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Covalency effects from mp̈ssbauer experiments on the hydrates of FeCl2
Volume
2. number
7
CHEMICAL
COVALENCY
EFFECTS GN
Department
M~SSBAUER . HYDRATES OF FeCr2
FL C. A2iITm of Chemical Princeton.
The results of Massbauer tion between the quadrupole
-
studies splitting
New Jersey
* Jr. **
University,
08540, USA 1968
on the hydrates of ferrous chloride and isomer shift. The correlation
show a nearly
linear
correla-
is discussed in terms of central field and symmetry restricted covalency. A comparative analysis of the present correlation and previous results on anhydrous ferrou s halides reveals information about the relative importance of u and r bonds in these compocnds.
= (f)e2Q(1
-
R)(Y~$--~)~F(A~, A.2, a2Ao, T), (1)
where Q is the quadrupole moment of the nuclear excited state and (1 -R) is a Sternheimer antishielding factor. The symmetry dependent reduction factor F is a function of the crystal field A2), the spin-orbit coupling consplitting (A stant, x = o!L x0, and the temperature, T. Covalency effects are represented in simplified form by a single parameter, the delocalization factor at2 = (r-3),‘(Y-3>o. Thus eq. (1) assumes that co* Supported b_vthe U.S. Atomic Enerm Commission. AEC Fellow, 1965-8.
** Special
440
Princeton
23 September
Theoretical attempts at describing the electron charge distributions in solids that contain transition metal ions are handicapped by the lack of an adequate set of electronic wave functions and by a dearth of relevant experimental results for the purposes of comparison [l-6]. A common approach is to use the wave functions of the free ion which is considered to be located at the center of a cluster of other atoms; interactions between the ligands and the ion’s second nearest neighbors are ignored. Typical results indicate that the effective ionic charge, z*, is very small [4]. Since Miissbauer measurements in iron compounds are sensitive to the electronic charge distribution at the site of the iron atom, they should provide a test Por- theoretical predictions. For high-spin ferrous compounds, the expression for the Mosabauer quadrupole splitting, QS( T) , exclusive of possible lattice contributions to the electric field gradient (EFG), is [7] Qs(T)
EXPERIMENTS
and J. W. HURLEY
Engineering,
Received
November 1968
LETTERS
FROM
THE
Y. HAZONY,
PHYSICS
valency affects the magnitudes of the EFG and the spin-orbit coupling constant in a similar fashion. Since most available QS(T) data comprise a small number of points over a constricted temperature range, eq. (1) cannot be uniquely determined without a prebri knowledge of the relative magnitudes of the different factors involved [8, 71. In order to gain that knowledge we are making measurements on several circumscribed series of high-spin ferrous compounds. In a recent communication we reported the results of one such study on the anhydrous ferrous halides [8]. In that series we observed linear correlations between the experimental values of QS in the paramagnetic phase, the corrected isomer shifts, $8, and the Pauling electronegativities of the ligand atoms. Although the magnitudes of the QS values vary widely, the changes with temperature in the range of 90- 300°K are all very similar and relatively small from which we concluded that the respective F factors are nearly the same and probably close to unity [9] at low temperature. As a consequence the major source of the differences in the QS values can be associated with differences in the delocalization factor e2. A scale for o2, established from the QS measurements and ESR data, indicates a surprisingly extensive 3d delocalization in the heavier halides. in the present letter we report the results of studies on a second circumscribed series of highspin ferrous compounds: FeC12, FeC12-H20, FeCl2 2H20, FeC124H20, FeSO4 7H20 and FeSiF6.6H20. The last two compounds have the
Volume 2, number 7
CHEMICALPHYSICSLETTERS
