Mössbauer studies of iron organometallic complexes—III

L Inorg. Nucl. Chem., 1966, Vol. 28, pp. 1001 to 1010. Pergamon Press Ltd. Printed in Northern Ireland

MOSSBAUER

STUDIES OF IRON ORGANOMETALLIC COMPLEXES~III OCTAHEDRAL COMPLEXES

R. L. COLLINS, R. PETTIT, The University of Texas, a n d W. A. BAKER, JR., Syracuse University, (Received 24 August 1965)

AImlraet--Fe57 M6ssbauer spectra and magnetic susceptibility data have been obtained on twentyeight complexes of ferrous and ferric iron. Ligands include cyanides, substituted phenanthrolines, substituted dipyridyls, halogens, water, isothiocyanates, isoselenocyanates and azides. The nephelauxetic effect of the ligands is clearly revealed in the quadrupole splitting of the ferrous and ferric complexes.

SEVERAL descriptive M~Sssbauer(1) studies of octahedral iron complexes have been published. ~-5) The octahedral complexes represent an attractive area for study; a) the high symmetry of the systems has led to several theoretical methods for treating the chemical bonding ts} and the aim of the present paper is to attempt to correlate the Mtissbauer spectra with the models purporting to describe the chemical structure. An understanding of the relation between MSssbauer parameters and structure would appear to be more straightforward in these symmetric complexes than for the less regular complexes such as ~r-bonded iron-tri- and tetra-earbonylsP -9) THEORY

M6ssbauer parameters. The spectral purity of the recoilless 14 keV y-rays from the nuclear transition in 57Fe is adequate that variations of the chemical environment produce observable effects on the spectrum. The MSssbauer spectrum is a plot of relative transmission of the 14 keV y-rays through an 57Fe containing absorber as the y-ray energy is varied. Doppler velocities of the source relative to the absorber of a few mm/sec are adequate in most cases to scan the spectral region of interest. The spectrum normally consists of one or two absorption lines but may consist of more when magnetic fields or chemically non-equivalent iron atoms are present. The centre of the absorption pattern relative to some absolute energy is termed the isomer (t~). This changes with s electron density at the iron nucleusJ 1°) ,~ =

-~

~ ze'ERe~ -- R~l[l~0)al ' - I,R0)el']

(1)

~x~R. L. M ~ B x t m R , Z. Pkysilc 121, 124 (1958); Naturwissensetmften 48, 538 (1958); Z. Naturforseh

14a, 211 (1959). (~) L. M. EvST~q, J. Chem. Phys. 36, 2731 (1962). m N. L. COSTA,J. DANONand R. M. XAVIER,J. Phys. Chem. Solid~ 23, 1783 (1962). (4) E. FLUCK,W. KERLERand W. NEUWIRTH,.4n~'. Chem. (International Edition) 2, 277 (1963). c6) p. R. BRADY,P. R. F. Wining and J. F. DUNCAN,Revs. Pure and Applied Chem. 12, 165 (1962). (6~ R. A. C o t t o n and G. W m ~ s o N , Advanced Inorganic Chemistry, Inter~ienee, New York (1962). tv~ R. L. COLI~S and R. P~Lllr, J. Amer. Chem. Soc. 85, 2332 (1963). (8~ R. L. COLLINS and R. Pel lxr, J. Chem. Phys. 39, 3433 (1963). c9) R. L. COLLISSand R. F~riH, J. Inorg. Chem. (submitted). clo) L. R. WALK~, G. K. WERTHEmand V. JACCARrNO,Phys. Rev. Letters 6, 98 (1961). 6

1001

1002

R.L. COLLIm,R. Pt~rru and W. A. BAKER,JR.

where Rex and Rgr are the nuclear radii in the 14 keY and ground states respectively, I,p(0)? is the density of wave function at the nucleus and " a " and " e " refer to absorber and emitter. It is important to note that Rex ~is less than Rgr2, so that the isomer shift is negative for increasing s electron density. The splitting (A = 2e) into two lines arises from an electric field gradient at the iron nucleus, m) which acts upon the quadrupole moment of the 14 keV state: e=

e=qQ

4•(2•-

1)

[3m = - I(I + 1)]

