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Electronic structure and spectra of tetrahalogenoferrates (II)
J. Inorg. Nucl. Chem., 1963, Vol. 25, pp. 159 to 163. Pergamon Press Ltd. Printed in Northern Ireland
ELECTRONIC STRUCTURE AND TETRAHALOGENOFERRATES
SPECTRA (II)
OF
C . FURLANI, E . CERVONE and V. VALENTI Institute of General Chemistry, University of R o m e
(Received 13 June 1962) A b s t r a c t - - T h e s p e c t r u m of the tetrahedral complex [FeCI4] = consists o f only one, rather intensive d-d b a n d in the near infrared region which is assigned to the transition 5E --~ 5T~ (Ta), a n d h a s ).m~ 2.47 /* with log Emol = 1,9. Suitable solvents for spectroscopic absorption m e a s u r e m e n t s in this region are CD.~NO~ or CICHzCN.
TETRAHALOGENOFERRATES(II), a member of the recently discovered family of tetrahedral halogenocomplexes of transition metal ions, have been prepared for the first time by NYr~OLM, GILL and coworkers (1,2'3) who demonstrated their tetrahedral structure and described also several physico-chemical properties of these compounds, but did not report their complete spectra, nor their interpretation. Continuing our previous researches on the absorption spectra of tetrahedral halogenocomplexes of transition metals, we independently prepared some new tetrachloroferrates (II) and measured their absorption spectra in the visible and near infra-red region. We found that all salts containing the anion [FeCI~]2- do not absorb in the visible region (they appear almost perfectly white in the solid state) but have one ligand field band with a maximum in the region around 4000 cm -1 (Fig. 1). Such a band is undoubtedly due to a transition between the two sublevels 5E (ground state) and 5T2, originated from the splitting of the ground term 5D of Fe e+ under a ligand field of symmetry T a. The best A value to fit the experimentally observed frequency in the scheme of intermediate coupling appears to be 4000 cm -1, and it corresponds perfectly to the theoretical expectation for a tetrahedral complex of magnetically normal Fe 2+ and CI- as the ligand (Aoctahea~l in [Fe(H20)6] 2+ is about 10400 cm -1(4,5), so [Atetrahedral[ is expected to be a little less than ~ of this value, owing to the position of C1- after H,,O in spectrochemical series). The ratio A(MeC142-)/A(Me aq62+) is 0"385, in perfect agreement with values quoted by COTTON e t al. (6) for other transition metals. It has to be remarked that such a A value for Fe z+ is among the highest values observed for similar tetrahedral complexes of the first row of transition metals* but nevertheless the only absorption band of [FeCI4]2- lies relatively far in the infrared. The intensity of the observed band is rather high (En~o~~ ~ 80 ~ 3 in chloroacetonitrile solutions), as in other tetrahedral halogenocomplexes, thus confirming the tetrahedral structure of [FeCI4]2-. The band is a single one, and appears to be * Typical values are: A = 3300 cm ~ (10) (~ N. (~ N. (3) N. ~) C. ~ C. ~ F.
[CoC14] ~- A = 3200 c m -~ (8); [NiCI~] ~-, A ~. 3700 cm ~ (9); [MnCI4] 2-,
S. GILL, R. S. NYHOLM a n d P. PAULING, Nature, Lond. 182, 168 (1958). S. GILL a n d R. S. NYHOLM, J. Chem. Soc. 3997 (1959). S. GmL, J. Chem. Soc. 3512 (1961). FURLAN1, Gazz. Chim. Ital. 87, 371 (1957). K. JORG~NSEN, Acta Chem. Scand. 8, 1502 (1954). A. COTTON, D. M. L. GOODGAME and M. GOODGAME, J. Amer. Chem. Soc. 84, 167 (1962). 159
160
C. FURLANI, E. CERVONE and V. VALENTI
symmetrical, although we cannot exclude the possible presence of satellites on its lowfrequency side; we take this fact as an indication that Jahn-Teller distorsions of the [FeCI4]2- tetrahedron, although theoretically expected, are probably not very conspicuous (see also Reference 3). Actually, the calculated scheme of energy levels according to the theory of intermediate coupling predicts a splitting of the upper level
.
water band
E 80
o
E W
60.
