- Email: [email protected]
Sign of the electric field gradient tensor and the nature of the low temperature phase transition in Fe(pyridine)2Cl2
.;
._;
.. '.
'. :
CHEMICAL PHYSi&
- :- ,‘.
‘Volume ia,‘number,I .. ,. :' :. v: .,, ..,~.
: .,: -. ~ -, I.
LETTERS
..:
'. . ', ..
:
: ..,_.
1 Septerpber1974 : ., :
.'. ., :
:
:
-.. _
:. -s~GN,oFTm&Ecnuc
.,
,THE
FIELD'GRAD~NTTENSORANDTHENATUREOF. LO!~TEMPERATUREPHAS~T~,SITION-IN Fe(PYRIbINEj&l;~ WM. FEIFF”
~
Dep&nent
. ‘;-.: ‘.
of C’hemi.stry, .Northeostern
Uniuqsify;
‘. Boston? Massochrrsetts 02115,.
USA
R.B. FRAkKEL Francis Bitter National Magner kboratbry, Massachusetts Institute Combridge, Massmhketis 02139, USA
,,.
of Techrrology.
and B.F. LITTLE and Gary J. LONG* Depaittnent
‘.
of Cfjemistry,
University of Missouri-Roll?.
Rolls, Missouri 65401,
USA
Received 25 January 1974 Revised manuscript received 29 April 1974
.,
Massbauer spectra of Fe(pyridine)gClz have been determined in large external magnetic fields at room temperatube and 78 K. The principal component OF the electric field >adient tensor is positive and consistent with nn electron .in the d+ drbital. The results.indicate no change in orbital ground state for the low tempctature phase transition of . . this material.
.,
‘. .ne
chloro-bridged
polymeric
octtihedral
transition from a room temperature $-nm~tricaL)r bridged species with a small quadrupo!e splitting to a low temperature asymrnetricajly bridged specie? with a laiger quzdrupole-splitting. The temperature region over .which there are four linei in the M&sbauer spectrum corresponds to the&existence of the orbital gound statec of both of these fcirms;‘ther’e is no change in qlin-rnultiplicity.at an$ temperature. We.6 lustrate this k fig. 1, where the $nunetric structure corresponds to tiiaf:obtained crystallogaphic$ly [2] for F]o(p~)~q~ and the asynG&c-structure correspmds to Cat found [2] for Circle. : I
complex.
‘,bis(pyridine)dichlor6iron(II), Fe(py)zCIS, has been the su$+ect of several recent investigations. A detailed study ,of this complex by Long et al. [I] demonstrated -‘that it contains high-&iron(H) with m.itidication df feridmagnetic bef?avior at 1oW temperature. Qis ,. sttidralso revealed that..Fe(py)2C12, when prepared by vacuu+ theJmoly:is of Fe(py)4C12, exhibited a ... four line ?+sbatier spectrum at intermediate tempera-. t’ures (1195 ind 78 K), which is iiidicatiie 6f two ‘. quadrupqle’split dotibkts; only o& koubiet is ob- ‘. ‘served at .room tempkrature. A direct preparation of Fe(py!aC12-iti
&l_ution yielded
a material
&&ibitina
a..,: .:- _-‘- ‘In a-subsequent
study,
Sanch:ez
et al. [3] deter-
:
“mined the detailed.temperaturedependencebf the_
‘singlequadrupoledoublet at both room,tergpera.tu:e
,.
and 78 K.lme MBssb#uer.sFec‘tral parameters at both the$e’tempekatu& are eskentiallj’the’same as tho& found in the' fo&&ie spectrum mehtioned.above. ‘. The&:results v+i@preted [l,]: in termsof a phase:
Mijssbatier’.;pedtruti of Fe(py)2C12 prepared in solu-. tion: They found ‘that.the_above.me?tidned $ynirnetrjc~. ’ and asymmetric &orb-b&lged ftims coexist in the :, solid State’i,v.er-thk,kmperature range of,i35 to 217 K;
..’
/.;,:
.0 A.<{hors t’6 kljok ‘, .I,.
,‘.
.:.