microstructure [Fe(H20)6]2+ and the entire series may be considered as the hydrates of ferrous chloride with the distorted octahedral structures of [FeCl6_,.nH2Op-4. Unlike the ferrous halides series, the present series involves significant differences in microsymmetry (cf. fig. 1) and the results will be discussed in terms of these differences. A preiiminary account of this work has been presented elsewhere [ 101. The studies were performed on reagent grade chemicals with the exception of FeC!l2.H20 and FeSiF6-6H20. FeC12eH20 was prepared by exposing anhydrous FeCl2 to atmospheric moisture; both FeCl2 and FeC12sH20 are difficult to prepare uncontaminated by the other. A subsidiary experiment [ll], however, established that measurements of QS(T) for both compounds were unaffected within f 0.010 mm/set by the presence of gross amounts of the other compound. FeSiFg-6H20 was crystallized from a solution of iron in H2SiF6. Two spectrometers were used, both of which have excellent longterm stability [12,13 J. Detailed thermal shit measurements over the range 90 - 300’K for each compound permitted the determination, within rt 0.005 mm/set, of 613 which is the isomer shift (IS) corrected for the second order Doppler shift and zer,-r>int motion_ This procedure removes substantially ali lattice dynamical and temperature effects from the IS [14]. The source was 57Co in a palladium
lo 1 Fe&
Novem5er
1968
foil and values for 61s are reported with respect to a natural iron absorber at room temperature. Fig. 2a is a plot of 6~s versus tz, the hydration number, which demonstrates that large differences in microsymmetry (cf. fig. I) do not signLficantly perturb the linearity between f~ and 61~. The latter relationship indicates that the concept of partial isomer shifts established by Herber et al. for covalent iron-organic compounds [15] is also valid for the ionic salts of the present study. The correlation is perhaps not so surprising when it is retailed that 31s is determined by the s electron density at the iron nucLeus. The spherical symmetry of s wave functions requires that their interaction with other electron clouds (whether via overlap or shielding effects) reflect a scalar average of the properties of the bonds between the iron atom and its six Ligurds. Some- Eartuitous compensation of the effects must be operative, however, since the distances (and hence the covalency) of all the Fe-Cl and Fe-O bonds in the series are not constant throughout the series. More surprising is the nearly linear correlation between QS and n that is demonstrated in fig. 2b. Here the details of the microsymmetry might be expected to play a predominant role via lattice contributions to the EFG, spin-orbit coupling and symmetry restricted covalency- Their combined effect, however, is too smaiL to perturb the linear correlation significantly and the implication is that the QS is also largely deter-
(b) FeCli2H,O
(c 1 FeCI,-4&O
0 -Fe l
--cl
o -0 (Hz01
(d 1 Fe SO;
7H,O
fe) Fe SiF,*GH,O
Fig. 1. Nominal octahedral configurations of five of the compoundswhose Miksbauer parameters are reported. 0 = = Fe2+, o = H20, l = Cl. a) FeCl2 [ZS]. b) FeC12-2H20 124,251, c) FeC12-4H20 f27.281. d) Fe.!W4.7%+ (311. a&d e) FeSiF6’6H20 [22]. 441
Volume
2. number
7
CHEMICAL
PHYSICS
LETTERS
November
1968
.
1.0
0.0 0
-
I.1
0.2
1.2
1.3 6,,
(b)
n
by the scalar average of the iron-&and bond properties. The nearly linear dependence of QS on 61s is illastrated in fig: 3a which shows that the c&r?+tions from linearity are smaller than the temperature variations in QS in the range 90- LX°K_ The implications of the correlation with respect to lattice effects. spin-orbit coupling, delocalization and bonc’kg, are discussed below. mined
fig.
3a are
occurs
for
effects. If only respective values of for the ferrous chloride hydrates in compared, FeClz-H20.
the greatest The
structure
discrepancy of the mc-
nohydrate, which seems to have been identified first in Mkksbauer studies fl6], is unstudied but undoubtedly posesses low symmetry since it apparently result5 from a replacement of a single chloride ion in the first coordination sphere with a water molecule. The rough agreement with the present correlation, however, indicates that the lowest t2g level for this compound is a singlet [ 171. Nozik and Kaplan have reccztly reported lattice sum calcUations that yield very small lattice contributions to the QS of F’eC12-2H20, 442
0.0 1.6
I mmlsecl
and
six
ferrous
hydrates
(clear bars).
The tops of the bars represent QS
Fig. 2. a) The corrected isomer shift. 618. with respect to natural iron at room temperature versus h dration number. n. for the series [FeC16_n-nH20]n -? _ b) Quadrupole splitting at SOoK versus hydration number n for the series [FeCl6_,M$O]n-%
tiffice QS (90’K)
1.5
Fig. 3. a) Quadrupole splitting over the range SO - 300°K vwsus corrected isomer shift for four ferrous halides (blackbars)
1.5I[
1.4
(gOoK) while the lower ends of the bars correspond to room temperature results; the uidths of the bars reflect the probable errors in 61s~QS values for FeI2. FeBr2 and FeC12 are doubled as discussed in ref. [8]. b) Trends in quadrupole splitting and isomer shift with the extent of U- and n-bonding. In all cases the arrows point in the direction for increasing influence.