80+ I
4
8
12
16~10= cm-1
dsz~ cir., \
. . . . , P I 40bo 4s~)o so'oo ss'oo 6ooo cm-~ F~G. l.--Absorption spectra of: (a) [C.HsCH2N(CH3)8]2[FeC14], solid in C4C16 mull;
ol 3soo
*"-~
(b) [(CoH~CH~N(CH3)3]C1solid in C4C16 mull; (c) [CeHsCH2N(CHs)z]2 [FeCI+] in CHaNO2; (d)[CoH~CH~N(CH3)]2[FeC1,] in CD~NO=; (e) [C+HsCHzN(CHz)3]2[FeCI~] in C1CH2CN; (a), (b): arbitrary ordinate scale; (b), (d), (e) in presence of excess [CoH~CHzN(CH3)a]C1. (STz under Ta), owing to spin-orbit coupling effects, into several sublevels grouped into three multiplets, over a range of approximately 500 cm -1 (see Fig. 2), but we think that all these sublevels are contained under the observed single band envelope. Minor variations in the position of the maximum according to the solvent used, to the nature of the cation bound to the [FeC14]~- anion or to the aggregation state of the complex (see Table), have been observed, but they are probably due to second-sphere environmental effects; they are in our opinion perfectly compatible with the scheme of interpretation discussed above. The absorption intensity follows satisfactorily the Lambert-Beer law only in the presence of an excess of free C1- ions, otherwise dilute solutions of [FeC14]2- salts exhibit decomposition, as is evident from the lowering of the apparent EnloXe value at
Electronic structure and spectra of tetrahalogenoferrates (II)
F
( r 'JT ~ ="~"" ']
161
l'a(STa]
ooo/ ..; "bTa (~l'a) aTa (r'T.,) E 2000 IM
re/" ~>
1ooo
,..¢>~"~-,,
v"
0
I
2500
I
3000
!
3500
I"I (cm-1)
I
4000
I
4500
FIG. Z - - T e r m system of the lowest energy levels (sublevels of~D) o f F e ~+ in tetrahedral complexes with spin-orbit coupling (2 =: - 9 0 cm-~); zero of the ordinate scale corresponds to E(SE)under Ta without spin orbit coupling. ABSORPTION SPECTRA OF TETRACHLOROFERRATES max
Compound
Solvent
(cm -1)
E . . . . (mole)
[C6HsCH2N(CH3)3]2FeC14 [C6HsCH~N(CHz)~]2FeCI4 [C6HsCH2N(CHs)3]2FeC14 [C6H~CH~N(CH3)3]2FeCI~ [C6HsCH~N(CH3)3]2FeC14
Reflexion spectrum C~C10 mull CHnNO2* CD3NO2* C1CHzCN*
4490 4440 4050 4050 4050
--78 ~ 5 75 zF 5 80 zF 3
[(CH~)4N]2FeCI4 [(CH3)4N]2FeCI4 [(CH3)~N]2FeC14
Reflexion spectrum C4C16, mull CICH2CN
4480 3880 4060
--61 T 10
[(C6H~)~As]~FeC14 [(C6Hs)4As]2FeCI4
Reflexion spectrum CICH~CN
4550 4060
77 zF 3
* In presence of excess [CoHsCH2N(CHa)3]CI.