”
correTonaen&
$o”id
be addressed.
:;.
:.:
,Volume 28; number 1
1 September 1974
‘CHEMICAL &+SICS.LE~~EIIS
‘.
: m-.-y
S)mmclric
Bridging
Asymmetric
:Lk-..-_
.____ ____L-___-.._
-__-.__
‘Bridging
;: ii Fig. 1. Schematic represeritations of the symmetric metric chtoro-bridged forms of Fe(py)zClz,
tid asym-
at ca. 150 & in the emission spectrum bf 57Co(py)2C12. This is the saine temperature at which an abrupt change of sign in the crystal magnetic anisotropy of Co(py)2C12 is pbserved [4] _ In this letter we report the tempera&e dependence -of the &ssbauer spectrum of Fe(py)ZCfZ, measured in a large external magnetic field. We have undertaken c 3 ” , this study to better understand the nature of the phase VELOCITY (rnml’secf transition and to ascertain whether 0; not a change in Fig. 2. The Mtissbauer spectrum ofFe(py)$12 at 30C,R wiih orbital ground state accompanies the structural trans(a) H= 0, a&(b) H = 4.5 kG. The soIid line is a computer formation..The sample of Fe(py)2C12 was prepared in s~~lation with H = 45 kG, q = 0 and &!SQ = 0.53 mm(s. solution by the method A,reported previously’ [I]. . The ,@issbauer spectra obtained in a zero and a non/ ,-i’&L 1 zero external magnetic field at room temperiture and A .**% $?o=*. I at 78 K are presented in figs..2 and 3. The Mijssbauer -
These
re$ts_may.be.i+~rstpod’in
telrnis df thd
,’
‘_.
-. ‘. ..‘... relative bondingability of the four bridging’&&&e; VEL0CIT.Y [rk-ri/zc~ ” : ,,,-. ,__ .’ ,,., at&-m and t-b. ~y~d~e-nitr~~en atoms bonde&to. :: : :,’ ,. ‘.... ‘:: ” _,( the cent@ iron(U) itin, ynd in ‘teims,of &e lcical‘sym-.::.. Fig. ?: ‘_The?&issbau& sgtitiup of Fee@Yl$2Iz at ?8 K,witii :mktiy about the iron;.As a.ligand, pyridtie is much .’ (a) H s.0, and (b!H = 30 kC; ‘I”tts &id line. is a.computkr wi?.e.= 30 kG, q = 0, and tin ~!:26 r&ifs. ‘* higher in’tfie &eeirqchem@aI se&eS than is’the,chJori(ie. ‘. fruition .. .. ...
:.,
..,( ‘,. -‘. *.
‘,.
,,.
,_‘;.
,..’ -.; ,__, ..‘L..’ ,: .. ,_, ,:., ._ .. .,
:.
,,
..
,.
,,
_; ,/-
.:,,,
..‘: ‘,.., /‘.._ .‘.., .. .. ..’.. :.: .” ,. _-.,, ,.:‘:. . . .’ ,, -,: __-_ ,. .., .,, _’ -’ .“,.,. :. ,; .?I ..’ ., ,,,I_’ .., ‘.., :s: :,_,... . ‘<.. _,.,: : :: : __ ,. .’ .
.
:y_ . . : -:, -., ;,, .-, :, _~:,
‘69
..’ ”
:
‘, _‘.. .._.
._.. ; .. . :
Volume 28,‘number 1.