FeS04-7H20 and FeSiF6-6H20, but -0.41 mm,.’ set for FeCl2-4H20 1183. These authors assumed that the Fe-O bond is c5mpleteIy ionic and that the Fe-Cl bond is 60 - 80% ionic. Mo:-ecular orbital calculations [4,6], however. indicate much smaller effective ionic charges than those used by Nozik and Kaplan. If such numbers are inserted into the lattice sum calculations, the latlice contributions to the EFG become negligible, even for FeC&-4H20, in agreement with the indication of the present experiments.
Spin-mbit co@ling_ Although the microstrucof the entire hydrate series is nominally oc-
ture
tahedral, the variations in structure should preclude any regularity with n of A or of A/(u2k,, the factor that determines the quenching of the QS by spin-orbit coupling. The quenching decreases rapidly with decreasing a2. As can be seen in fig. 3a, the relative changes in QS( T) in the range 90- 300°K are nearly the same for all the compounds except Fe(Jlr4H20. They are also closely similar to those for the ferrous halides, thus supporting the conclusion that the effects of spin-orbit coupling on the QS are small for both series.
Volume 2. number 7
CHEMICAL PHYSICS LETTERS
Electron d&localization In discussing the nephelauxetic or “cloud-expansion” effect, Schaffer and Jbrgensen distinguish between central field covalency (CFC) and symmetry restricted covalency (SRC) [19]. CFC is a consequence of the expansion of the radial portion of the 3d wave functions due to the reduction of the metal ion’s effective charge via cr- and u-bonding; SRC results, in the present case, from delocalization of the lowest level t2 electrons by D overlap with the Iigand orbitals. # eretofore it has been difficult to separate the two contributions to electron delocalization [20]. Comparison of the &Rissbauer results for the two pairs of compounds, FeSiF6.6H20 versus FeS04-7H20 and FeCl2-2H20 versus FeC12, however, permits an assessment of the relative influence of the two effects. The change in microstructure of the first coordination sphere on going from FeSiF66H20 to FeS04-7H20 (fig. 1) involves the declination of the z axis [21,22]. The positive sign.of the EFG in FeS04‘7H20 1231 requires that the Iv> be the lowest t2 orbital. The degree of overlap between the lowes 4 tag orbital and the orbitals of the four coplanar ligands in thexy plane govern the contribution of the SRC which should thus be the same for both compounds. The fact that both the QS and 6fS shift along the correlation curve (fig. 3a) indicates that Iroth properties measure the average bonding of the six ligands and that, since the SRC could not have changed, the difference in delocalization must be due to a difference in CFC. In the case of FeC122H20, the lowest t2 orbital is coplanar with the orbitals of four c fi orl-ne atoms [23-251 with approximately the same Fe-Cl bond lengths as those in FeC12 [26]. Once again the delocalization reflected in the increase in both QS and 6fS for FeC122H20 vis 2 vis FeC12 must reflect primarily a change in the CFC. The predominant influence of CFC on the covalency of the ferrous chloride hydrates indicates that for the ferrous halides [8], for which the differences in microsymmetry are quite minor, the CFC must also be controlling. Actually, the fact that the delocalization factors for the heavier halides are less than 0.5 in itself may demonstrate a strong contribution from CFC since SRC gives a2 2 0.5 even for a fully covalent compound [27], provided that 4d contributions to tg MO are negligible. In FeC12-4H 0 [2!,29], the Fe-Cl bonds are much shorter ( 1 -38 A) than those in FeC12 (2.53 A) whi$e two of the Fe-O bo_nds are much longer (2.59 A) than normal (2.14 A). Such a reduction in the Fe-Cl bond length indicates a substantial increase in the covalency of the bond. The fact
November 1968
that the covalency difference may be compensated, insofar as the linear correlation between QS and 6= is unperturbed, by the increase in bond length of a single pair of H20 molecules must mean that the normal Fe-O bond is appreciably covalent. Thus the FeC124H20 M&sbauer data confirm, in a qualitative manner, the covalency indicated by the ESR ass;gnment of cu2 - 0.8 for FeSiFg-6H20 [27,7] and by the o2 scale proposed in our previous studies [S]. z-bonding versus o- bondi=. In view of the importance of the central field covalency, a decrease in the extent of u-bonding should be accompanied by a decrease in both QS and 6fS. A decrease in n-bonding should result in increased QS but decreased 6 Fig. 3b illustrates these two trends as well as ‘Kv o intermediate cases for which both a- and n-bonding change. A comparison with the experimental data in fig. 3a makes clear that the primary trend in both hydrate and halide series isone
of
decreasing
cr-bondingwith
Littlechange
in a-bonding in the direction n = 0 h 11= 6 and I > Br > Cl y F. The juxtaposition cf the data for [Fe(H20)6]2+ and FeF2. however, reIreals a striking increase in a-bonding in the fIuoride relative to that of the hexahydrate. Coucbrsions. By distinguishing between the effects of SRC and CFC, the present data cIearly demonstrate that it is chiefly general field covalency that accounts for differences in bonding in high-spin ferrous compounds. The work thus ccjnfirms the reasonableness of In&L’s proposal [7] that all covalency effects of the MGssbauer QS for these compounds may be lumped into a singLe parameter, the delocalization factor (~2 = ~3)~(~3)~ In addition. these studies provide a basis for hope that additional Mijssbauer studies on carefully chosen series may ultimateLy furnish quantitative measures of the magnitudes of the X- and u-bond strengths from a calibration of the QS- 61S correlation plane.