(2)

where eq denotes ~V[rSz ~, eQ is the quadrupole moment, I = ] and m takes on values --{, --½, ½ and a. The electric field gradient is the gradient of a vector and is hence a tensor. The e.f.g, is produced by electronic charges external to the nucleus. Thus, the Laplaeian of the potential vanishes:

0=V 0sV 0=V V = V = ~ + ~ + ~----- 0

(3)

This electronic charge distribution is also affected by the nuclear quadrupole moment. ~x2~ It is always possible to choose a co-ordinate system in which

Ox~y= ~y~z = ~zOx = 0 and we orient these axis such that

IV==l • IV.I • IV=l,

(4)

where V=, = O=V/~z2, etc. The e.f.g, is then specified by two parameters: cz3~ e q = V~=

v=-

v~,

(5)

If ~/5~ 0, the energy shift in (2) is multiplied by (1 + ~/2/3)t.tm The quadrupole splitting can frequently be reconciled with chemical structure on classical grounds. (s) Six identical ligands about octahedral iron should cause no splitting while substitution of a single ligand by a weaker or stronger bonding ligand results in splitting. ~4~ However, even for those cases in which the ligands have cubic ~xl~O. C. KJs~m~ and A. W. SUNYAR,Phys. Rev. Letters 4, 412 (1960). ~m R. S ~ E a ~ and H. M. Fo~Y, Phys. Rev. 102, 731 (1956) and references therein. {xs~M. H. Cotton and R. Rmr, Solid State Physics 5, 321 (1957). u~}T. P. DAs and E. L. HAHN,Nuclear Quadrupole Resonance Spectroscopy. Academic Press, New York (1958).

Mt~ssbauer studies of iron organometallie complexes--IH

1003

symmetry, non-binding electrons may cause splitting when a sub-shell, such as the Tes 3d-electrons, is partially filled. Ultimately, the electric field gradient must be described by the distribution of electronic wave function about the nucleus:

/

eq = e \ 3

cos 2 0 -- 1 \

ra

//

(6)

Octahedral bonding. Formally Fe 2+ has an electron configuration of ls 2, 2s z, 2pe, 3s', 3p6, 3d e. Only the 3d-dectrons participate in bonding in Fe ~- complexes with the other 18 electrons retaining the closed shell configuration of argon. The major theoretical treatments of octahedral complexes are based on the effect on these 3d-dectrons of electrostatic interaction (crystal field theory) or covalent bonding (molecular orbital theory) with the ligands. As will be shown later, the crystal field treatment is inadequate to account for the Mt~ssbauer spectral results. The molecular orbital method begins by classifying the iron 3d wave functions according to the symmetry of the octahedral groups. Linear combinations of abonding ligands of like symmetry are chosen and from these metal and ligand orbitals are constructed the molecular orbitals. Electrons from the ligands then occupy these molecular orbitals; the extent of "metal" character these electrons adopt is described as forward co-ordination. "Back donation" refers to the extent of off-loading of certain metal d-electrons into available unoccupied orbitals of the ligands in an attempt to preserve electrical neutrality. Electrons in molecular orbitals formed with strong-bonding ligands tend to acquire large "metal" character while those in weak-bonding ligands retain much "ligand" character. This extent of "metal" character for the Alg molecular orbital, which involves the metal 4s atomic orbital, largely determines the isomer shift in the M~Sssbauer spectrum. It is precisely this "partial metal character" which is needed to properly account for the isomer shifts with ligand strength. The electric field gradient is determined by the order of occupancy of the molecular orbitals in addition to the metal character of the orbitals. It is important to note that the Mtissbauer results are applicable solely to the ground state of the molecule, whereas optical measurements measure the differences between excited states and the ground state. EXPERIMENTAL The MiSssbauer spectra were taken using a constant acceleration electromagnetic drive. The source

consists of 5 mc of 57Comelted into copper and rolled to 0.001 in. thickness. The source was operated at room temperature. Samples were formed into 1 in. diameter disks with hand pressure and weighed from 150 to 500 mg. A Dewar flask with berylliumwindowsallowed spectra to be obtained at 770K. The 14 keV y-ray was detected with a NaI (T1) scintillationcrystal and photomultiplier. A R.I.D.L. 400 channel analyser, operated in the time mode, was used for data accumulation. The analyser was stepped from channel to channel at a constant rate of 400 channels/secby a crystal controlled oscillator. The triangnlar voltage which determinesthe velocityof the source, through a high-gainnetwork of operational amplifiers,is derived directly from the analyser.~xS~ The spectra obtained consist of one spectrum in the first 200 channels and its mirror image in the second 200 channels. The zero of velocity is not readily determined; the calibration of the channel numbers in terms of velocity is, however, readily achieved using an enriched 5VFemetal absorber. A {m E. KANKELE1T,Rev. Sci. Instrum. 35, 194 (1964).