CH~MlCAL~HYSJCSLEl?ER~~~;~.,
ion Ii]. Thus it is not su~rising-that ii Co(py),Cl,, the cob~lt-chlerine distzr.cC i2] is-?.49 8,while the .:cobalt-nitioneh distance is%‘iiJ A, Similar results are .; .obtain’ed (21 for Cu(p~)~Clj in which the copperchlorine’distances are 3.05 and 2,28 A and the coppernitrogen.distance.is 2102 A: The room temperature ,forin ofFe(py)2C,12 is iscmorphous [I] with, C~$py)~Cl;, and it is rkasonable to assume shorter iron-nitrogen.:than iron-chlorine bond lengths in the .iron.complex. The pyridine ligan’ds in each of thesecomplexes are trans and the angles between the bonds to the metal are all ca. 90”. Thus in Chicle and --the room temperature form of Fc(py)2C12, the local symmetry of the coordination environment is approximately D,,. From a’crystal field point of view, the.tetragonal compression of the octahedral ligand ‘. field by the frms pyridine ligands destabilizes.the dXi ‘ami $2 orbitals relative to the dXY orbital, which becomes the ground state orbital. For this ground state orbital; a positive quadrupole coupling constant is expecteg [6], and is observed for the complex under investigation. If the low temperature form of Fe(py)iClz is analogous to Coracle, in which the .coppei-chlorine distances are no longer all equivaient, the local symmetry about the metal ion is Dzh. How-
(
..v
ever, the complex will still have its strongest ligand field along the pyridine-metal-pyridine axis, and VZZ, .-although large;, should still be positive and correspond to a d,,, ground state orbital. Our observation of a positive Vi, at both room temperature.and 78 K sup-
:’ ..
:
:
I,
1’ . .’ ..
.’
.-1 September.1974 ....”
.irrfrared,and visible spectrum.of Fe(py),Cl, suggests that signia and pi effects’are, probably not readily’ separable. ” Finalht,it-is also possible,th$ the structure, and symmetrj, of the iow. temperature form of Fe(py)2C12 are sigriiiicantly different from that of Cu(py)zC1z’ such that there.is significant inixing of the t ?g and eE orbitals co;riponents. This woU;d reqilire a drfferent descripticn of the orbital grounc! state than the simple one we hnWproposed_ However, consideration of such a possibility must await detailed !ow temperature crystallographic studies of Fe(py)?C12. On the basis_‘ofthe nearly two-fold increase in the ,quadrupole splitting observed for Fe(py)zCIZ at 78 K as cornpar-ed to room temperature, the change in quadrupole splitting has been interpreted [3] as car-. responding to’s transformation from a ground state orbital doublet (d,,, dy,) at room temperature to a ground state orbital singlet (d,) at low temperature. From a simple crystal field point of view, this is reasonable, because the quadrupole splitting of the doublet is one-half that,of the singlet [lo]. However, for such a transformation, the sign of V,, should chge [ 11, 121, contrary to the results of our work. In addition, there ia the inescapable fact that the runs pyridine ligands produce a strqnger ligand field and this field would be expected_to destabilize the d,,
and d,, orbitals at all temperatures. Rather than a
change in orbital ground state with temperature, the increase in quadrupole splitting with decreasing temperature observed [I] in.Fe(pyjzC12 seems more ports the foregqing analysis. compatible with a larger low-symmetry component in One can also attempt to analyze the resuits from a molecular orbital point of piew [8], If differential pithe ligand field for the low temperature asymmetric bridged form of the complex, and -of the same sign bonding effect-s are more important than sigma-bonding effects; then the dxz,yz pair can become the ground for both forms. In spite of the larger low-symmetry orbitals. That is, pi donation by the in-plane chloride component at low temperature, both fomls of the cbmpldx have.the same orbital singlet ground state .. ions and pi delocalization by the axial pyridine molecules can act in concert to stabilize a. 5E.ground term (cl,;,) and hence, the same sign of VZZ:It is worthcomptised’@r&ily of d,,,;, and for which V,, is : while to point out-that,in an analysis of the low tern-. ’ negative. ,This does not appear to be the case’sinck V,, ..’ perature (9.2.K), magnetically hype&e split spectrum ofF~(py)~‘Cl~, V,, is indicated.to be positive ,, is observed to be positive. Thjs approach,is one in[3]. In further supp.ort of,the foregoing interpretation; : v&rig separation of sigma- and pi-bonding effects, the magnetic anisotropy i:e!, the f$ orbitals are influenced only, by differences : : the temperatumdependence-of _: for Co(py)2,Cli can best be interpreted with one sense i;l pi bo:riding,while tile eg orbitals are affected only y.by- differences in sigma bonding. Such an approach.is ‘.‘, (negqtive) to’the tetragoriality [4] 1This-corresponds valid_only ,&,;t_hehit of sniall distortions from cubic : ;: to a tetragonally,compressed octal@ral ligand field. (Oh),symmetry, [9]. !Jl& iarge.s$itting [I] -(ca.‘The larger;lo.~s~m~try~~gahd~field,component in 39Wcm+~).pf the es orbitals derived fromthe near _‘.:‘:. .-the loti‘ternperature @it;) form &,y have as its origin
; II.’ ,. -; _’ __I ., : ,: ,,;yo_ ~.:~.;;r.:,,‘;,:] :,,_ _ ,..,.