162
C. FURLANI, E. CERVONE and V. VALENTI
the m a x i m u m . Therefore, a d d i t i o n o f halide ions in excess a p p e a r s to have a beneficial effect on the stability o f the t e t r a h e d r a l structure in solution, as has been observed also with other t e t r a h a l o g e n o c o m p l e x e s (e'v's,a~, a n d prevents also other possible ways o f d e c o m p o s i t i o n . I n this respect, while solid salts c o n t a i n i n g the [FeC14] z- a n i o n are r e m a r k a b l y stable in d r y air, their solutions in p o l a r organic solvents, which are colourless or only slightly yellowish w h e n freshly p r e p a r e d , b e c o m e sometimes b r o w n a n d u n d e r g o evident a l t h o u g h n o t i m m e d i a t e d e c o m p o s i t i o n , especially in c o n t a c t with air. A m o n g the d e c o m p o s i t i o n p r o d u c t s are [FeC14]- salts, which can be detected b y the presence o f their characteristic a b s o r p t i o n p a t t e r n in the 500-800 m/~ region, b u t the d e c o m p o s e d solutions o f [FeCla] 2- are a b r o w n c o l o u r which is quite different f r o m the greenish-yellow c o l o u r o f [FeC14]-, a n d therefore m u s t c o n t a i n also other d e c o m p o s i t i o n p r o d u c t s , e,g. p s e u d o - o c t a h e d r a l solution products. However, this t y p e o f d e c o m p o s i t i o n occurs m u c h less seriously a n d m u c h m o r e slowly, o r can even be c o m p l e t e l y prevented, in the presence o f an excess o f free chloride ions. EXPERIMENTAL Preparation of tetrachloroferrate (ll) salts. On mixing stoicheiometric amounts of anhydrous FeC12 and of an oniurn chloride in absolute ethanol, usually the chloroferrate(II)precipitatesspontaneously. Since the compounds are not extremely sensitive to oxidation, exclusion of air is not strictly necessary, but it helps in obtaining purer and white products; this is important, since it is often difficult if not impossible to recrystallize tetrachloroferrates (II). Following this procedure we prepared: bistrimethylbenzylammonium tetrachloroferrate (II) [CeHsCH2N(CHa)a]~[FeC14], a cream white solid (Found C, 48"06; H, 6-55; N, 5.13; Fe, 11"14; C1, 28"70 Calc. for C20Hs~N2FeCI~; C, 48"21; H, 6-43; N, 5.62; Fe, 11"21 ; C1, 28.53 ~o); bis-tetramethylammonium tetrachloroferrate (II) [(CH3)4N]~[FeCI4], which is perfectly white (Found C, 27-86; H, 6-74; N, 7.92; Fe, 16.05; C1, 40.70. Calc. for CsH24NzFeC14; C, 27-76; H, 6.94; N, 8.10; Fe, 16.14; C1, 41.06~) and bis-tetraphenylarsoniumtetrachloroferrate (II) [(CeHs)4As]~[FeC14],pale yellow (Found: C, 60"30; H, 4"01; Fe, 5"46; C1, 14.33. Calc for C48H40As2FeCl~: C, 59'76; H, 4.15; Fe, 5.79; C1, 14.73~). For the sake of comparison with the spectra of decomposed solutions, we prepared also a complex of Fe(III), namely trimethylbenzylammonium tetrachloroferrate (III) [CsHsCH~N(CH3)a][FeCI4], pale green needles crystallizable from alcohol (Found: C, 33.86; H, 4"81; N, 4'20; Fe, 16'26; C1,40.40. Calc. for C10H16NFeC14 C, 34-50; H, 4.60; N, 4.02; Fe, 16.04; C1,40-83~). All tetrachloroferrates (II) are readily and completely decomposed by water, and to a smaller extent also by alcohols, but are soluble in polar organic solvents, including CH3NO2, CHaCN, dimethylformamide, dimethylsulphoxide, and chloroacetonitrile. As in other series of tetrahalogenometallates (II) of transition metalstS,9~ the tetramethylammoniurn salt is by far the least soluble of all. All tetrachloroferrates (II) we prepared behave as 1 : 2 electrolytes in nitromethane and have a magnetic moment which is a little higher than the spin-only value for four unpaired electrons, as reported by GILLtal. Spectroscopic measurements. Preliminary confirmation of the existence and position of an absorption band in the near infra-red region (4000-5000 cm-1) was obtained by the spectra of solid tetrachloroferrate (II) salts, either reflection spectra of the powdered substances, or spectra in hexachlorobutadiene mulls. More exact and quantitative observation of the absorption spectra in solution was difficult because of two factors which are always a serious obstacle to spectrophotometricmeasurements in the region below 5000 cm-l: (a) a strong disturbance caused by water absorption at 2.7-2-8/~, which can be removed only by extreme care in purging both the apparatus and the solution from traces of water; (b) opacity of the solvents. Most common solvents contain hydrogen, and their stretching modes correspond to more or less intense absorptions below 4-5000 cm-1; for this reason the use of solvents like dimethylsulphoxide, acetonitrile and dimethylformamide had to be discarded. Nitromethane absorbs less strongly than the previous solvents and can be used within certain limitations as a solvent, especially in thin layers (absorption cells of 1 ram. or less). An improvement can be made by the use of deuterated nitromethane CDaNO~, which we prepared from CHaNO2 and D20 by WILSON'Stx0~method; it still absorbs in the region 2"7-2"8/~, as does nitromethane, but the t7~ F. A. CoTroN, D. M. L. GOODGAMEand M. GOODGAME,.J'. Amer. Chem. Soc. 83, 4690 (1961). la~ C. FURLANIand G. MORPURGO,Z. Phys. Chem. N.F. 28, 93 (1961). tg~ C. FURLANIand A. FURLANI,J. Inorg. Nucl. Chem. 19, 51 (1961). tlo~ T. P. WILSON,J. Chem. Phys. 11, 361 (1943).
