The absorption spectrum of potassium hexafluoronickelate(IV)

JOl-tLN.\LOF MOLECULARSPECTROSCOPY 29, IO%-119(1969)

The Absorption Spectrum of Potassium Hexafluoronickelate( IV) Electronic MARTIX University

J. REISFELD,

of California,

LARNED

Los dlamos

Spectra’

B. ASPREY,

ScienliJic

~NII ROBERT

Laboratory,

,4. PENNEMAX

Los z4lumos, New Mexico

8Y544

Compounds of the formula MzNiF6 containing tetravalent nickel and the three heavy alkali metals were prepared and their electronic spectra studied. The observed peaks were assigned to the crystal-field energy levels for an octahedral low-spin d6 configuration. The crystal-field parameters for KzNiFs are Dy = 2090 cm-1 and B = 485 cm-l. The fine strrlcture component,s appearing on the ‘Al, + ‘T, transition have been correlated with data for the ground state vibrational frequencies, and t,he occurrence of a Jahn-Teller distortion is postulated for t.his level. I. 1NTl:OI)UCTIOiY There

is a large body

of information

t#he first) ro\v transit’ion understanding avaiktble

data

covering was

plexe_; in either

from

was decided and

Si(

IV).

conq)ound

The

t’o examine The

st)udies of transition

of

part, of t,he in their

metal

com-

states have been published,

In

work

complexes describes

of nickel, with emphasis the results

it

on X(111)

on the tetravalent,

nickel

&Nilie

fluoro-complex

fluorination

are a number

of the struct#ure and spect,ra of such complexes,

fluoride

present

fen

low or high valence

the course of our investigation

and there

of

for a clearer

in \vhich the ions are found

stat)es, s,nd relat,ively

abnormally

of complexes

useful

Ho\\.ever, the greatest

(I-,$)

studies

spectra

has proved

stat#es of the ions;

the subject.

derived

oxidation

on t’he electronic

element’s which

of the electronic

genera.1 reviews common

metal

Ii2SiFa

was first

of a 2: 1 stoichiometric

prepared

mixture

by Klemm

of KC11 and

nnd Huss

Sic12

(5)

at, 275°C.

by The

resulting product nas found t)y them to he cubic ( l~m:Sm) of the K21’tC:ls type’ structure with a = S.11 ,!, Z = 4. l’he mntrria! used for the present m)rk QXS prepared iu a differcut, manner. Fluorin:Ltion of a single divalent nickel complex already cont:Cng the desired

1This work was sponsored

by t,he U. H. Atomic 109

ISnergy Cr)rnrnission.

110

REISFELD,

ASPREY,

AND

PENNEMAN

ratio of I< to Ni is a preferable method for the preparation of K2NiF6, since the stoichiomek-y of the starting material is t,hereby well-defined and results, after treatment, in a pure product whose yield may be readily ascertained. Ko restrict’ions are present as to the uniformity of mixing a combination of starting materials, as occurs when one begins with a heterogeneous mixture of react.ant.s. It was decided to employ I<&( CN), as a starting material, since it is easy to prepare in high purity and its chemical and optical properties are well known; t,hese may be utilized as a check upon the completeness of t)he reaction. The material also fluorinates relatively easily \\-ithout excessive overheating. A stoichiometric yuant,ity of Ni(CiS)2 was added t,o a solution of I
MEASUREMENTS

Speckoscopic data in the range from 1870 to 36 000 A&were taken on a Cary model 14 RIR Spectrophotometer. The samples of IGNiF6 obtained by direct fluorination as well as by recryst,allization from liquid anhydrous HF were intimately ground with Fluorolube’ oil for studies in the visible and near-infrared 2 Fluorolube

M-000, Hooker

Electrochemical

Co.

ELECTRONIC

SPECTRA

111

OF &NiF,

The grinding operations were carried out, in an inert atmosphere because the compounds are extremely sensit’ive to decomposition by small amount’s of \\-atervapor. The mulls were placed between IiRr discs, ut,ilizing tantalum spacers of various thickness to optimize the spectra. These spacers ranged in thickness from 0.001 t’o 0.020 in. A Cary liquid nitrogen De\var \\-a~used for the low temperature spectra.

regions.

