3d electron delocalization from Mössbauer measurements on ferrous halides

Volume 2: number 8

3d

ELECTRON

CHEMICAL PHYSICS LETTERS

DELOCALIZATION ON

FROM

FERROUS

December

Mi3SSBAUER

HALIDES

1968

MEASUREMENTS

*

R. C. AX!fXANN.

Y. HAZONY and J. W. HURLEY Jr. Department of Chemical Engitleering. Pn’nceton University. Princeton. X J. 08540, USA

**

Received 9 September 1968

Measurements on the anhydrous ferrous halides indicate (a) that the Miissbauer quadrupole split&g (QS) and isomer shift (IS) have a common origin and (b) that both properties may provide estimates of 3d electron delocalization. rous halides is linearly

As in the case of similar series of Au. related to the Pauling electronegativity.

. The substantial number of reported correlations of QS versus IS [l-4] and of IS versus ii-i gand electronegativity 15-71 suggest that undel; sufficiently selective circumstances these twq MSssbauer measurements may probe the samk property of chemical bonds. Collins and Petit. [I], D3non (21, Brady et al. [3 ], and Dale et $1. [4] have all found approximately linear relationships between QS and IS in circumscribed series of low-spin ferrous compounds. Linear relations between IS and ligand electronegativities have been demonstrated for aurous halides [5], tetrahedrad+stan ic halides 61 and octahedral halides of Sn , Sb‘+, and Ted+ 171. Lees and Flinn’s illuminating study of bonding in stannous compounds that begins from a linear QS-IS relationship [8] as well as the available molecular orbital calculations on transition metal halides [9,10] have prompted tis to reexamine the situation for the high spin, anhydrous, ferrous halides. An earlier report of a &S-IS correlation for Fez+ compounds [ll] was later criticized 1121. The extent of the 3d delocalization in ferrous compounds may be described by the parameter a2 E (r3)/(r3)o where (r3> and (Y-~>, are proportional to the charge densities in the solid and free ion, respectively. In high-spin ferrous compounds QS is a strong function of (~2 and is given, according to Ingalls 1131, by QS = ($)e2dl

- R)(Y~{Y-~),

F(al,

4,

dXo,

7’) (1)

where Q is the quadrupole moment of the nuclear * Supported by the U.S.Atomic Energy Commission. ** A.E.C. Special Fellcw, 1965-8.

Sn, Sb and Te compounds.

the IS in fer-

excited state, (1 -R) is a Sternheimer antishielding factor, X0 is the spin-orbit coupling constant for the free ion, 81 and AZ give the separation of the three tzg levels and T is the temperature. Es. (1) ignores all lattice contributions to the electric field gradient (EFG) at the iron nucleus. If this assumption is justified and if differences in the effects of crystal field splitting (Al, AZ), spinorbit coupling and temperature are small in a given series of compounds, then QS should increase approximately linearly with m2. In the interpretation of QS results. compiications arise that depend upon the crystal field splitting. Very small distortions from octahedral symmetry suffice to remove the degeneracy of the tripIet tzg level. In a rhombic field both 51 and A2 are non-zero and the deviation of F in eq. (1) from unity depends upon the relative magnitudes of its four arguments. For the case where (A/(u2Xo) ‘> 1 then F - 1 as T - 0. On the other hand, axial distortion alone only partl?lly lifts the tag degeneracy and a singlet and doubkt result. If (y2Xo is small and the singlet is luwea (Al =A$, F - 1 as T - 0; ifthe doublet is Lower (Al =0) then F - 0.5 as T - 0 1141. Ln a compacison of 3d delocalization in a series of compouxis that include axial only-and axial plus rhombic fields, the results must be normalized by doubling the observed QS for those compounds in which a doublet tzg level lies below a singlet We have performed careful measurements on the anhydrous halides of divalent iron: Fetz. FeBr2 and F&l2 in the paramagnetic phase ***. **I Footnote see next page. 613

