melts formed from sublimed PyHCl gave only the single TMA+ reference signal hence the sublimed pyridinium
Discussion The results quoted above can be used to make three useful statements about the chemistry of these, and related, systems. First, considering the >”+ proton, we show how the removal of the chemically basic, protonattracting C1- ions by complexation permits the pyridinium cation to display the characteristics expected €or the “free” cation. The same is found for the ring proton shifts. Second, we use the way in which this shift approaches the free ion value with changing composition to deduce that the chlorine bridged [PyH]&l+ cation is an important species stabilized with respect to the isolated ions by some 50 kJ mol-l. Finally, we will relate our observations on the binary systems to some of the more interesting features of PyHCl itself. (1)Chemical Shifts for the “Free” Pyridinium Cation. ( a ) >NH+ Proton. The chemical shift exhibited by the proton reflects the efficiency of shielding of the proton magnetic moment by electrons in its vicinity. In PyHCl itself this is rather poor due to the movement of the proton away from the nitrogen lone pair under the attractive influence of the chloride ion. It can be said that a strong hydrogen bond is formed between PyH+ and Cl-.l The deshielding influence of the chloride ion will be reduced if the chloride ion is itself complexed in some way. To emphasize the systematic manner in which the nitrogenic proton responds to interaction of the chloride ion with Lewis acids of different strengths, we have included in Figure 2 data for the system PyHCl + HC1, described in an earlier paper.l Here HC1 acts as a rather weak Lewis acid, tending to form HC12- by hydrogen bonding. It is clear from Figure 2 that the stronger the Lewis acid the smaller is the residual attraction of the chloride species for the proton, hence the more unperturbed or “free” is the pyridinium cation. An immediate consequence is the resolution of the >NH+ proton signal into a triplet (due to interaction with the three spin states of the nitrogen) characteristic of stable (nonexchanging) >NH configurations, such as the amines. In the chemistry of strongly acid media a striking similarity to chemical processes observed in vacuo (studied by mass spectrometry) frequently emerges, thus it is of interest to compare the chemical shift for the pyridinium >NH+ proton with that calculated for the “free” cation in our previous paper,’ viz. 5.4-ppm downfield from benzene or -8.9 ppm from TMA’, the reference in the present study. This value is shown as a dashed line in Figure 2 , and is in good accord with the value observed in the most acid solution of this study corresponding approximately to saturation with AlCl,. AlC1, is generally recognized as a very strong Lewis acid surpassed only by the “superacids” based on SbF5 and related compounds. A corollary of the above discussion is that the proton chemical shift of the pyridinium ion can be used as an
The Journal of Physical Chemistry, Vol. 84, No. 5, 1980 541
PyCl and ZnCI, or AI& Acid-Base Reactions
indicator of Lewis acid strength in acid-base systems. ( b ) Ring Protons. An approach to theoretical shifts under highly acid conditions is also found for the p and y ring protons, as shown in Table I. In 70 mol % AlCl, solutions the 0and y shifts, which have been shown to be mainly dependlent upon the P cloud polarization, correspond well with theoretical value^.^>^ The oppositely directed behavior of the 15y proton is interesting; it is probably best interpreted as due to error in the theoretical consideration of the c donation of the N upon protonation or to the evaluation of the anisotropic paramagnetic contribution. For the corresponding a proton in the a-picHC1 + ZnC1, system, the chemical shifts show a different composition dependence, paralleling those of the y proton (Figure 4). Thiis may be due to the increased electron density on the initrogen side of the ring in the case of the a-picolinium cation. The AlCl, system shows the most structured ring proton behavior, a reversal in composition dependence being noted as the (PyH-Cl-.HPy) stoichiometry discussed in the next section is approached (Figure 3). (2) Competitive (Complexing Processes and t h e (PyH-.Cl.-HPy)+ Cation. We return to the set of curves displayed in Figure 2 and ask why the chemical shift for the >NH+ proton remains essentially unchanged during the first 30% of added AlC13. We argue that this is a further consequence of the removal, from competition for the proton, of the chloride species due to AlC1,- formation. Because the residual charge density on the chlorides in this complex is so small, the remaining chlorides at -30 mol % AlCl, are unirnpeded in their interaction with the PyH+ cations. Each chloride can orientate two PyH+ species, giving the [PyH...Cl--HPy]+ cation, in which the proton environment is little different from that in PyHCl itself, see section 3. The same tendency is present in the ZnClz-based system but is less pronounced because the chlorides in ZnC1:- are not as strongly polarized hence still compete with the uncomplexed chlorides for the >NH+ protons. Belief in the stability of the [PyH-.Cl-.