IV. RESULTS

AND

l>ISCUSSIOX

In I?gs. 1 and 2 are given the spectra of I<,NiE’6 at) liquid nitrogen temperature. A t abuMion of the observed frequencies is given in Table I for I<&ili’e as well as for Iib,NiFe and CsiKiF6 . Xo new structure was found at SO”K as compared to room temperature spectra; the only effect was to narrow the peaks. Tetravalent8 nickel, as is present in the NiPi- complex, possesses a 32 electronic configuration, which, under the influence of a cubic field generat,ed by the ligands, splits into two levels & and rly, separat,ed by :m energy 10 Dq (the cryatalline-field pxramet,er). Thus we have t,he situation in which t,he various electronic configurations c(cn&6--n are to he regarded as t)he starting point)s, and t)he positions of t’he multiplets arising from each configuration are to be ascert)ained. Such calculations have been carried out by numerous authors ( l-,4 ). lcor the rl” system considered here, there are two possibilities for t,he structure of the ground atate; one :t quintuplet of symmetry 67’2 and the other :L singlet of t,ype “11, . The

so-

i

40.-

t iii

$30~-i

izo0

IO-

I

I

50

I

FIG. 1. Ahsorption Sure.

spertrum

40 I

I

30

FREQUENCY Iid CM“l

of Fluorolube

I

I

20

-

mull of K,NiF,

nt liclllid nitrogen

I

IO

1

tempcr:t-

112

REISFELD,

46?0

I

48;O

ASPREY, AND PENNEMAN

I

50?0

I

52;O

I

5400

I

56;O

FIG. 2. Absorption spectrum of Fluorolube mull of &NiFs at liquid nitrogen temperature; the *A, + ‘2’1 transition.

multiplicity of the ground state depends on two main influences. The first is the of the ligand field, which at high values will overcome the exchange stabilization energy for parallel spins, causing spin-pairing and a concomitant decrease in the multiplicity of the ground state. Thus, Fe’* forms high-spin complexes with weak ligands such as F-, HzO, ethylenediamine, etc., but forms low-spin complexes with dipyridyls, phenantholines, and cyanides (2). Similarly, Co3+ forms low-spin complexes with the strong cyanide ligand, but high-spin complexes with fluoride ion (2). The second effect arises from the charge and radius of the metal ion. Within a given transition series (restricting ourselves again to the d6 configuration) the ease of formation of low-spin complexes increases as we increase the valence of the metal ion. The magnetic susceptibility measurements by Scholder and Klemm (7) have demonstrated that the ground state of &NiFs has no net electron spin and is thus the singlet state. Thus we have the progression that Fe’+ forms complexes with quintet ground states with weak ligands such as water and fluoride, Co3+ forms such complexes only with fluoride, and Ni’+ apparently possesses no high-spin complexes. strength

ELECTRONIC

SPECTRA

OF K2NiF6

113

Table I. Observed frequenciesin cm -' for severalNiF62-complexes. hi and &

representthe splittings of the absorption bands. Rb,NiF,

K,NiF, 61 -

"

61

v

CstNiF, -

62

-

61

62

lJ

354 654 17507

17467

17757 453

455 105 18242<::::I 499

18636 18741/ \

490 19231/ \ 493

18864 19124 19342 19607

17920

123

\

20235< 482

19861 20101

20313 20602

20717 / \ 483

20827 21091

483

21324 21567

504 107 111 476 117 137 501 125 78 486 115 110 453 109 124

108 18508 13769 13_ / J 18904 100 '19004 19242 138 / 19380 \ 87 19467 19739 142 / 19881 \ 19943 Go 20202 145 / 20367 \ GG 20433 20691 129 20820/ \