1004

R . L . COLLINS, R. Pt~rl~r and W. A. BAKER, JR.

16 nag absorber of powdered STFe,bound to a beryllium disk I in. in diameter with Duco cement, is very useful, t16) The STFemetal spectrum appears from a separate experiment to be centred at --0.168 mm/sec relative to the 57Co -Cu source. The reproducibility is nominally +0.01 mm/sec. An iron calibration spectrum is run both before and after each compound. The isomer shifts are reported in relation to the centre of the iron spectrum. DATA

The compounds studied are listed in Table 1. All of the compounds were measured at liquid nitrogen temperature and some were also measured at room temperature. The pattern of this data is made more apparent in Fig. 1, in which the quadrupole TABLE 1.--MOSSBAUERPARAMETERSOF OCTAHEDRALIRON COMPLEXESAT 77°K No unpaired electrons Compound

tSFe(mm/see)

A(mm/sec)

1. Fe(II)(phen*)2(CN)2 2. I~Fe(II)(CN)¢3H20

+0.199 --0-016

0.60 0.00

3. Fe(II)(dipyD~(CN)~.3H~O

+0.191

0.61

+0.341 +0.325 +0-341 +0.333 +0.334 +0.311 +0-346 +0-323 +0.373 +0"334 +0.334 +0-350

0.23 0.39 0.19 0.18 0.19 0.00 0-24 0.24 0.61 0"19 0.19 0.23

+0.100 +0.067 --0.102

1.67 1.76 0.27

+1.000 +1.025 + 1.052 +1.008 +1.009 +1.240 +1.013 +1.077 +1.017

2.88 3.15 2.80 2.82 2.81 3.05 2.61 2-40 1.50

+0.573

0.63

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Fe(II)(phen)8(C10~)~ Fe(ID(dipy)8(C10,)2 Fe(II)(5-NO~-phen),(CIO,)8 Fe0I)(4,7-diCHa-phen).(CIO,), Fe(II)(4,7-diPh-phen)8(C104)2 Fe(II)(4,4'-diPh-dipy)s(CIO~)2 Fe(II)(5-Cl-phen)3(CIO4)s Fe(II)(4,4'-diCHa-dipy)a(C104)2 Fe(II)(4,7-diOH-phen)a(ClO4)s Fe(II)(5-CHa-phen)a(C104)2 Fe(II)(5,6,-diCHa-phen)s(C104)~ Fe(II)(5-Br-phen)8(C10,)2 One unpaired electron

16. Fe(III)(phen)3(C104)s 17. Fe(III)(dipy)8(CIO~)8 19. KsFe(III)(CN)s Four unpaired electrons 20. 21. 22. 23. 24. 25. 26. 27. 28.

Fe(II)(phen)2(Na)a Fe(II)(phen)2C12 Fe(II)(phen)ili Fe(II)(phen)2(SCN)8 Fe(lI)(phen)sBr~ Fe(II)Cll.4H20 Fe(II)(phen)a(SeCN), Fe(II)(dipy)2(SCN)a Fe(II)(2-Cl-phen)8(C104)a Five unpaired electrons

18. FeOIDCI3"6HIO

* phen = 1,10-phenanthroline I" dipy = dipyridyl ~xe)R. S. PRESTON,S. S. HANNAand J. I-I~BERLE,Phys. Reo. 128, 2207 (1962).

1005

M6ssbauer studies of iron organometallic complexe, s--III

splitting (A) is plotted against the isomer shift relative to metallic iron (~). Representative spectra are illustrated in Figs. 2-8. The difference in spectra obtained upon replacement of two weak ligands (C1-) by strong ligands (CN-) is shown in Figs. 2 and 3. The small splitting of the low spin (diamagnetic) complexes is illustrated in Figs. 4 and 5. Oxidation of the Fe(II) to the Fe(III)(phen)33+ complex results in a large increase in splitting as seen in Fig. 6. This i

I

I

I

I

Io21

I 025

02O 2 4 , 2 3 0 o 22 O26 027

2

-

0 17 016

& mm/s

028

012

3~)1

018

O5 019

II ~

I

o -0.2"

o9

0.2

0

4,13,14,15

7 " " ' 6, O, lO

I

I

I

0.4 0.6 0.8 mm/s relative to iron

I

I

1.0

1.2

FIo. 1.--M/~ssbaucr parameters of iron octahcdral complexes at --196°C.