ACKNOWLEDGEMENTS This work made use of computer facilities supported in part by Gi-ant GP-579 from the National Science Foundation. One of us (Y-H.) gratefully acknowledges support from the Sloan Foundation.
443
Volume
2. number
7
CHEMICAL
REFERENCES [l]
R. E. Watson and A. J. Freeman, Phys.Rev. 134 (1964) 1526. [2] E. Simanek and Z. Sroubeck, Phys. Status Solidi 4 (1964) 251. [3] S. Sugano and Y. Tanabe. J. Phys. Sot. Japan 20 (1965) 1155. [4] H.Basch. A. Viste and H.B.Gray, J. Chem. Phys. 44 (1966) 10. [5] P.Offenhartz. J.Chem.Phys. 47 (1967) 2951. [6] C. J. Ballhausen and H. B. Gray, Molecular Orbital Theory (W.A.Beujamin Inc.. New York-Amsterdam, 1965). [7l R.Ingalls, Phys.Rev. 133 (1964) A787. [8] R; C.Axtmann. Y. Hazony and J. W. Hurley Jr., Chem. Phys _Letters. in press. i9] Except for a degeneracy factor cf. discussion in -ref. [S] . [IO] Ii:C. Axtmann, J. W. Hurley Jr. and Y. Hazony. BuU.Am.Phys_Soc. 13 (1968) 691. [ll] J.W.Hurleybr., Y.Ha&ny &d R.C.Axtmann, Bufl.A+Phys. Sot. 13 (1968) 690. [12] Y.Hazony. Rev.Sci.Inet. 38 (1967) 1760. 1131 D-E-Earls, R.C.Axtmann, Y.Hazony and I.Lefkowitz, J. Phys. Chem. Solids, in press. 1141 Y.Hazony, J.Chem.Phys. 45 (1966) 2664.
PHYSICS
LETTERS
November
1968
[15] R.H.Herber, R.B.King ad G.K. Wertheim. Inorg. Chem. 3 (1964) 101. [IS] J. W. Hurley Jr.. R. C. Axtmann and Y. Hazony. BulL.Am.Phys.Soc. 12 (1967) 654. [17] See discussion in ref. [a]. [la] &3J_ Nozik and M-Kaplan. Phys. Rev. 159 (1967) [I91 C.E’.Schaffer and C.K.J&gensen. J.Nucl.Chem. 8 (1958) 143. [ZOJ C.K. Jbrgensen, Absorption Spectra and Chemical Bonding in Complexes (Pergamon Press, 1962) p. 143. [ZIJ W. H.Baur, Acta Cry& 17 (1964) 1167. 1221 W. C. Hamilton, Acta Cryst. 15 (1962) 353. [23] R. W-Grant. H.Wiedersich, A. H-Muir Jr., U. Gonser and W.N.Delgass, J.Chem.Phys. 45 (1966) 1015. 1241 B. Morosin and E. J.Graeber. J.&em. Phys. 42 (1965) 898. [25] S. Chandra and G. R. Hoy, Phys. Letters 22 (1966) [26] i?errari, A.Celeri and F.Giorgi:Rend.Accad. Lincei 9 (1929) 782. r271 J.Owen. Proc. Roy. Sot. (London) A (1955) 183. [ZS] B.R.PenfoLd and J.A.Grigor, Acta Cryst. 12 (1959) 850. [29] P. Zory, Phys.Rev. 140 (1965) Al401.