,:_. ;, ;
: .,-I
: ., ‘1
,;. _; ; .‘,
:, : ..
.‘.:
1 September
LETTERS
CHE~WALPHYSICS
an even *.&ak& in-plane bonding relative to the axial bonding when, the. iron-chloride bridging bond distances become asymmetric at low temperature. Thus .for &xh.rnplethe average metalTchldrine~distance in WPY@‘2. @2fJ IS . somewhit longei than t& corre.. spending distance in’Co(py)ZClt . ‘The inequivalent iron_chIorine distances in thk low temperature form are expected to resuit in, a nonzero asymmetry parameter n. However, it is difficult to estimate the ma~itude of 17without an elaborate calculation extending beyond the first coordination sphere. As mentioned previously, simulations of the data in fig. 3 were acceptable with non-zero but reIatively small values of rl (GO.4). In any event the increase in quadrupo!e splitting from an increase in 7 to as large as one (wk.ich does nbt occur) can increase the quadrupole e,Lfect by only of the order of 10%; much less than rhe observed increase. A previous Pfassbauer spectral study [3] indicates that the structllral transformation involved takes place over a teInpe;&ure range of about 18 R for Fe(py]ZCIZ prepared in solution, and about 3 K for. 57Co(py)2C12. As a resul?, one might consider the transformation to be second order because the transition is not sharp. However, the temperature dependence of both the Mijssba~ler spectra [3] and the magnetic anisotropy [4] shc,w hysteresis which is more characte’ristic of a first order than a second order-transformation. A more definitiye and reliable analysis of the order of this phase.transition must await more detai!sci thermal measurements. A latent heat is expected il’the process is indeed first order.
1974
W.M. Reiff $ pleas’ed to acknowledge the support. of the Research Corportition’and the NationaI Science Foundation [Grant Number GH-39010). B J. LittIe, would like to thank NSF for a FacuIty Science Fellow‘ship. G J. Lang would like to’ thank NSF for Grant Number GP-8653. The Francis’Bitter NationaI Magnet ‘Lboratory iS supported by the Natiohai Science ‘Foundatiw.
Reference&
,.
[l] G.J. Long, D.L. ~~~itney and J.E. Kenrredy, Inorg. C:KXTL10 (1971) 1406. [2] j.D: Dunitz, Acta Cryst. 10 (1957) 207. 131 J.P. Sarxhez, L. Asch and J.&l. Friedt, Chem. Phys. Letters 18 (1973)250. [4] R.B. Bentley, BI. Gerioch, 1. Lewisand’P.N. Qtiested,J. Chem. Sot. A (1971) 3751. [51 R.L. Collins, J. Chem. Phys. 42 (1965) 1972. 161 R.L. Collins and J.C. Travis, &Iiissbaucr Effect hlethodo(ogy 3 (1967) 123. L71 F.A. Cotton and G. Wilkinson, Advanced inorganic chemistry,‘3rd Ed. (Wiley-interscicnce~, New York, 1972). ra1 D.S. hlcClure. Advances in the theory of coordination compounds (hia~~an, New York, 1461) p. 493. (Elscvicr. 191 A.P.B. Lever, Inorga.nic electronic spectroscspy Amsterdam, 196Ei) p. 205. [lOI FL Ingalls, Phys. Rev. A133 (1964) 787. [Ill W.hl. Reiff, R.B. Frankel and C.R. Abeledo, Chcm. Phys. Letters 22 (1973) 124. Abelcdo, R.B. Frazket,J.A. Costama_rr;a, 1121 R. Latorre,C.R. WM. Reiff and E. Frank, J. Chem.‘Phys. 59 (1973) 2580.