Electronic structure and spectra of tetrahalogenoferrates (II)
163
absorption bands of CH3NOz at 2.2-2-3/~ and at 2.4 p are greatly reduced in intensity, so the opacity of the solvent is also greatly reduced. Good results are obtained also by the use of chloroacetonitrile as a solvent; although C1CH2CN has several intense absorption bands between 2 and 3 # (at 2.20, 2.35, 2-54, 2.67/~), they are very narrow, and obscure only a few narrow frequency intervals in that region, so that the contour of a rather broad absorption band, as is that of [FeCI~] ~-, can be completely identified; chloroacetonitrile has the additional advantage of being a very good solvent, where fairly concentrated solutions of halogenocomplexes can be prepared, thus allowing use of very thin absorption cells and further improving the total transparency of the solvent. The vibrational spectrum of the onium cations can be sometimes observed as a system of narrow bands superimposed to the envelope of the much broader d-d band of the anion. Most measured spectra were of the trimethylbenzylammonium salt in the presence of the corresponding chloride, which are both easily soluble in the solvents mentioned above; the tetramethylammonium salt is much less soluble, and the tetraphenylarsonium tetrachloroferrate is itself well soluble, but tetraphenylarsonium chloride is not, and the complex could be measured only in presence of a very small excess of free C1- ions. Spectroscopic results could be obtained for all three mentioned tetrachloroferrates (II), and no significant difference between them was detected (see Table) except a shift of ~ 4 0 0 cm -1 in passing from solutions to the solid state and a somewhat lower extinction coefficient for the tetramethylammonium salt. Ligand-fieMcalculations. The term system of Fe 2~ was calculated only for the split terms of 5D under a field of tetrahedral symmetry with spin-orbit coupling, in two weak-field approximations, i.e. (SLMsM~) --~ (SLJMa) --+( F M F M j ) and (SLMsML) -~ (S FMsMI,) ~ (I'Mr,Mj), which gave exactly coincident results; matrix elements for the first reckoning scheme can be found in Griffith ~1~ and for the second one in CONDON and SHORTLEY~1~ "Theory of the atomic spectra", Cambridge Univ. Press 1953. Since the ligand field interactions due to tetrahedral environment represent a very weak perturbation we are confident that the adoption of a weak-field scheme and our neglecting the higher levels of lower spin multiplicity are fully justified. We assumed ~ = 360 cm -~ or 2 = --90 cm 1 (a slightly smaller value than in free Fee+), and varied Atetr" between 2500 and 4500 cm 1. As is evident from Fig 2 the tetrahedral ground state 5E(Ta) is left still practically degenerate by the spin-orbit perturbation, whereas the upper state (~T2, Ta) is split to a much larger extent; the best A value which can fit the average experimental frequency is 4000 cm -1.
Acknowledgement--The present work has been carried out under financial support by NATO through a NATO Research grant. 01! j. S. GRIFFITH, Theory of Transition Metal Ions. Camb. Univ. Press (1961). ~:~ CONDON and SHOTLEY, Theory of Atomic Spectra. Camb. Univ. Press (1953).