.~....... ". ...'"...'.'.'"'"'"'"'"'".''"".'... ...

.............."."..""".'"'".'.."..........

~s

I

l

-3

-2

-I 0 Velocity in m ~ s , relative to iron

FIo. 2.--Fe(1D(phenanthroline)~C1, at -- 196°C.

1.4

1006

R.L.

COLIaNS, IL I ~ r r i r and W. A. B ~ ,

_u~............i........'" ....'.....'." ..',.".'..'" .'"'.".'....

.,.

JR.

.,........"'".'. ".,' .'.....".'"..'..... " . . . . . . . . . . . . . .

,.".~.'.',.,

g N

m n m I0 m

I

I

I -I

0

Velooity

in ~mb/s, r e l a t i v e

to iron

"

FIG. 3 . ~ F e ( I I ) ( p h c n a n t h r o l i n e ) , ( C N ) , at -- 196°C.

.......,...'..".....'...'..."." ,. • ....'....'...,...,..... .......

...

o~

~s ......

I

I0

1

-I

I

I I

0 Velocity

in a~/s,

relative

to

1~o. 4.--Fe(ID(4,4'-diphenyl-dipyridyl)s(C10~) at - - 196°C.

., ..,'.,'..

o~ 4'

~s m m

..

I

,

I

-I

,...,.

,

0 Velocity

in ~ s ,

I !

~elatlve

to iron

FIO. 5.--Fo0I)(5-nitro-phenanthrolinc)a(ClO4)s

at -- 196°C.

I

MOsstmuer

. .... o~

--

...,.'""'" ..

,...,.....'...., "'""'"'"' "'"'"'"/".'...." . ' . . ,.

.'" ',.

m

studies of iron organometalfic c o m p l e x e s - - H I

1007

.~. ..,.,' ......'..'.. "...'"',..""..." ".

/ ,..,."

o 5

) -|

o Veloolty In mm/sp r~latlYe to ll'on

I

FIo. 6 . - - F e ( I I I ) ( p h e n a n t h r o l i n e ) s ( a O , ) s at -- 196°C.

...' '. ,.,'"'"'""'" .".'..' ,...' ".....'"..'.. '. '. './..'. '..'. o.,..~'..' ..,.. ,..'

.,...

5--

i° ,

-!

j

i

o Velooit¥ in w~/sj ~lati.ve to

I

i

I~C. 7 . - - I ~ F c ( I ~ ( C N ) ¢ 3 H , O

at --196°C.

,...,,.,'...'..'.".'"",'"'..'.'-'",..",*.....- .. ,.... ". ..j. -. .,.'

• ....

). ) ..,,,"'""...."

iO

J -I

a

I

,

[

o

Velooit7 in m~/8;

m ~lat~v~

Fxo. 8 . - - K s F ~ I I I ) ( C N ) e at

t o :~'.on

--

196°C.

1008

R.L. COLt.n~, R. P~rxar and W. A. BAKER,JR.