Volume

2. number

8

CHEMICAL

PHYSICS LETTERS

December

1968

Data for Fe??2 were obtained from similar work !3y Sahnsaa and Dish [15)_ Ac&O>rding to Xanamuri, FeC12 and Fe12 have identical t2g orbital

.W&em=%wi% #s? dwW& Igir>g IzWer :>q. kpparentlg no data are available for FeBr2 but this compound is expected to be similar [l?] and we &tam consjsren2 r~?.Su2Ls Wit-hthe assumption tlrat if aIsso has a dnuhry de<~%erdte loz?er IeW%L %eccause 01 tie rurj1.e S~UC~II-.E?of FeFz. the three tag levels are split in this compound [18]. The studies were performed on reagent grade cchemicaSs, Femz 35 easjly c0z&?zzz~z?~ateb by atmospheric water to eiue FeCLZ-K20 but a subsidiary experiment established that values of IS(T) an& GqT;s fw -w Fiz_qi =-?I$_-&-I-I2.&F- ??mve unaffected within experimental error by the pre%EnW..‘@ &?zW~ L%EGWz~.& L%Y? G&?~ lLRmrpocrrrd [19]. The Mijssbauer spectrometer, which has excellent long-term stability, is described elsewhere [20]. Detailed thermal shift measurements over the rage 25 - 3%3% permitteti ‘the determination. within * 0.005 mm.“sec, of 61s which is the IS corrected for the second order Doppler shift and zero-point motion. This procedure removes substantialiy all lattice dynamical and temperature effects from the isomer shift [21,22J. The source was 57Co in a palladium foil and values for 61s are reported with respect to natural iron at room tcemperalbre. The mezsuremetrts of BS(T> ha ~‘e Lz probable error within f 0.005 mm ;sec. Fig. 1 is a plot of 61s for the ferrous halides versus the Pauling electronegativity [23] of the halogen atoms. As mentioned above, the AuX8 series [5]j’ discloses a similar linearity as do the data reported for Snq2-. SbXk- and TeX,$,[7’j’zrm’for ah% (I?{ (‘iY = r”. Cf. Sr, I). The respective u bonds involve 6~2 orbitafs in the aurous series. 5~5~3 for SnX4. 5s5p3Ed2 for the octahedral compounds of Sn, Sb and Te and 3d24s4p3 for the FeX2 series. In the latter case tile 3b electrons are strongly delocalized even in FeF2 [18] and the delocalization increases in the order F -:: Cl < Br x I, i.e.. in the nephelauxetic series [26J. It is thus seen that the relative in-

*** Al) three compounds shon’ anomalous

changes in low temperature (G 2fi°K) magnetic phnse transitions. We restrict the present drscussion to the pammagnetic phase only. t Bhide et al. [5] plotted their IS result- versus ionicity rather than efectronegativity. For the range of electronegativity differences in the compounds they studied. tte ionicity and the elcctronegetivity differences are. themselves. linearly relnted. cf. Pauling [23]. p. 98. QS

6w4

at

PAULING

ELECTRONEGATIVITY

Fig. 1. The corrected isomer shift. 61s. with respect

to nntura1 iron at room temperature versus PauIing electronegativitv for the ferrous halides. Error limits on 6fS are represented hy the size of the dots.