ELECTRONIC STRUCTURE AND TETRAHALOGENOFERRATES
SPECTRA (II)
OF
C . FURLANI, E . CERVONE and V. VALENTI Institute of General Chemistry, University of R o m e
(Received 13 June 1962) A b s t r a c t - - T h e s p e c t r u m of the tetrahedral complex [FeCI4] = consists o f only one, rather intensive d-d b a n d in the near infrared region which is assigned to the transition 5E --~ 5T~ (Ta), a n d h a s ).m~ 2.47 /* with log Emol = 1,9. Suitable solvents for spectroscopic absorption m e a s u r e m e n t s in this region are CD.~NO~ or CICHzCN.
TETRAHALOGENOFERRATES(II), a member of the recently discovered family of tetrahedral halogenocomplexes of transition metal ions, have been prepared for the first time by NYr~OLM, GILL and coworkers (1,2'3) who demonstrated their tetrahedral structure and described also several physico-chemical properties of these compounds, but did not report their complete spectra, nor their interpretation. Continuing our previous researches on the absorption spectra of tetrahedral halogenocomplexes of transition metals, we independently prepared some new tetrachloroferrates (II) and measured their absorption spectra in the visible and near infra-red region. We found that all salts containing the anion [FeCI~]2- do not absorb in the visible region (they appear almost perfectly white in the solid state) but have one ligand field band with a maximum in the region around 4000 cm -1 (Fig. 1). Such a band is undoubtedly due to a transition between the two sublevels 5E (ground state) and 5T2, originated from the splitting of the ground term 5D of Fe e+ under a ligand field of symmetry T a. The best A value to fit the experimentally observed frequency in the scheme of intermediate coupling appears to be 4000 cm -1, and it corresponds perfectly to the theoretical expectation for a tetrahedral complex of magnetically normal Fe 2+ and CI- as the ligand (Aoctahea~l in [Fe(H20)6] 2+ is about 10400 cm -1(4,5), so [Atetrahedral[ is expected to be a little less than ~ of this value, owing to the position of C1- after H,,O in spectrochemical series). The ratio A(MeC142-)/A(Me aq62+) is 0"385, in perfect agreement with values quoted by COTTON e t al. (6) for other transition metals. It has to be remarked that such a A value for Fe z+ is among the highest values observed for similar tetrahedral complexes of the first row of transition metals* but nevertheless the only absorption band of [FeCI4]2- lies relatively far in the infrared. The intensity of the observed band is rather high (En~o~~ ~ 80 ~ 3 in chloroacetonitrile solutions), as in other tetrahedral halogenocomplexes, thus confirming the tetrahedral structure of [FeCI4]2-. The band is a single one, and appears to be * Typical values are: A = 3300 cm ~ (10) (~ N. (~ N. (3) N. ~) C. ~ C. ~ F.
[CoC14] ~- A = 3200 c m -~ (8); [NiCI~] ~-, A ~. 3700 cm ~ (9); [MnCI4] 2-,
S. GILL, R. S. NYHOLM a n d P. PAULING, Nature, Lond. 182, 168 (1958). S. GILL a n d R. S. NYHOLM, J. Chem. Soc. 3997 (1959). S. GmL, J. Chem. Soc. 3512 (1961). FURLAN1, Gazz. Chim. Ital. 87, 371 (1957). K. JORG~NSEN, Acta Chem. Scand. 8, 1502 (1954). A. COTTON, D. M. L. GOODGAME and M. GOODGAME, J. Amer. Chem. Soc. 84, 167 (1962). 159
160
C. FURLANI, E. CERVONE and V. VALENTI
symmetrical, although we cannot exclude the possible presence of satellites on its lowfrequency side; we take this fact as an indication that Jahn-Teller distorsions of the [FeCI4]2- tetrahedron, although theoretically expected, are probably not very conspicuous (see also Reference 3). Actually, the calculated scheme of energy levels according to the theory of intermediate coupling predicts a splitting of the upper level
.
water band
E 80
o
E W
60.