2. number
7
CHEMICAL
COVALENCY
EFFECTS GN
Department
M~SSBAUER . HYDRATES OF FeCr2
FL C. A2iITm of Chemical Princeton.
The results of Massbauer tion between the quadrupole
-
studies splitting
New Jersey
* Jr. **
University,
08540, USA 1968
on the hydrates of ferrous chloride and isomer shift. The correlation
show a nearly
linear
correla-
is discussed in terms of central field and symmetry restricted covalency. A comparative analysis of the present correlation and previous results on anhydrous ferrou s halides reveals information about the relative importance of u and r bonds in these compocnds.
= (f)e2Q(1
-
R)(Y~$--~)~F(A~, A.2, a2Ao, T), (1)
where Q is the quadrupole moment of the nuclear excited state and (1 -R) is a Sternheimer antishielding factor. The symmetry dependent reduction factor F is a function of the crystal field A2), the spin-orbit coupling consplitting (A stant, x = o!L x0, and the temperature, T. Covalency effects are represented in simplified form by a single parameter, the delocalization factor at2 = (r-3),‘(Y-3>o. Thus eq. (1) assumes that co* Supported b_vthe U.S. Atomic Enerm Commission. AEC Fellow, 1965-8.
** Special
440
Princeton
23 September
Theoretical attempts at describing the electron charge distributions in solids that contain transition metal ions are handicapped by the lack of an adequate set of electronic wave functions and by a dearth of relevant experimental results for the purposes of comparison [l-6]. A common approach is to use the wave functions of the free ion which is considered to be located at the center of a cluster of other atoms; interactions between the ligands and the ion’s second nearest neighbors are ignored. Typical results indicate that the effective ionic charge, z*, is very small [4]. Since Miissbauer measurements in iron compounds are sensitive to the electronic charge distribution at the site of the iron atom, they should provide a test Por- theoretical predictions. For high-spin ferrous compounds, the expression for the Mosabauer quadrupole splitting, QS( T) , exclusive of possible lattice contributions to the electric field gradient (EFG), is [7] Qs(T)
EXPERIMENTS
and J. W. HURLEY
Engineering,
Received
November 1968
LETTERS
FROM
THE
Y. HAZONY,
PHYSICS
valency affects the magnitudes of the EFG and the spin-orbit coupling constant in a similar fashion. Since most available QS(T) data comprise a small number of points over a constricted temperature range, eq. (1) cannot be uniquely determined without a prebri knowledge of the relative magnitudes of the different factors involved [8, 71. In order to gain that knowledge we are making measurements on several circumscribed series of high-spin ferrous compounds. In a recent communication we reported the results of one such study on the anhydrous ferrous halides [8]. In that series we observed linear correlations between the experimental values of QS in the paramagnetic phase, the corrected isomer shifts, $8, and the Pauling electronegativities of the ligand atoms. Although the magnitudes of the QS values vary widely, the changes with temperature in the range of 90- 300°K are all very similar and relatively small from which we concluded that the respective F factors are nearly the same and probably close to unity [9] at low temperature. As a consequence the major source of the differences in the QS values can be associated with differences in the delocalization factor e2. A scale for o2, established from the QS measurements and ESR data, indicates a surprisingly extensive 3d delocalization in the heavier halides. in the present letter we report the results of studies on a second circumscribed series of highspin ferrous compounds: FeC12, FeC12-H20, FeCl2 2H20, FeC124H20, FeSO4 7H20 and FeSiF6.6H20. The last two compounds have the
Volume 2, number 7
CHEMICALPHYSICSLETTERS
microstructure [Fe(H20)6]2+ and the entire series may be considered as the hydrates of ferrous chloride with the distorted octahedral structures of [FeCl6_,.nH2Op-4. Unlike the ferrous halides series, the present series involves significant differences in microsymmetry (cf. fig. 1) and the results will be discussed in terms of these differences. A preiiminary account of this work has been presented elsewhere [ 101. The studies were performed on reagent grade chemicals with the exception of FeC!l2.H20 and FeSiF6-6H20. FeC12eH20 was prepared by exposing anhydrous FeCl2 to atmospheric moisture; both FeCl2 and FeC12sH20 are difficult to prepare uncontaminated by the other. A subsidiary experiment [ll], however, established that measurements of QS(T) for both compounds were unaffected within f 0.010 mm/set by the presence of gross amounts of the other compound. FeSiFg-6H20 was crystallized from a solution of iron in H2SiF6. Two spectrometers were used, both of which have excellent longterm stability [12,13 J. Detailed thermal shit measurements over the range 90 - 300’K for each compound permitted the determination, within rt 0.005 mm/set, of 613 which is the isomer shift (IS) corrected for the second order Doppler shift and zer,-r>int motion_ This procedure removes substantially ali lattice dynamical and temperature effects from the IS [14]. The source was 57Co in a palladium