-HPy]+ species is encouraged by the existence of a congruent-melting crystalline compound at this stoichiometry in the related system a-MePyHC1 C AlC13, a phase diagram of which4 is shown as an insert to Figure 3. Although the crystal structure of thiis compound is so far unknown, it will be surprising indeed if it proves not to have the chlorodipyridinium cation as a structural element. There is a less stable compound in the PyH ZnC1, system at 20 mol %, which may well have a corresponding structure, as this composition corresponds to 33 equiv % and may be formulated as (PyH-~C1--HPy+),-ZnC14z-. There is no corresponding crystalline compound in the HC1-based system, though it has bieen noted earlierlJOthat HC1 is easily lost from melts of stoichiometry [PyH'] [HCL-] and that melt compositions tlend to stabilize around stoichiometry of Py.1.4HCl. This corresponds to [PyH-Cl-.HPy+] [HClJ, indicating a t least a pronounced drop of HC1 activity associated with the complex cation. We may quantify these considerations and obtain a crude first approximation value for the free energy of formation of the complex cation from the "free" ions PyH+ and C1- by writing equations for the normal chloroaluminate acid -base equilibrium
+
C1-
+ AlzC17-e 2AlC14-
(1) and for the equilibrium in the presence of the competitive complexation process
[PyH-Cl-.HPyl+
+ AlzCl7- + 2PyH+ + 2AlC14-
(2)
The equilibrium constant Kcl for the first of these can be guessed a t from data obtained by electrochemical studies on related chloroaluminate systems. For instance, for the alkali halide AlCl, systems, values of Kcl between lo4 and lo8 are obtained depending on cation type and, to a lesser extent, temperature.ll For the "weaker" tetrabutylpyridinium cations this increases to 10l2at room temperature,12 hence an estimate of 1O1O would seem reasonable for the present case. For the second process Kcz can be estimated from the proton chemical shift observations as discussed below. Combination of processes 1 and 2 yields
+
-
2PyH+
+ C1- + [PyH--Cl**.HPy]+ Kc3 = KcdKcz
(3) (4)
hence, at 148 OC (421 K)
AGtZ1 = -RT In (Kcl/Kcz)
(5)
We assign 6 values to the pure (unperturbed) states PyH+ and [PyH.-.Cl.-PyH]+ on the basis of the extreme acid value, -9.0 ppm, and the plateau value, -13.5 ppm, respectively, and estimate the fraction f of protons present as PyH+ at 50 equiv % AlC13 from the observed shift, -9.90 ppm, by using 6,,bsd
= -9.Of - 13.5(1 - f )
(6)
which yields f = 0.8. Recognizing that each mole of [PyH.-Cl-.PyH]+ carries two protons, we obtain a mole ratio of the two cations of 8:l. Thus 1mol of PyHAlCl, dissociates, at 148 "C, according to [PyH.-.Cl...HPy]+ + Al,Cl,- + 2PyH+ 0.1 mol 0.1 mol 0.8 mol
+ 2AlC14-
0.8 mol
For a molar volume of 0.2 L mol-14 we find Kcz = lo3. Hence Kc3 = Kcl/KC2 = lo7 From eq 5, therefore, AGf = -56 k J mol-l, which implies a well-defined chemical species, since RT = 3.5 kJ/mol at 148 "C. (3) Correlation with Previous Data. It remains to comment further on the implications of this work for the structure and dynamic properties of PyHCl and a-PicHC1. It is perhaps surprising a t first sight that the magnetic environment of the proton does not change during removal of almost half of the original interacting chlorides by formation of AlC14-. This could be taken to indicate that the pure liquids already contain the species [PyH.-CI.HPy]+ and should be formulated [PyH-Cl-HPy]+Cl-. However, since [PyH-.Cl-.HPy]+ is disrupted even in the presence of the relatively inert species ZnC14,-, it seems rather unlikely that it should enjoy any special integrity in the presence of free C1-. Therefore, in the pure liquid the way in which the PyH+ species experiences a [PyH. -.Cl-.HPyl+ like environment is probably through a dynamic equilibrium in which the pyridinium cation is continuously reorientating between its various C1- near neighbors. Indeed, it would be very surprising if such dynamic equilibrium was not present in the liquid since even in the solid phase, in which the pyridinium cations are stacked in parallel sheets,13 it is found by wide-line NMR14 that rapid reorientation of the rings sets in some 50" below the normal melting point. The fact that the density changes by only 2.2% on fusion15 implies similarity
542
J. Phys. Chem. 1980, 84, 542-547
of liquid and high temperature solid structures and presumably reorientation dynamics (except for frequency increases). A rapid reorientation of pyridinium cations provides a mechanism for an extra contribution to the total electrical conductance via a Grotthus-like proton transfer between Py and C1- species. This should be more effective in the case of PyHCl than a-PicHC1 because of steric hindrance to rotation by the a-CH3 group in the latter case, and it may be sufficient to explain the 100% difference in conductivity between the two hydrochlorides at 150 0C.2,4The decrease of rotational freedom for PyH+ as Lewis acids are added and the [PyH.-Cl-.HPy]+ species becomes prominent satisfactorily explains why the discrepancy in conductance disappears with increasing acid (only the ZnC12 solutions have been studied for both cation^)^^^ and why the conductivity in the case of PyHC1-ZnC12 solutions gives little indication of the complexation reaction. In the case of a-PicHC1, in which we would argue the conductance is more nearly ionic in character, a weak conductivity maximum is observed a t the ZnC1:- stoichiometry, due to a minimum in cohesion at this comp~sition.~ Acknowledgment. This work was supported by the Advanced Research Project Agency of the Department of
Defense and by the National Science Foundation, MRL program GH33574Al. The potential interest content of a ‘H NMR sutdy of complexing reactions involving pyridinium salts was originally suggested to us by C. T. Moynihan. We thank him and also I. M. Hodge and J. B. Grutzner for many helpful discussions. The assistance of J. Barnes with instrumental problems was invaluable.