80+ I
4
8
12
16~10= cm-1
dsz~ cir., \
. . . . , P I 40bo 4s~)o so'oo ss'oo 6ooo cm-~ F~G. l.--Absorption spectra of: (a) [C.HsCH2N(CH3)8]2[FeC14], solid in C4C16 mull;
ol 3soo
*"-~
(b) [(CoH~CH~N(CH3)3]C1solid in C4C16 mull; (c) [CeHsCH2N(CHs)z]2 [FeCI+] in CHaNO2; (d)[CoH~CH~N(CH3)]2[FeC1,] in CD~NO=; (e) [C+HsCHzN(CHz)3]2[FeCI~] in C1CH2CN; (a), (b): arbitrary ordinate scale; (b), (d), (e) in presence of excess [CoH~CHzN(CH3)a]C1. (STz under Ta), owing to spin-orbit coupling effects, into several sublevels grouped into three multiplets, over a range of approximately 500 cm -1 (see Fig. 2), but we think that all these sublevels are contained under the observed single band envelope. Minor variations in the position of the maximum according to the solvent used, to the nature of the cation bound to the [FeC14]~- anion or to the aggregation state of the complex (see Table), have been observed, but they are probably due to second-sphere environmental effects; they are in our opinion perfectly compatible with the scheme of interpretation discussed above. The absorption intensity follows satisfactorily the Lambert-Beer law only in the presence of an excess of free C1- ions, otherwise dilute solutions of [FeC14]2- salts exhibit decomposition, as is evident from the lowering of the apparent EnloXe value at
Electronic structure and spectra of tetrahalogenoferrates (II)
F
( r 'JT ~ ="~"" ']
161
l'a(STa]
ooo/ ..; "bTa (~l'a) aTa (r'T.,) E 2000 IM
re/" ~>
1ooo
,..¢>~"~-,,
v"
0
I
2500
I
3000
!
3500
I"I (cm-1)
I
4000
I
4500
FIG. Z - - T e r m system of the lowest energy levels (sublevels of~D) o f F e ~+ in tetrahedral complexes with spin-orbit coupling (2 =: - 9 0 cm-~); zero of the ordinate scale corresponds to E(SE)under Ta without spin orbit coupling. ABSORPTION SPECTRA OF TETRACHLOROFERRATES max
Compound
Solvent
(cm -1)
E . . . . (mole)
[C6HsCH2N(CH3)3]2FeC14 [C6HsCH~N(CHz)~]2FeCI4 [C6HsCH2N(CHs)3]2FeC14 [C6H~CH~N(CH3)3]2FeCI~ [C6HsCH~N(CH3)3]2FeC14
Reflexion spectrum C~C10 mull CHnNO2* CD3NO2* C1CHzCN*
4490 4440 4050 4050 4050
--78 ~ 5 75 zF 5 80 zF 3
[(CH~)4N]2FeCI4 [(CH3)4N]2FeCI4 [(CH3)~N]2FeC14
Reflexion spectrum C4C16, mull CICH2CN
4480 3880 4060
--61 T 10
[(C6H~)~As]~FeC14 [(C6Hs)4As]2FeCI4
Reflexion spectrum CICH~CN
4550 4060
77 zF 3
* In presence of excess [CoHsCH2N(CHa)3]CI.