lo 1 Fe&
Novem5er
1968
foil and values for 61s are reported with respect to a natural iron absorber at room temperature. Fig. 2a is a plot of 6~s versus tz, the hydration number, which demonstrates that large differences in microsymmetry (cf. fig. I) do not signLficantly perturb the linearity between f~ and 61~. The latter relationship indicates that the concept of partial isomer shifts established by Herber et al. for covalent iron-organic compounds [15] is also valid for the ionic salts of the present study. The correlation is perhaps not so surprising when it is retailed that 31s is determined by the s electron density at the iron nucLeus. The spherical symmetry of s wave functions requires that their interaction with other electron clouds (whether via overlap or shielding effects) reflect a scalar average of the properties of the bonds between the iron atom and its six Ligurds. Some- Eartuitous compensation of the effects must be operative, however, since the distances (and hence the covalency) of all the Fe-Cl and Fe-O bonds in the series are not constant throughout the series. More surprising is the nearly linear correlation between QS and n that is demonstrated in fig. 2b. Here the details of the microsymmetry might be expected to play a predominant role via lattice contributions to the EFG, spin-orbit coupling and symmetry restricted covalency- Their combined effect, however, is too smaiL to perturb the linear correlation significantly and the implication is that the QS is also largely deter-
(b) FeCli2H,O
(c 1 FeCI,-4&O
0 -Fe l
--cl
o -0 (Hz01
(d 1 Fe SO;
7H,O
fe) Fe SiF,*GH,O
Fig. 1. Nominal octahedral configurations of five of the compoundswhose Miksbauer parameters are reported. 0 = = Fe2+, o = H20, l = Cl. a) FeCl2 [ZS]. b) FeC12-2H20 124,251, c) FeC12-4H20 f27.281. d) Fe.!W4.7%+ (311. a&d e) FeSiF6’6H20 [22]. 441
Volume
2. number
7
CHEMICAL
PHYSICS
LETTERS
November
1968
.
1.0
0.0 0
-
I.1
0.2
1.2
1.3 6,,
(b)
n
by the scalar average of the iron-&and bond properties. The nearly linear dependence of QS on 61s is illastrated in fig: 3a which shows that the c&r?+tions from linearity are smaller than the temperature variations in QS in the range 90- LX°K_ The implications of the correlation with respect to lattice effects. spin-orbit coupling, delocalization and bonc’kg, are discussed below. mined
fig.
3a are
occurs
for
effects. If only respective values of for the ferrous chloride hydrates in compared, FeClz-H20.
the greatest The
structure
discrepancy of the mc-
nohydrate, which seems to have been identified first in Mkksbauer studies fl6], is unstudied but undoubtedly posesses low symmetry since it apparently result5 from a replacement of a single chloride ion in the first coordination sphere with a water molecule. The rough agreement with the present correlation, however, indicates that the lowest t2g level for this compound is a singlet [ 171. Nozik and Kaplan have reccztly reported lattice sum calcUations that yield very small lattice contributions to the QS of F’eC12-2H20, 442
0.0 1.6
I mmlsecl
and
six
ferrous
hydrates
(clear bars).
The tops of the bars represent QS
Fig. 2. a) The corrected isomer shift. 618. with respect to natural iron at room temperature versus h dration number. n. for the series [FeC16_n-nH20]n -? _ b) Quadrupole splitting at SOoK versus hydration number n for the series [FeCl6_,M$O]n-%
tiffice QS (90’K)
1.5
Fig. 3. a) Quadrupole splitting over the range SO - 300°K vwsus corrected isomer shift for four ferrous halides (blackbars)
1.5I[
1.4
(gOoK) while the lower ends of the bars correspond to room temperature results; the uidths of the bars reflect the probable errors in 61s~QS values for FeI2. FeBr2 and FeC12 are doubled as discussed in ref. [8]. b) Trends in quadrupole splitting and isomer shift with the extent of U- and n-bonding. In all cases the arrows point in the direction for increasing influence.