:
:
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.
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-
._;
.. '.
'. :
CHEMICAL PHYSi&
- :- ,‘.
‘Volume ia,‘number,I .. ,. :' :. v: .,, ..,~.
: .,: -. ~ -, I.
LETTERS
..:
'. . ', ..
:
: ..,_.
1 Septerpber1974 : ., :
.'. ., :
:
:
-.. _
:. -s~GN,oFTm&Ecnuc
.,
,THE
FIELD'GRAD~NTTENSORANDTHENATUREOF. LO!~TEMPERATUREPHAS~T~,SITION-IN Fe(PYRIbINEj&l;~ WM. FEIFF”
~
Dep&nent
. ‘;-.: ‘.
of C’hemi.stry, .Northeostern
Uniuqsify;
‘. Boston? Massochrrsetts 02115,.
USA
R.B. FRAkKEL Francis Bitter National Magner kboratbry, Massachusetts Institute Combridge, Massmhketis 02139, USA
,,.
of Techrrology.
and B.F. LITTLE and Gary J. LONG* Depaittnent
‘.
of Cfjemistry,
University of Missouri-Roll?.
Rolls, Missouri 65401,
USA
Received 25 January 1974 Revised manuscript received 29 April 1974
.,
Massbauer spectra of Fe(pyridine)gClz have been determined in large external magnetic fields at room temperatube and 78 K. The principal component OF the electric field >adient tensor is positive and consistent with nn electron .in the d+ drbital. The results.indicate no change in orbital ground state for the low tempctature phase transition of . . this material.
.,
‘. .ne
chloro-bridged
polymeric
octtihedral
transition from a room temperature $-nm~tricaL)r bridged species with a small quadrupo!e splitting to a low temperature asymrnetricajly bridged specie? with a laiger quzdrupole-splitting. The temperature region over .which there are four linei in the M&sbauer spectrum corresponds to the&existence of the orbital gound statec of both of these fcirms;‘ther’e is no change in qlin-rnultiplicity.at an$ temperature. We.6 lustrate this k fig. 1, where the $nunetric structure corresponds to tiiaf:obtained crystallogaphic$ly [2] for F]o(p~)~q~ and the asynG&c-structure correspmds to Cat found [2] for Circle. : I
complex.
‘,bis(pyridine)dichlor6iron(II), Fe(py)zCIS, has been the su$+ect of several recent investigations. A detailed study ,of this complex by Long et al. [I] demonstrated -‘that it contains high-&iron(H) with m.itidication df feridmagnetic bef?avior at 1oW temperature. Qis ,. sttidralso revealed that..Fe(py)2C12, when prepared by vacuu+ theJmoly:is of Fe(py)4C12, exhibited a ... four line ?+sbatier spectrum at intermediate tempera-. t’ures (1195 ind 78 K), which is iiidicatiie 6f two ‘. quadrupqle’split dotibkts; only o& koubiet is ob- ‘. ‘served at .room tempkrature. A direct preparation of Fe(py!aC12-iti
&l_ution yielded
a material
&&ibitina
a..,: .:- _-‘- ‘In a-subsequent
study,
Sanch:ez
et al. [3] deter-
:
“mined the detailed.temperaturedependencebf the_
‘singlequadrupoledoublet at both room,tergpera.tu:e
,.
and 78 K.lme MBssb#uer.sFec‘tral parameters at both the$e’tempekatu& are eskentiallj’the’same as tho& found in the' fo&&ie spectrum mehtioned.above. ‘. The&:results v+i@preted [l,]: in termsof a phase:
Mijssbatier’.;pedtruti of Fe(py)2C12 prepared in solu-. tion: They found ‘that.the_above.me?tidned $ynirnetrjc~. ’ and asymmetric &orb-b&lged ftims coexist in the :, solid State’i,v.er-thk,kmperature range of,i35 to 217 K;
..’