162
C. FURLANI, E. CERVONE and V. VALENTI
the m a x i m u m . Therefore, a d d i t i o n o f halide ions in excess a p p e a r s to have a beneficial effect on the stability o f the t e t r a h e d r a l structure in solution, as has been observed also with other t e t r a h a l o g e n o c o m p l e x e s (e'v's,a~, a n d prevents also other possible ways o f d e c o m p o s i t i o n . I n this respect, while solid salts c o n t a i n i n g the [FeC14] z- a n i o n are r e m a r k a b l y stable in d r y air, their solutions in p o l a r organic solvents, which are colourless or only slightly yellowish w h e n freshly p r e p a r e d , b e c o m e sometimes b r o w n a n d u n d e r g o evident a l t h o u g h n o t i m m e d i a t e d e c o m p o s i t i o n , especially in c o n t a c t with air. A m o n g the d e c o m p o s i t i o n p r o d u c t s are [FeC14]- salts, which can be detected b y the presence o f their characteristic a b s o r p t i o n p a t t e r n in the 500-800 m/~ region, b u t the d e c o m p o s e d solutions o f [FeCla] 2- are a b r o w n c o l o u r which is quite different f r o m the greenish-yellow c o l o u r o f [FeC14]-, a n d therefore m u s t c o n t a i n also other d e c o m p o s i t i o n p r o d u c t s , e,g. p s e u d o - o c t a h e d r a l solution products. However, this t y p e o f d e c o m p o s i t i o n occurs m u c h less seriously a n d m u c h m o r e slowly, o r can even be c o m p l e t e l y prevented, in the presence o f an excess o f free chloride ions. EXPERIMENTAL Preparation of tetrachloroferrate (ll) salts. On mixing stoicheiometric amounts of anhydrous FeC12 and of an oniurn chloride in absolute ethanol, usually the chloroferrate(II)precipitatesspontaneously. Since the compounds are not extremely sensitive to oxidation, exclusion of air is not strictly necessary, but it helps in obtaining purer and white products; this is important, since it is often difficult if not impossible to recrystallize tetrachloroferrates (II). Following this procedure we prepared: bistrimethylbenzylammonium tetrachloroferrate (II) [CeHsCH2N(CHa)a]~[FeC14], a cream white solid (Found C, 48"06; H, 6-55; N, 5.13; Fe, 11"14; C1, 28"70 Calc. for C20Hs~N2FeCI~; C, 48"21; H, 6-43; N, 5.62; Fe, 11"21 ; C1, 28.53 ~o); bis-tetramethylammonium tetrachloroferrate (II) [(CH3)4N]~[FeCI4], which is perfectly white (Found C, 27-86; H, 6-74; N, 7.92; Fe, 16.05; C1, 40.70. Calc. for CsH24NzFeC14; C, 27-76; H, 6.94; N, 8.10; Fe, 16.14; C1, 41.06~) and bis-tetraphenylarsoniumtetrachloroferrate (II) [(CeHs)4As]~[FeC14],pale yellow (Found: C, 60"30; H, 4"01; Fe, 5"46; C1, 14.33. Calc for C48H40As2FeCl~: C, 59'76; H, 4.15; Fe, 5.79; C1, 14.73~). For the sake of comparison with the spectra of decomposed solutions, we prepared also a complex of Fe(III), namely trimethylbenzylammonium tetrachloroferrate (III) [CsHsCH~N(CH3)a][FeCI4], pale green needles crystallizable from alcohol (Found: C, 33.86; H, 4"81; N, 4'20; Fe, 16'26; C1,40.40. Calc. for C10H16NFeC14 C, 34-50; H, 4.60; N, 4.02; Fe, 16.04; C1,40-83~). All tetrachloroferrates (II) are readily and completely decomposed by water, and to a smaller extent also by alcohols, but are soluble in polar organic solvents, including CH3NO2, CHaCN, dimethylformamide, dimethylsulphoxide, and chloroacetonitrile. As in other series of tetrahalogenometallates (II) of transition metalstS,9~ the tetramethylammoniurn salt is by far the least soluble of all. All tetrachloroferrates (II) we prepared behave as 1 : 2 electrolytes in nitromethane and have a magnetic moment which is a little higher than the spin-only value for four unpaired electrons, as reported by GILLtal. Spectroscopic measurements. Preliminary confirmation of the existence and position of an absorption band in the near infra-red region (4000-5000 cm-1) was obtained by the spectra of solid tetrachloroferrate (II) salts, either reflection spectra of the powdered substances, or spectra in hexachlorobutadiene mulls. More exact and quantitative observation of the absorption spectra in solution was difficult because of two factors which are always a serious obstacle to spectrophotometricmeasurements in the region below 5000 cm-l: (a) a strong disturbance caused by water absorption at 2.7-2-8/~, which can be removed only by extreme care in purging both the apparatus and the solution from traces of water; (b) opacity of the solvents. Most common solvents contain hydrogen, and their stretching modes correspond to more or less intense absorptions below 4-5000 cm-1; for this reason the use of solvents like dimethylsulphoxide, acetonitrile and dimethylformamide had to be discarded. Nitromethane absorbs less strongly than the previous solvents and can be used within certain limitations as a solvent, especially in thin layers (absorption cells of 1 ram. or less). An improvement can be made by the use of deuterated nitromethane CDaNO~, which we prepared from CHaNO2 and D20 by WILSON'Stx0~method; it still absorbs in the region 2"7-2"8/~, as does nitromethane, but the t7~ F. A. CoTroN, D. M. L. GOODGAMEand M. GOODGAME,.J'. Amer. Chem. Soc. 83, 4690 (1961). la~ C. FURLANIand G. MORPURGO,Z. Phys. Chem. N.F. 28, 93 (1961). tg~ C. FURLANIand A. FURLANI,J. Inorg. Nucl. Chem. 19, 51 (1961). tlo~ T. P. WILSON,J. Chem. Phys. 11, 361 (1943).