increase in splitting is much less upon oxidation when all six ligands are CN- as is shown in Figs. 7 and 8. DISCUSSION The compounds as plotted in Fig. 1 tend to group about three positions: (1) the Fe(II) A~(B-)a compounds with four unpaired electrons group at 6 + 1.0 ram]see and A ~ 2.7 mm/sec, (2) the Fe(II) A3a+ compounds are diamagnetic and group about 0 ~ +0-33 and A ~ 0.25, and (3) the Fe(III) A3~- compounds occur near 0 ~ +0.08 and A ~ 1.7. Aside from ferrous and ferric chloride, the deviations from this trend are the cyanide complexes Fe(II)(phen)a(CN-)a and Fe(II)(dipy)2(CN-)a.3HaO, which are diamagnetic, and the potassium ferro- and ferricyanides which exhibit a reduced sensitivity of A to change in oxidation. The magnetic properties of the compounds Fe(phen)a(SCN)2, Fe(phen)a(SeCN)a and Fe(dipy)a(SCN)a have shown a curious temperature dependence in that the moment at liquid nitrogen temperature is drastically lower than that at room temperature. ¢x7) This change, however, is not paralleled in the M6ssbauer parameters; the M6ssbauer values for these complexes at liquid Na appear to be essentially the same as for the other high spin Fe(II) complexes reported here. The varying sensitivity of ~ and A to oxidation suggests that the electrons about Fe have variable "metal" character which cannot be adequately described by crystal field theory. The compounds of groups (2) and (3) are essentially oetahedral except for small strains imposed by the bidentate ligands. The group (1) compounds are approximately octahedral, however, the detailed stereochemistry is not known for certain, although certain evidence suggests that the ligands are cis. ¢17) Fe(II)(2-chlorophen)3$+ is a high spin complex and except for a smaller quadrupole splitting lies with the group (1) complexes. The M.O. theory of octahedral complexes provides a basis for discussion of the data. Figure 9 illustrates the pattern of the levels for both weak and strong ligands. The paramagnetism of group (1) compounds is evidently caused by low-lying antibonding Eg* orbitals which fill by Hund's rule. The strong ligands raise Eg*, and so the Tag levels become filled and have zero net spin. In the ferric complexes with strong ligands, one Tag electron is removed and this hole in the otherwise symmetric group results in the observed splitting. The extent to which this hole results in an electric field gradient at the iron nucleus depends inversely on the extent of back donation of the Tag orbital electrons to the ligands. With ferro- and ferricyanides extensive back donation occurs. These T2g electrons have less metal character and consequently have a diminished effect on the splitting. The relative importance of various metal orbitals for effeeting splitting is readily calculated:

V,,le

/ 3 cosa 0 -- 1\ r" /

=\

=- (3 cosa 0 --1)(r ~ ) nvj W. A. BAKER,JR. and H. M. BoeomcH,/norg. Chem. 3, 1184(1964).

M6ssbauer studies of iron organometallic complexes--III

! Molecularorbital

Metal

~p

I'

I/

1009

Ligand

Eg

4s 3d

\I

\~ StrongIIgands

I ]

4p

Tlu/J (a) Diamagnetic I,,

t I~

(/

~8

~I JJ

Eg*

3d

\

!i~

//~Weak llgands

I~-{' I

Alg#///

r

FIe. 9.--Molecular orbitals of oetahedral iron 01). which results, for hydrogenic orbitals: 8:

Pz:

0

~
p= =

d,:

d,, = dy=: dx, = dx,_~,:

$(r -S)

{(r -S) --¢(r-S)

Thus, a filled 4p shell results in zero splitting, as does a spin-unpaired half filled shell. The 3d-orbitals form two subgroups: E~* contains d, 2 and d~,,_,,, which yield zero splitting when filled or half filled, and T2E which contains d=,, d,, and d~, and which likewise yields zero splitting upon being filled or half-filled. An electron in a molecular

I010

R.L. COLHNS,R. Pgrrrr and W. A. B ~ g , JR.

orbital of partial metal character will cause an e.f.g, somewhat smaller than that of a "metal" electron. The tendency ofligands to expand the T2gelectrons, e.g. to diminish the metal character has been termed the nephelauxetic effect.~e~ The extent of this effect is revealed by the relative change of splitting on oxidation from Fe(II) to Fe(III) for various systems; the larger nephelauxetic effect leads to a smaller change of splitting. The ligand strength also directly affects the isomer shift. Phenanthroline might reasonably be expected to form a stronger bond than does 2-chlorophenanthroline, and the metal character of the Alg electrons would be correspondingly greater. This increased s electron density is revealed in the isomer shift, this being more negative with phenanthroline ligands. Removal of a Tag electron upon oxidation from Fe(II) to Fe(III) results in an isomer shift to the left (see Reference 9). Several interesting correlations emerge between isomer shift and quadrupole splitting for the substituted Fe(II)(phen)ss + c om p l e x es. The total splitting is small and a computer curve-fit program is needed for reliable values of A. This is now under development. Aeknowledgements--Wo are pleased to acknowledgesupport from the Donors of the Petroleum

Research Fund, adrnini.~teredby the AmericanChendcalSociety,The Un/versityof Texas Research Institute and the National ScienceFoundation.