FeS04-7H20 and FeSiF6-6H20, but -0.41 mm,.’ set for FeCl2-4H20 1183. These authors assumed that the Fe-O bond is c5mpleteIy ionic and that the Fe-Cl bond is 60 - 80% ionic. Mo:-ecular orbital calculations [4,6], however. indicate much smaller effective ionic charges than those used by Nozik and Kaplan. If such numbers are inserted into the lattice sum calculations, the latlice contributions to the EFG become negligible, even for FeC&-4H20, in agreement with the indication of the present experiments.
Spin-mbit co@ling_ Although the microstrucof the entire hydrate series is nominally oc-
ture
tahedral, the variations in structure should preclude any regularity with n of A or of A/(u2k,, the factor that determines the quenching of the QS by spin-orbit coupling. The quenching decreases rapidly with decreasing a2. As can be seen in fig. 3a, the relative changes in QS( T) in the range 90- 300°K are nearly the same for all the compounds except Fe(Jlr4H20. They are also closely similar to those for the ferrous halides, thus supporting the conclusion that the effects of spin-orbit coupling on the QS are small for both series.
Volume 2. number 7
CHEMICAL PHYSICS LETTERS
Electron d&localization In discussing the nephelauxetic or “cloud-expansion” effect, Schaffer and Jbrgensen distinguish between central field covalency (CFC) and symmetry restricted covalency (SRC) [19]. CFC is a consequence of the expansion of the radial portion of the 3d wave functions due to the reduction of the metal ion’s effective charge via cr- and u-bonding; SRC results, in the present case, from delocalization of the lowest level t2 electrons by D overlap with the Iigand orbitals. # eretofore it has been difficult to separate the two contributions to electron delocalization [20]. Comparison of the &Rissbauer results for the two pairs of compounds, FeSiF6.6H20 versus FeS04-7H20 and FeCl2-2H20 versus FeC12, however, permits an assessment of the relative influence of the two effects. The change in microstructure of the first coordination sphere on going from FeSiF66H20 to FeS04-7H20 (fig. 1) involves the declination of the z axis [21,22]. The positive sign.of the EFG in FeS04‘7H20 1231 requires that the Iv> be the lowest t2 orbital. The degree of overlap between the lowes 4 tag orbital and the orbitals of the four coplanar ligands in thexy plane govern the contribution of the SRC which should thus be the same for both compounds. The fact that both the QS and 6fS shift along the correlation curve (fig. 3a) indicates that Iroth properties measure the average bonding of the six ligands and that, since the SRC could not have changed, the difference in delocalization must be due to a difference in CFC. In the case of FeC122H20, the lowest t2 orbital is coplanar with the orbitals of four c fi orl-ne atoms [23-251 with approximately the same Fe-Cl bond lengths as those in FeC12 [26]. Once again the delocalization reflected in the increase in both QS and 6fS for FeC122H20 vis 2 vis FeC12 must reflect primarily a change in the CFC. The predominant influence of CFC on the covalency of the ferrous chloride hydrates indicates that for the ferrous halides [8], for which the differences in microsymmetry are quite minor, the CFC must also be controlling. Actually, the fact that the delocalization factors for the heavier halides are less than 0.5 in itself may demonstrate a strong contribution from CFC since SRC gives a2 2 0.5 even for a fully covalent compound [27], provided that 4d contributions to tg MO are negligible. In FeC12-4H 0 [2!,29], the Fe-Cl bonds are much shorter ( 1 -38 A) than those in FeC12 (2.53 A) whi$e two of the Fe-O bo_nds are much longer (2.59 A) than normal (2.14 A). Such a reduction in the Fe-Cl bond length indicates a substantial increase in the covalency of the bond. The fact
November 1968
that the covalency difference may be compensated, insofar as the linear correlation between QS and 6= is unperturbed, by the increase in bond length of a single pair of H20 molecules must mean that the normal Fe-O bond is appreciably covalent. Thus the FeC124H20 M&sbauer data confirm, in a qualitative manner, the covalency indicated by the ESR ass;gnment of cu2 - 0.8 for FeSiFg-6H20 [27,7] and by the o2 scale proposed in our previous studies [S]. z-bonding versus o- bondi=. In view of the importance of the central field covalency, a decrease in the extent of u-bonding should be accompanied by a decrease in both QS and 6fS. A decrease in n-bonding should result in increased QS but decreased 6 Fig. 3b illustrates these two trends as well as ‘Kv o intermediate cases for which both a- and n-bonding change. A comparison with the experimental data in fig. 3a makes clear that the primary trend in both hydrate and halide series isone
of
decreasing
cr-bondingwith
Littlechange
in a-bonding in the direction n = 0 h 11= 6 and I > Br > Cl y F. The juxtaposition cf the data for [Fe(H20)6]2+ and FeF2. however, reIreals a striking increase in a-bonding in the fIuoride relative to that of the hexahydrate. Coucbrsions. By distinguishing between the effects of SRC and CFC, the present data cIearly demonstrate that it is chiefly general field covalency that accounts for differences in bonding in high-spin ferrous compounds. The work thus ccjnfirms the reasonableness of In&L’s proposal [7] that all covalency effects of the MGssbauer QS for these compounds may be lumped into a singLe parameter, the delocalization factor (~2 = ~3)~(~3)~ In addition. these studies provide a basis for hope that additional Mijssbauer studies on carefully chosen series may ultimateLy furnish quantitative measures of the magnitudes of the X- and u-bond strengths from a calibration of the QS- 61S correlation plane.