volvement of the (n - 1)d. ns, rtp and nd electrons in the bonds of the several series may vary widely without affecting the linear dependence of 6fS on the electronegativities of the ligands. While the contribution of the s electrons to $5 is via the electrostatic interaction between the nucleus and the s electron density at the nucleus, the contribution of Ihe p and d electrons is via shieIding of the s electrons and hence has the opposite sign although comparable magnitude. The implication of the present data in the context of those previously reported [5-73 is that the separate contributions of the s, p and d electrons, the latter whether or not involved_ in_;T.hnn&. axe Qavkp?.m!~d!~~ linear with the electronegativity of the halide ligands. The indication of separable contributions from the individual electrons to 6 S bears out the conclusions of earlier work [25 i , a detailed discussion of which will be the subject of a later communication [26]. This study disclosed a nearly linear relationship between SIS and Z, the hydration number in the series FeCl2.jzH20, in spite of significant changes in the microsymmetry of the Fe2+ ion. Fig. 2 summarizes the relationship between QS and 6fs for the four halides. The tops of the bars in the figure represent QS (gOoK) while the lower ends correspond to the room temperature results: the widths of the bars reflect the probable errors in $s_ An examination of the detailed therma data curves (QS versus T) disclosed that QS is a monotonically increasing function of 6~ at every temperature in the range

Volume 2. number 8

CHEMICAL PHYSICS LETTERS

Fig. 2. Quadrupole splitting over the range 90-300% -%wsuS eorreo* -%xiin+ar*i%t %izr-&h-a %eriix %&*ee

The significance of the rectangular data points is de=s_F_&xL, in.%P,f&.1_.2s i-5&L r%tLmn&e tic &uW_WGtie observed

_

values of QS for FeC12, FeBr2 and Fe12.

25 - 300°K. Further, QS ls a monotonically decreasing function of temperature for FeI2, FeBr2 and FeCL2, and for FeF2 1153 it changes very little in the region below 9O0K (N&e1 temperature = 85OK). Since bIs is an extremely linear function of ligand electronegativity (cf. fig. 1) and a nearly linear function of QS, it seems reasonable to deduce that the major contributions to QS are from delocalization effects (cY~) rather than, say, differences in spin-orbit coupling or lattice contributions to the EFG at the 57Fe nucleus. Accordingly we have made a least squares fit of the QS (gOoK) data to give the straight line indicated in fig. 2. From analysis of ESR data, Tinkham estimates (Y3 = O-.6for FeFZrc181, If this value is used for normalization, extrapolation of the line in fig. 2 to (Ye = 0 at QS = 0 establishes the cu2 scale shown on the right side of the figure. This scale then gives (~2 = 0.34, 0.39 and 0.42 for the iodide, bromide and chloride, respectively, in the expected order for the nephelauxetic series E241Previous workers have fitted QS( T) data to eq. (1) to determine values of a2 = 0.7 for Fe12 [27] and 0.9 for FeC12 [14] in conflict with the nephelauxetic series and the present work if Tinkham’s value for FeF2 [18] is accepted. Because of the large number of parameters in eq. (1) and the paucity of QS(T1 data elnployed in the fits, the choices for cu2 were not unique [27,14] and the discrepancy is perhaps not too surprising_ It is interesting that if the value for QS (gooK) of FeSiF6.6H20 1251 is taken in conjunction with the LYEscale of fig. 2, then o2 = 0.8 is indicated for this compound in excellent agree-

December

1968

The weigth of the evidence presented here in&&?&es ibai ibere is P &DsX?EDrPs?1a%%%, aL Xeast for the ferrous halides, between Che eIectronegativity of the ligand and de!ocalization of the metal iQn’s3d elechwn.5.232ecDrreh'rir>n is crube in the sevse that it &&es not *c&n fhe riecacaEfz&ion amozz,g Ike pssjb2e mo2ecular ortim>s or. in the language of Jorgensen’s nephelauxetic effect, between the effects of central field coval&WY and s~~_m&r~ r~&rjted cova3ency >%?>. The reason for this. of course, is thatallfour halides have nearly the same mLcro structure ..- L-i-Z3 &_&i&?~ &&n&r& e tion. In a later communication 126 =T we shallre_gfur& simi?ar dat-a QR &?zesec>,es [FeC16_n-nH20]tr-4 (~2= 0, 1, 2, 4 and 6) for which the distinction can be drawn. It may be said, however, that the present results indicate surprisingly extensive 3d delocaIization, particularly for the ‘nigher ha%es. Sh~-u-e + -3) depends strongly on the efiective charge, Z*, of the transition metal ion$. the experiments thus support Basch et al.‘s recent molecular orbital calculations that give effective charges of transition metal ions that are generally well below +l. irrespective of the nominal valence [lo]. Ii is a p2easure 2~ acknowledge a helphd &iscussion of these matters with Prcfessor R. H. Herber. R. G. Persing gave expert assistance with the Mossbauer apparatus. We are grateful fcr generous grants from the Mobil Oil Corn@ny (to R. C. A.) and from the Sloan Foundation (to Y-H.). The work made use of computer facirltfes supported in pari t3y @am GJ?--S:T from the National SCience Foundation. REFERENCES [l] R. L. Collins and R. Pettit. J. Chem. Phys. 39 (1963)