/.;,:
.0 A.<{hors t’6 kljok ‘, .I,.
,‘.
.:.
”
correTonaen&
$o”id
be addressed.
:;.
:.:
,Volume 28; number 1
1 September 1974
‘CHEMICAL &+SICS.LE~~EIIS
‘.
: m-.-y
S)mmclric
Bridging
Asymmetric
:Lk-..-_
.____ ____L-___-.._
-__-.__
‘Bridging
;: ii Fig. 1. Schematic represeritations of the symmetric metric chtoro-bridged forms of Fe(py)zClz,
tid asym-
at ca. 150 & in the emission spectrum bf 57Co(py)2C12. This is the saine temperature at which an abrupt change of sign in the crystal magnetic anisotropy of Co(py)2C12 is pbserved [4] _ In this letter we report the tempera&e dependence -of the &ssbauer spectrum of Fe(py)ZCfZ, measured in a large external magnetic field. We have undertaken c 3 ” , this study to better understand the nature of the phase VELOCITY (rnml’secf transition and to ascertain whether 0; not a change in Fig. 2. The Mtissbauer spectrum ofFe(py)$12 at 30C,R wiih orbital ground state accompanies the structural trans(a) H= 0, a&(b) H = 4.5 kG. The soIid line is a computer formation..The sample of Fe(py)2C12 was prepared in s~~lation with H = 45 kG, q = 0 and &!SQ = 0.53 mm(s. solution by the method A,reported previously’ [I]. . The ,@issbauer spectra obtained in a zero and a non/ ,-i’&L 1 zero external magnetic field at room temperiture and A .**% $?o=*. I at 78 K are presented in figs..2 and 3. The Mijssbauer -
These
re$ts_may.be.i+~rstpod’in
telrnis df thd
,’
‘_.
-. ‘. ..‘... relative bondingability of the four bridging’&&&e; VEL0CIT.Y [rk-ri/zc~ ” : ,,,-. ,__ .’ ,,., at&-m and t-b. ~y~d~e-nitr~~en atoms bonde&to. :: : :,’ ,. ‘.... ‘:: ” _,( the cent@ iron(U) itin, ynd in ‘teims,of &e lcical‘sym-.::.. Fig. ?: ‘_The?&issbau& sgtitiup of Fee@Yl$2Iz at ?8 K,witii :mktiy about the iron;.As a.ligand, pyridtie is much .’ (a) H s.0, and (b!H = 30 kC; ‘I”tts &id line. is a.computkr wi?.e.= 30 kG, q = 0, and tin ~!:26 r&ifs. ‘* higher in’tfie &eeirqchem@aI se&eS than is’the,chJori(ie. ‘. fruition .. .. ...
:.,
..,( ‘,. -‘. *.
‘,.
,,.
,_‘;.
,..’ -.; ,__, ..‘L..’ ,: .. ,_, ,:., ._ .. .,
:.
,,
..
,.
,,
_; ,/-
.:,,,
..‘: ‘,.., /‘.._ .‘.., .. .. ..’.. :.: .” ,. _-.,, ,.:‘:. . . .’ ,, -,: __-_ ,. .., .,, _’ -’ .“,.,. :. ,; .?I ..’ ., ,,,I_’ .., ‘.., :s: :,_,... . ‘<.. _,.,: : :: : __ ,. .’ .
.
:y_ . . : -:, -., ;,, .-, :, _~:,
‘69
..’ ”
:
‘, _‘.. .._.
._.. ; .. . :
Volume 28,‘number 1.