References and Notes (1) (2) (3) (4) (5) (6)
(7) (8) (9) (10) (11) (12) (13) (14) (15)
J. W. Shuppert and C. A. Angell, J. Chem. Phys., 07, 3050 (1977). A. J. Easteal and C. A. Angell, J . Phys. Chem., 74, 3987 (1970). H. A. Oye and D. M. Gruen, Inorg. Chem., 3, 836 (1964). C. A. Angell, I. M. Hodge, and P. A. Cheeseman In “Molten Salts”, J. P. Pemsler, Ed., The Electrochemical Society, London, 1976, p 138. V. C. Reinsborough, Aust. J . Chem., 23, 1473 (1970). H. K. Hofmeister and J. R. Van Wazer, J. Phys. Chem., 09, 791 (1965). D. S. Newman, R. T. Tillback, D. P. Morgan, and Wal-Ching Wan, J. Electrochem. Soc., 124, 856 (1977). G. Kotewycz, T. Schaefer, and E. Bock, Can. J . Chem., 09, 791 (1985). J. N. Murrell and V. M. S. Gll, Trans. Faraday SOC.,00, 248 (1964). M. Goffman and G. W. Harrington, J. Phys. Chem., 07, 1877 (1963). G. Torsi and G. Mamantov, Inorg. Chem., 11, 1439 (1972). R. J. Gale and R. A. Osteryoung, Inorg. Chem., 18, 1603 (1979). C. Rerat, Acta Cryst., 15, 427 (1962). C. H. Mathews and D. F. R. Giison, Can. J. Chem., 48, 2625 (1970). H. Bloom and V. C. Reinsborough, Aust. J . Chem., 21, 1525 (1968).
A Study of the Iron Borides. 2. Electronic Structure Oliver Johnson, Davld J. Joyner,? and Davld M. Hercules” Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received October 12, 1979) Publication costs assisted by the Petroleum Research Fund
On the basis of ESCA and magnetic spin data, electronic structures are proposed for Fe, Fe2B, and FeB which specify numbere of localized d electrons, itinerant d electrons, conduction electrons, and the configuration of boron. These formulations show the small changes in distribution of both d electrons and conduction electrons for the iron borides which account for the absence of chemical shifts in ESCA and the observed Mossbauer isomer shifts, as well as the observed differences in ESCA valence bands between Fe and the borides. The chemical bonding in these lower borides is metallic with boron atoms or chains occupying interstitial regions in the metallic lattice of Fe.
Introduction The electronic structure and chemical bonding of metal borides have been discussed in recent reviews.lP2 The literature includes differing concepts of chemical bonding in transition metal borides, and for iron borides there are bonding descriptions involving large electron transfer from boron to iron3 as well as band structure models including a B-B covalent bond.4 The data on Fe core level binding energies (BE) obtained from X-ray photoelectron spectroscopy (ESCA), given in part 1,5showed no appreciable change in BE between Fe, Fe2B, and FeB. This provides clear evidence that there is nearly the same electron density around Fe in the iron borides as in Fe metal. Thus, the new data provide a firm basis for proposing electronic structures for FezB and FeB when the variety of data in the literature are also considered. The following sequence will be used in establishing electronic structures for the iron borides. First, the ESCA data will be evaluated with consideration of effects on BE t Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, England.
0022-3654/80/2084-0542$0 1.OO/O
of volume changes in the solid state6 and of extra-atomic relaxation.’ The data from Auger electron spectroscopy (AES) and the differences in ESCA valence bands will also be discussed. Second, electron density maps based on X-ray diffraction studies8 of FezB and spin density maps from polarized neutron diffraction studies of Fe2B8and FeB9 will be used to formulate unique d-orbital degeneracies and occupancies which fit the ESCA, electron density, and magnetic data. The structures will be presented in terms of a metallic modello which denotes numbers of localized and itinerant d electrons and conduction electrons; the model emphasizes the role of the electron density of conduction electrons in interstitial regions of the metallic lattice. No band structure calculations have been published for iron borides, but when the d orbitals of the present structures are broadened into overlapping bands, there is obtained a partial density of states for the Fe(3d) valence band. Finally, other data on FezB and FeB, such as the quadrupole coupling parameter data from NMR,I1 isomer shifts from Mossbauer data,12and electronic specific heat9,13will be considred in further elucidation of the electronic structure of transition metal borides. 0 1980 American Chemical Society