3433. [Z] J.Danon. J. Chem.Phys. 41 (1964) 3378 [3] P. R. Brady. J. F. Duncan and K. F. BIok. Proc. Roy. Sot. (London) A287 (1965) 343. [4] B. W. Dale. R. J. P. Williams. P. R. Edwards and C. E. Johnson. Trans. Faraday Sot. 64 (1968) 620_ [S] V. G. Bhide. G. K. Sheno? and 31. S. Multani. Sol. State Comm. 2 (1964) 221. [6] A. Y. Aleksandrov, K. N. Delyagin, K. P. Mitrofanov. L. S. Polak and V.S;Shpinel. Zhur. Eksp. Teor. Fiz. 43 (1962) 1242. [7] V. S. Shpinel. V. A. Bryukhanov. V. Kothekov, B. 2. Jofa and S. I. Semenov. Disc. Faraday Sot.. to be published. f See ref. [27]! p. Il.

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2. number_ 8

CHEMICAL

{8] J.K. Lees and P.A. Flinn, J. Chem.Phys. 882. [9] R. Bersohn

and R. G. ShuIman,

PHYSICS

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J. Chem. Phys.

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860: Rev.Mod.Phys. 36 (1964)-352. _ 1121 A. H.Mufr, U. Gonser _ _ R. W.Grant. H. kieclerslich. and W. N. Delgass. J. Chem..Phys. 45 (1966) 1015. 1131 R.IngaIIs, Phys. Rest*. 133 (1964) A787. 114) K.Ono, A. Ito and T. Fujita, J.Phys.Soc. (Japan) 19 (1964) 2119. [15] 1~. P. Johnson and J. G. Dash, Bull. Am.Phys. Sot. 12 (1967) 378, and private communication, December 1967. [16] J. Kanamori. Frogr. Theor. Phys. 20 (1958) 890. 1171 T. Fujita. private communication, December 196’1.

lETTERS

[18] M. Tinkhhm, [19] [ZO] [21] [22) [23]

[24]

December

1968

Proc. Roy. Sot. (London) A236 (1956)

549. J_ W. Hurley Jr.. Y. Hazony and R. C. Axtmann, Bull. Am. Phys. Sot. 13 (1968) 690. D. E. Earls, R. C.Axtmann, Y. Hazony and I. Lefkowitz. J. Phys. Chem.Solids, to be published. Y. Hazony, J. Chem. Phys. 45 (1966) 2664. M. G. Clark, G. M. Bancroft and A. G. Stone, J. Chem. Phys. 47 (1967) 4250. L. Pauling. The Nature of the Chemical Bond, 3rd Ed. (CornelI University Press, Ithaca. N.Y., 1960) p. 93. C. E. SchLffer and C. K. J$rgensen. J. Inorg. Nucl.

Chem. 8 (1958) 143.

[25] R. C. Axtmann. J. W. Hurley Jr. and Y. Hazony, Bull. Am. Phys. Sot. 13 (1968) 691. [ZS] Y.Hazony, R. C.Axtmann and J. W.Hurley Jr.. to be published. 127) C. K. J$rgensen, Orbitals in Atoms and Molecules (Academic Press. London. 1962) p- 59.