457

20899

7g

\ 511

473

19399<

\

453

21763

22650

461

50980

87 116 94 152 88 143

21114

163

21277 21372 21575

95 159

21734 / \ 21858

22247

25830

32785

139

20816

484

497

101

491

118

22153

19960 20182

20325/

457

486

19493 19720

100

19872/

116 130

18529 18800

18939 / \ 19026 460 19283

21277 21813 22035

18077 18328 18428/

15400 \

125

101

\ 452

105

21683 470

18038

480

21200/ \

17976 32

\

19724/ 511

469

33330

124

114

REISFELD,

ASPREY,

AN11 PENNEMAN

An energy level diagram for the d6 configuration appears in Pig. 3. The diagram was constructed by utilization of the matrices given by Tanabe and Sugano (8). The energy levels of the multiplets are functions of the crystal-field parameter Dp,

d6

(C/B=

5 00)

r

I I

/’ , ,’

‘S

/I / //

E/B

I

‘G

I

%:I

'G ‘T.?

J I

-I/ /

7-l / / /

\/

\/

?O Dq/B

FIG. 3. Energy level diagram for d6 configuration in a cubic field: f = 0, C/B = 5; (- - -), levels of higher energy arising from the same symmetry manifold as those of prime interest.

ELECTKONIC

SPECTHA

OF KyNiF,

I 1*-I

and the int,erelect’ronic repulsion Raeah parameters B anti C. These latter qu:~nt ities in turn may be expressed as linear combinations of the Slater integrals I<‘, for a ratio of (‘/‘I? = 5.0, corrc~s~~o~ltland F1 . The diagram has been constructed ing to a value of Fs/F1 = 12.0. Tanabe and Sugar10 have derived values ot’ (’ IZ Watson (lo), utjilixing for Nvi” of 4.7 and for Yi3+ of 4.9. For comparison, Hnrt,ree-Foali SCF wave functions hau calculated a value for Si4+ of 3.i.jfor tlw y:wcous at,om, and it is well recognized that (r/f3 lies higher in the cor~~piczc~cl species (2). As may be noted from t,he ls‘ig. 3, the strongest transition:: \vhich may he expected to appear in the spectrum arise from t,he allo\vcd ewitatiotl; from the ‘aI ground state to l)hc ITI and ‘I’, multiplets ( all arising from tlw ~‘IXY ion ‘I level ). Transitions to the lower “Z’l :mtl “Tr levels are spirl-forbiddcltr, :~n(l, to first order will not, appear. As may be seen by inspect.ion of Fig. 1 and Table I, several high intcnsit!peaks are noted in the speckurn of I\2NiE’6 . These have nxmirna located at 19 240, 25 S30, and 33 785 cm-‘. There is also evidence for another peak Iowtctl difficulties in this spectral rclgioll at :Ipproxinlat,ely 51 000 cm-‘, but instrumental preclude further analysis. The very strong peak locat.ed at 32 7S5 cnl? is assigtwd The corrwlwtltlto :t charge-transfer band arising from the It:, - L’P,,transition. ing charge-transfer tr:ansitjions for isoelectronic Co(III ) complexes occur :rt approximately 5 EV (10 000 cm-‘). For the strong field tl” case of E’e( C’S )i the first charge-transfer band is found at 3S 000 cm-’ (,9 ). .Jergensen ( 10 ) r(:htw the energy of t.he elec.tron-t.rnnsfer band t.o the optical clectronegativit.~~ tlifI’t~l~11:~s cnce, xcpr , between the ccntrnl metal ion and the ligantl, after correctiotl been made for the spin-pairing correlat’ion energy and the crysta-field stabilizat icItI energy. Such a relst,ion leads to a value of x,,~, for Ki4+ of 3.25 for the so-slcsni considered here. Jgrgensen derived a value of xliPl = 2.S for the +-I ositlatic~tl state of It/! configuration ions and suggests that xliPl should be about O.C~highc~r in the M shell. Thus t’hc value found here of 33.25 is in good ugreemcnt \\.ith tlr:tt to be expected on the b:ws of such considerat.iotls. We therefore :wign the s;troIIg peal< at 3’1 33 cm-’ t,o an clcct,ron-tra.nsfer b:mtl. The peaks of lower intensity, centered at I9 30 and 2.5 8.30 cm-’ are thw 10 be correlated with transitions from the ‘A, grouncl state to the ‘?‘I1 and ‘7’, Irwl~, respectively. Transit,ions to the lower lying triplet states 31’1 and ‘1’? arc spinforbidden and were not, observed in either absorption or fluorescence. Tht, higher frequency band, l.ving at 25 S30 cm-’ may be reasonably assigned to the ‘TL’1~~~1, but an interesting compli&ion arises in the case of the lower energy peal;. -1s shown in Fig. 2, this lowr lying peak possesses :L considerable :rmount. of well-defined fine st,ructure. ;% large number of :LpprcJxinwtd? rqu:tlly spawd peaks are superimposed upon t’he main absorption, \vit h an average sep:ir:itiori of Mi =t I:! c&. Such structured pcnks have been ob+wved in man?- case>\ of electronic transitions and are typical of :I vibronic mechanism involving eledronic escitation accompanied by simultaneous phonon excitation. For the centro~s~~rw metric complexes discllssctl hew, the I,:tportc rule, \\-hich forbids transition