Electronic structure and spectra of tetrahalogenoferrates (II)
163
absorption bands of CH3NOz at 2.2-2-3/~ and at 2.4 p are greatly reduced in intensity, so the opacity of the solvent is also greatly reduced. Good results are obtained also by the use of chloroacetonitrile as a solvent; although C1CH2CN has several intense absorption bands between 2 and 3 # (at 2.20, 2.35, 2-54, 2.67/~), they are very narrow, and obscure only a few narrow frequency intervals in that region, so that the contour of a rather broad absorption band, as is that of [FeCI~] ~-, can be completely identified; chloroacetonitrile has the additional advantage of being a very good solvent, where fairly concentrated solutions of halogenocomplexes can be prepared, thus allowing use of very thin absorption cells and further improving the total transparency of the solvent. The vibrational spectrum of the onium cations can be sometimes observed as a system of narrow bands superimposed to the envelope of the much broader d-d band of the anion. Most measured spectra were of the trimethylbenzylammonium salt in the presence of the corresponding chloride, which are both easily soluble in the solvents mentioned above; the tetramethylammonium salt is much less soluble, and the tetraphenylarsonium tetrachloroferrate is itself well soluble, but tetraphenylarsonium chloride is not, and the complex could be measured only in presence of a very small excess of free C1- ions. Spectroscopic results could be obtained for all three mentioned tetrachloroferrates (II), and no significant difference between them was detected (see Table) except a shift of ~ 4 0 0 cm -1 in passing from solutions to the solid state and a somewhat lower extinction coefficient for the tetramethylammonium salt. Ligand-fieMcalculations. The term system of Fe 2~ was calculated only for the split terms of 5D under a field of tetrahedral symmetry with spin-orbit coupling, in two weak-field approximations, i.e. (SLMsM~) --~ (SLJMa) --+( F M F M j ) and (SLMsML) -~ (S FMsMI,) ~ (I'Mr,Mj), which gave exactly coincident results; matrix elements for the first reckoning scheme can be found in Griffith ~1~ and for the second one in CONDON and SHORTLEY~1~ "Theory of the atomic spectra", Cambridge Univ. Press 1953. Since the ligand field interactions due to tetrahedral environment represent a very weak perturbation we are confident that the adoption of a weak-field scheme and our neglecting the higher levels of lower spin multiplicity are fully justified. We assumed ~ = 360 cm -~ or 2 = --90 cm 1 (a slightly smaller value than in free Fee+), and varied Atetr" between 2500 and 4500 cm 1. As is evident from Fig 2 the tetrahedral ground state 5E(Ta) is left still practically degenerate by the spin-orbit perturbation, whereas the upper state (~T2, Ta) is split to a much larger extent; the best A value which can fit the average experimental frequency is 4000 cm -1.
Acknowledgement--The present work has been carried out under financial support by NATO through a NATO Research grant. 01! j. S. GRIFFITH, Theory of Transition Metal Ions. Camb. Univ. Press (1961). ~:~ CONDON and SHOTLEY, Theory of Atomic Spectra. Camb. Univ. Press (1953).