ACKNOWLEDGEMENTS This work made use of computer facilities supported in part by Gi-ant GP-579 from the National Science Foundation. One of us (Y-H.) gratefully acknowledges support from the Sloan Foundation.
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Volume
2. number
7
CHEMICAL
REFERENCES [l]
R. E. Watson and A. J. Freeman, Phys.Rev. 134 (1964) 1526. [2] E. Simanek and Z. Sroubeck, Phys. Status Solidi 4 (1964) 251. [3] S. Sugano and Y. Tanabe. J. Phys. Sot. Japan 20 (1965) 1155. [4] H.Basch. A. Viste and H.B.Gray, J. Chem. Phys. 44 (1966) 10. [5] P.Offenhartz. J.Chem.Phys. 47 (1967) 2951. [6] C. J. Ballhausen and H. B. Gray, Molecular Orbital Theory (W.A.Beujamin Inc.. New York-Amsterdam, 1965). [7l R.Ingalls, Phys.Rev. 133 (1964) A787. [8] R; C.Axtmann. Y. Hazony and J. W. Hurley Jr., Chem. Phys _Letters. in press. i9] Except for a degeneracy factor cf. discussion in -ref. [S] . [IO] Ii:C. Axtmann, J. W. Hurley Jr. and Y. Hazony. BuU.Am.Phys_Soc. 13 (1968) 691. [ll] J.W.Hurleybr., Y.Ha&ny &d R.C.Axtmann, Bufl.A+Phys. Sot. 13 (1968) 690. [12] Y.Hazony. Rev.Sci.Inet. 38 (1967) 1760. 1131 D-E-Earls, R.C.Axtmann, Y.Hazony and I.Lefkowitz, J. Phys. Chem. Solids, in press. 1141 Y.Hazony, J.Chem.Phys. 45 (1966) 2664.
PHYSICS
LETTERS
November
1968
[15] R.H.Herber, R.B.King ad G.K. Wertheim. Inorg. Chem. 3 (1964) 101. [IS] J. W. Hurley Jr.. R. C. Axtmann and Y. Hazony. BulL.Am.Phys.Soc. 12 (1967) 654. [17] See discussion in ref. [a]. [la] &3J_ Nozik and M-Kaplan. Phys. Rev. 159 (1967) [I91 C.E’.Schaffer and C.K.J&gensen. J.Nucl.Chem. 8 (1958) 143. [ZOJ C.K. Jbrgensen, Absorption Spectra and Chemical Bonding in Complexes (Pergamon Press, 1962) p. 143. [ZIJ W. H.Baur, Acta Cry& 17 (1964) 1167. 1221 W. C. Hamilton, Acta Cryst. 15 (1962) 353. [23] R. W-Grant. H.Wiedersich, A. H-Muir Jr., U. Gonser and W.N.Delgass, J.Chem.Phys. 45 (1966) 1015. 1241 B. Morosin and E. J.Graeber. J.&em. Phys. 42 (1965) 898. [25] S. Chandra and G. R. Hoy, Phys. Letters 22 (1966) [26] i?errari, A.Celeri and F.Giorgi:Rend.Accad. Lincei 9 (1929) 782. r271 J.Owen. Proc. Roy. Sot. (London) A (1955) 183. [ZS] B.R.PenfoLd and J.A.Grigor, Acta Cryst. 12 (1959) 850. [29] P. Zory, Phys.Rev. 140 (1965) Al401.