116

REISFELD,

ASPREY,

AND

PENNEMAN

between levels of the same parity may break down due to several possible mechanisms. (a) Magnetic dipole radiation will lead to a nonvanishing transition probability for the ‘Al + ‘7’1 transition under Oh symmetry. Typically these intensities may be expected to be quit’e weak for polyatomic molecules unless an accompanying molecular vibration permits enhancement by the stimulation of electric dipole radiation. (b) Vibronic interactions, in which vibrations of the central ion and its associated ligands introduce odd-parity electronic-vibrational states will lead to nonvanishing matrix elements for the transition probabilities of radiation transfer between forbidden states. If a particular component of the transition moment with respect to space-fixed axes is to be nonvanishing, the vibronic species symmetry must be different from that of the electronic species. For example, under Oh symmetry, when antisymmetric vibrations of type T,,, are excited, nuclear configurations occur which do not have the full symmetry of the point group, and since the electronic wave function varies slightly with the configuration, the transition moment will t>henbe nonzero. (c) The excited state may have a different equilibrium configuration than does the ground state, thus allowing transitions which were forbidden under the assumption of an invariant point group. The appearance of excited state vibrational progressions whose frequencies differ from those of ground state fundamental vibrations may be considered evidence of such changes in configuration. A particular case of this phenomenon is the Jahn-Teller effect’; the instabilit’y of an electronically degenerate complex wit’h respect t)o dist’ortions t,hat remove t’he degeneracy. Thus, for example, vibrations of symmetry species E, and T,, will product Jahn-Teller instability in the ‘T,, state of an octahedral complex, leading in the case of E’, mode t,o a tetragonal dist,ortion of the complex. Of course, combinations of effects (a), (b ), and ( c ) \vill also lead t’o t’he appearance of “forbidden” transitions with accompanying vibrational fine structure. Spin-orbit coupling &e&s, rotationalelectronic int#eractions and state-mixing are also possible mechanisms leading to such transitions but these effects are usually small in the case of 3d ions. We will now turn our attention to the question of which of these mechanisms might, account for the present observat’ions regarding the structure of the ‘A1 + ‘Tl transition. For the octahedral ground state complex NiFi-, or more properly, for the unit cell, there are several u-species vibrations which will permit electronic transit,ions. The nondegenerate ‘A1 ground state, as well as the excited state may combine with these vibrations to form a progression of vibronic levels of odd parity. In absorption, there may then occur electric-dipole transitions from the pure electronic state t,o t#heupper vibronic states, as well as t)ransitions from the vibrational-ele&ronic ground state to t,he electronic excited state. In t,he second case, a marked temperature effect’ in t’he int’ensities should be noted as depopulation of the lower vibronic state occurs. No apparent diminut,ion of t,he int,ensity of the vibrational progression was observed as the temperature was lowered from 298 to SO%, indicating that the progressions observed should be