CH~MlCAL~HYSJCSLEl?ER~~~;~.,
ion Ii]. Thus it is not su~rising-that ii Co(py),Cl,, the cob~lt-chlerine distzr.cC i2] is-?.49 8,while the .:cobalt-nitioneh distance is%‘iiJ A, Similar results are .; .obtain’ed (21 for Cu(p~)~Clj in which the copperchlorine’distances are 3.05 and 2,28 A and the coppernitrogen.distance.is 2102 A: The room temperature ,forin ofFe(py)2C,12 is iscmorphous [I] with, C~$py)~Cl;, and it is rkasonable to assume shorter iron-nitrogen.:than iron-chlorine bond lengths in the .iron.complex. The pyridine ligan’ds in each of thesecomplexes are trans and the angles between the bonds to the metal are all ca. 90”. Thus in Chicle and --the room temperature form of Fc(py)2C12, the local symmetry of the coordination environment is approximately D,,. From a’crystal field point of view, the.tetragonal compression of the octahedral ligand ‘. field by the frms pyridine ligands destabilizes.the dXi ‘ami $2 orbitals relative to the dXY orbital, which becomes the ground state orbital. For this ground state orbital; a positive quadrupole coupling constant is expecteg [6], and is observed for the complex under investigation. If the low temperature form of Fe(py)iClz is analogous to Coracle, in which the .coppei-chlorine distances are no longer all equivaient, the local symmetry about the metal ion is Dzh. How-
(
..v
ever, the complex will still have its strongest ligand field along the pyridine-metal-pyridine axis, and VZZ, .-although large;, should still be positive and correspond to a d,,, ground state orbital. Our observation of a positive Vi, at both room temperature.and 78 K sup-
:’ ..
:
:
I,
1’ . .’ ..
.’
.-1 September.1974 ....”
.irrfrared,and visible spectrum.of Fe(py),Cl, suggests that signia and pi effects’are, probably not readily’ separable. ” Finalht,it-is also possible,th$ the structure, and symmetrj, of the iow. temperature form of Fe(py)2C12 are sigriiiicantly different from that of Cu(py)zC1z’ such that there.is significant inixing of the t ?g and eE orbitals co;riponents. This woU;d reqilire a drfferent descripticn of the orbital grounc! state than the simple one we hnWproposed_ However, consideration of such a possibility must await detailed !ow temperature crystallographic studies of Fe(py)?C12. On the basis_‘ofthe nearly two-fold increase in the ,quadrupole splitting observed for Fe(py)zCIZ at 78 K as cornpar-ed to room temperature, the change in quadrupole splitting has been interpreted [3] as car-. responding to’s transformation from a ground state orbital doublet (d,,, dy,) at room temperature to a ground state orbital singlet (d,) at low temperature. From a simple crystal field point of view, this is reasonable, because the quadrupole splitting of the doublet is one-half that,of the singlet [lo]. However, for such a transformation, the sign of V,, should chge [ 11, 121, contrary to the results of our work. In addition, there ia the inescapable fact that the runs pyridine ligands produce a strqnger ligand field and this field would be expected_to destabilize the d,,
and d,, orbitals at all temperatures. Rather than a
change in orbital ground state with temperature, the increase in quadrupole splitting with decreasing temperature observed [I] in.Fe(pyjzC12 seems more ports the foregqing analysis. compatible with a larger low-symmetry component in One can also attempt to analyze the resuits from a molecular orbital point of piew [8], If differential pithe ligand field for the low temperature asymmetric bridged form of the complex, and -of the same sign bonding effect-s are more important than sigma-bonding effects; then the dxz,yz pair can become the ground for both forms. In spite of the larger low-symmetry orbitals. That is, pi donation by the in-plane chloride component at low temperature, both fomls of the cbmpldx have.the same orbital singlet ground state .. ions and pi delocalization by the axial pyridine molecules can act in concert to stabilize a. 5E.ground term (cl,;,) and hence, the same sign of VZZ:It is worthcomptised’@r&ily of d,,,;, and for which V,, is : while to point out-that,in an analysis of the low tern-. ’ negative. ,This does not appear to be the case’sinck V,, ..’ perature (9.2.K), magnetically hype&e split spectrum ofF~(py)~‘Cl~, V,, is indicated.to be positive ,, is observed to be positive. Thjs approach,is one in[3]. In further supp.ort of,the foregoing interpretation; : v&rig separation of sigma- and pi-bonding effects, the magnetic anisotropy i:e!, the f$ orbitals are influenced only, by differences : : the temperatumdependence-of _: for Co(py)2,Cli can best be interpreted with one sense i;l pi bo:riding,while tile eg orbitals are affected only y.by- differences in sigma bonding. Such an approach.is ‘.‘, (negqtive) to’the tetragoriality [4] 1This-corresponds valid_only ,&,;t_hehit of sniall distortions from cubic : ;: to a tetragonally,compressed octal@ral ligand field. (Oh),symmetry, [9]. !Jl& iarge.s$itting [I] -(ca.‘The larger;lo.~s~m~try~~gahd~field,component in 39Wcm+~).pf the es orbitals derived fromthe near _‘.:‘:. .-the loti‘ternperature @it;) form &,y have as its origin
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1 September
LETTERS
CHE~WALPHYSICS
an even *.&ak& in-plane bonding relative to the axial bonding when, the. iron-chloride bridging bond distances become asymmetric at low temperature. Thus .for &xh.rnplethe average metalTchldrine~distance in WPY@‘2. @2fJ IS . somewhit longei than t& corre.. spending distance in’Co(py)ZClt . ‘The inequivalent iron_chIorine distances in thk low temperature form are expected to resuit in, a nonzero asymmetry parameter n. However, it is difficult to estimate the ma~itude of 17without an elaborate calculation extending beyond the first coordination sphere. As mentioned previously, simulations of the data in fig. 3 were acceptable with non-zero but reIatively small values of rl (GO.4). In any event the increase in quadrupo!e splitting from an increase in 7 to as large as one (wk.ich does nbt occur) can increase the quadrupole e,Lfect by only of the order of 10%; much less than rhe observed increase. A previous Pfassbauer spectral study [3] indicates that the structllral transformation involved takes place over a teInpe;&ure range of about 18 R for Fe(py]ZCIZ prepared in solution, and about 3 K for. 57Co(py)2C12. As a resul?, one might consider the transformation to be second order because the transition is not sharp. However, the temperature dependence of both the Mijssba~ler spectra [3] and the magnetic anisotropy [4] shc,w hysteresis which is more characte’ristic of a first order than a second order-transformation. A more definitiye and reliable analysis of the order of this phase.transition must await more detai!sci thermal measurements. A latent heat is expected il’the process is indeed first order.
1974
W.M. Reiff $ pleas’ed to acknowledge the support. of the Research Corportition’and the NationaI Science Foundation [Grant Number GH-39010). B J. LittIe, would like to thank NSF for a FacuIty Science Fellow‘ship. G J. Lang would like to’ thank NSF for Grant Number GP-8653. The Francis’Bitter NationaI Magnet ‘Lboratory iS supported by the Natiohai Science ‘Foundatiw.
Reference&
,.
[l] G.J. Long, D.L. ~~~itney and J.E. Kenrredy, Inorg. C:KXTL10 (1971) 1406. [2] j.D: Dunitz, Acta Cryst. 10 (1957) 207. 131 J.P. Sarxhez, L. Asch and J.&l. Friedt, Chem. Phys. Letters 18 (1973)250. [4] R.B. Bentley, BI. Gerioch, 1. Lewisand’P.N. Qtiested,J. Chem. Sot. A (1971) 3751. [51 R.L. Collins, J. Chem. Phys. 42 (1965) 1972. 161 R.L. Collins and J.C. Travis, &Iiissbaucr Effect hlethodo(ogy 3 (1967) 123. L71 F.A. Cotton and G. Wilkinson, Advanced inorganic chemistry,‘3rd Ed. (Wiley-interscicnce~, New York, 1972). ra1 D.S. hlcClure. Advances in the theory of coordination compounds (hia~~an, New York, 1461) p. 493. (Elscvicr. 191 A.P.B. Lever, Inorga.nic electronic spectroscspy Amsterdam, 196Ei) p. 205. [lOI FL Ingalls, Phys. Rev. A133 (1964) 787. [Ill W.hl. Reiff, R.B. Frankel and C.R. Abeledo, Chcm. Phys. Letters 22 (1973) 124. Abelcdo, R.B. Frazket,J.A. Costama_rr;a, 1121 R. Latorre,C.R. WM. Reiff and E. Frank, J. Chem.‘Phys. 59 (1973) 2580.
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