ELECTRONIC

SPECTRA

OF K,NiF,

117

correlated with vibrational modes of t#he complex in an excited state configuration. A complete analysis of the ground stat’e vibrational spectrum of IGNiF~ has been made (11) and we may ut’ilize these data in t’he consideration of t’he elcctronic spectra. For the vibronic mechanisms considered above, it is the odd vibration of T,, symmeky which enables transitions, and thus the observed spacing of 486 cm-’ in the excited state may be correlated with the direct observution of the antisymmetric stretching frequency ~3of 6.33 cm-’ in the ground stnt,e. The progression would then represent the transitions 0 + va , 0 -+ ‘2~2, 0 --f 3~~ , et,c., wit.h the origin of the band (no-phonon line) being absent. This assignment ivould then place the parent elect.ronic transition at 4% cm-’ below the first observed member of the vibronic sequence, leading t,o the ‘T, level at 17 301 cm-‘. Alternatively, we may consider the 4SG cm-l progression t,o arise from t,he I:‘!, mode which has been observed in the ground state at 520 cm-‘. This would suggest that’ the Jahn-Teller distortion is represented and that the upper state devintcs in configuration from the ground state octahedral geometry. The activity of the Tl,, vibrational mode would then make itself felt, not as a progression forming mode but, as an enabling vibration. In this case, the band maximum represent,s the most probable configuration and is the proper energy at which to :wsigtl crystal-ficltl levels, i.e., 19 230 cK’. Of the t’\vo possible alternatives, it is difficult, without available single crystal polariz:~tion data to make a11 uncyuivocal choice between the t\\-o foregoing alternntivcs, hut we prefer the second mechanism involving a ,Jahn-Teller distoltioll for the following reasons: ( i ) the vibrational progression seems so regulat in
:LLKIfor the temperatures of 7’1 = 29S”I<, T, = sO”I<, and ; = 4Mi cn?, the r:itic) is 1.21: 1. This intensitychangcof ’ 21 5 which was not observed could, ho\~ever, be accounted for by thermal contraction of the mulling agent at t’he lower temperatuw which would increase the effective concentration of the complex. We thus locate the ‘1’1 level :at 19 230 cm-’ , mcl this assignment, together with the previously obtained value for the energy of t.hc ‘A, - I?‘:! tr:msit,ion at ‘75 MO cn? allows a determination of both the crystal-field parameter Dy and the interelectronic repulsion parameter B. Two met,h& were emplo.ved for fit’ting the dat:t.

11s

UEISFELD,

ASPIZEY,

AND

PENNEMAN

The first involved a graphical iterative technique utilizing the energy level diagram of Fig. 3, and the second employed analytical expression for t,he energies to first order as given by Tanabe and Sugano (8). The desired parameters are Dq = 2090 cm-’ and B = 4% cm-‘. The assumption that C/B = 5.0 gives C = 3425 cm-‘. From a series of studies on isoelect’ronic cZ6Co( III) complexes, Wentworth and Piper (la) derive a value of Dq for t,he F‘- ligand of IS49 cm and an average value of B of approximately 500 cm-‘. If for the case of tetravalent nickel we calculate a value for t’he gaseous interelectronic parameter Bo by use of the expressions given by Jorgensen (13) and Tanabe and Sugano (S), we obtain B. = 1220 cm-l from which we may evaluate the nephelauxetic ratio p = B/B0 . The rat’io found is @ = 0.40. For the analogous 4d6 complex PdF$, a value of /? = 0.33 has been obtained (IS), and for the similar 5d6 species PtFi-, p = 0.53 (2). We, therefore, may with some confidence assume the above assignments for the elect’ronic levels of IGSiF6 to be correct. From recent work by Westland et ad. (Ii) on the reflectance spectrum of K&F6 we have calculated values of Dq = 1990 cm-’ and B = 450 cm-l. Ko vibrational fine structure was observed, and t,he electronic transitions were placed at the band maxima. Considering the difficulties of reflectance type measurements, the results are in good agreement with our own. A further feature of the complicated band at 19 230 cm-l may also be noted. Superimposed on each of t’he vibronic components is a further splitting, which in the case of I<,KiF6 , amounts to 115 f 9 cm’ about each component. The origin of this component is not clear, but there are several possibilities. Firstly, the spin-orbit coupling interact,ion may serve to reduce the degeneracy of the vibronic species, allowing further transitions to occur. Secondly, the infrared-inactive bending frequency vg of symmetry T?,, will also serve as an enabling mode for vibronic kansition in the octahedral cl6 configuration. Frequencies ranging from 140 to 240 cm-l have been assigned to this mode for a number of t~ransitionmetal hexafluorides (Is), and a value for vgof 220 cm-’ in the ground st’ate of IGKiFs has been calculated ( 11). The spacing between members of the observed progression is 245 f 19 cm-l. Another possibility is a further Jahn-Teller component arising from interactions with a Tr, vibration which has been placed at 310 cm-l in the ground state (11). Finally, there remains to be considered the occurrence of a crystal lat’tice mode due to translations of the complex wit’hin the unit cell. For &?;iFG , Reisfeld (11) has found a lat’tice vibration at’ 13s cm-’ and Hiraishi and Shimanouchi (16) have locat’ed such a lattice frequency at 90 cm-’ in KZPtC16 . However, the latter authors also observed that for the series of compounds I<,PtC16 , RbzPtCls , and CszPtCls , the lattice vibrat’ion decreased in frequency from 90 to 68 cm-l, a change of 25%. The data here shown for the potassium, rubidium, and cesium hexafluoronickelate(IV) complexes indicate that the small splittings remain essentially constant regardless of the cation. Therefore, the origin of the splitting probably lies in an intermolecular vibration, possibly of the excited state T2, mode ~6 .

ELECTRONIC

SPECTRA

OF K&iFG

119

ACPSOTVLEDGMENTS The authors are indebted to Mr. J. Bloor for having obtained some of the preliminnr! spectra and to Mr. D. Armstrong for t,he preparation of the figures. RECEIVEI):

June 5, 196s REFERENCES

1, J. 8. (:RIFFIYTH “Theory of TransitiowMetal Ions.” Cambridge Univ. Press, Londott and New Yolk, 1961. .4ddison9. C. K. J~~RGKNSEN,“ Bbsorption Spectra and Chemical Bonding iu Complexes.” Wesley, Reading, Massachusetts, 1962. to Trnnsitiou-Metal Chemistry Ligand-Field Theory.” Wile?., 3. L. ORGEI., “Introduction New York, 1960. to Ligand Field Theory.” McGraw-Hill, New York, 4. C. J. BALHAUSEN, “Introduction 196Y2. 5. W. KLEMM IND E. Huss, %. rlnorg. C’he?n. 253, 221 (1949). 6. 1~. L. MCCULLOUGH, L. H. JONISS,:\ND G. A. CROSBY, Spectrochim. data 16, 929 (1960). 7. K. SC'HOLDEK AND W. KIXMM, dngew. C’hem. 16, 461 11954). 8. 1.. T.~N.\~%E AND S. SCGANO, J. Phvs. Sac. Japan 9, i53, 766 (1954). 9. 1). S. MCCLURE, Solid St&e Phys. 9, 399 (1959). 10. C. K. J@RGENBGN, Mol. Phgs. 6, 43 (1963). !/. 11. J. ~~EISFELD, J. Mol. Speclry. 29, 120 (1969). I?. R. A. 1). WKNT~OKTH AND T. S. PIPER, Tnorg. (‘hem. 4,709 (1965). 1:3. c’. K. J$KGKNSEN, Hell!. C'hinz. dcla, Sepuratrcna, Facicctlus Extraordinarizts .4lfrerl MTe/,ner 1967, 131. f4. A. WXSTL.\ND, 11. HOPPE, -INU S. K.~sI,:No, Z. Anorg. AIlgem. C’henr. 338, 319 (1965). 15. 8. X. TH.KCR AND D. K. R.u, J. Mol. Spectq/.19, 341 (1966). 16. J. Hrn.~~wr-rr 0-1) T. SHIM\NOUCHI, Spw2rochiw~. .-I& 22, 1483 (1966).