QUARTET-DOUBLET TRANSITION IN HEXACYANOCHROMATE(III) 605
VoZ, 2, No. 3, June, 1963
CONTRIBUTION No. 1108 FROM THE DEPARTMENT OF CHEMISTRY, INDIANA UNIVERSITY, BLOOMINGTON, INDIANA
A Quartet-Doublet Transition in Hexacyanochromate (I11 ) Anion BY R. KRISHNAMURTHY
AND
WARD B. SCHAAP
Received October 6, 1962 During the course of other studies concerned with Cr(CN)s3- complex ion, a new weak band a t Xmax = 530 mp has been detected in the visible absorption spectrum of its aqueous solution. This band is hidden in the tail of the much more intense first spin-allowed absorption band, 4 A ~-+ g 4T2g(F), and has a molar extinction coefficient of 0.44. The band has been interpreted from the standpoint of the Tanabe-Sugano ligand field model, and assigned to the spin-forbidden 4A2, + *Tzg (G) transition. The significance of this transition is discussed and correlated with the nephelauxetic effect of the cyanide ligands in the complex.
The Laporte-forbidden d-d absorption spectra of chromium(II1) with many ligands have been interpreted by the ligand field theory.l-* However, in the case of hexacyanochromate(II1) ion in aqueous solution, only the first two spin-allowed bands, located a t 375 and 310 m p , have been o b s e r ~ e d . ~These have been assigned t o the 4A2, + 4T2,(F) and 4A2, 4T1,(F) transitions, respectively (Fig. 1 and 2 ) . There is no previous report of either the third spin-allowed band, 4A2g+ 4T1,(P), or the spin-forbidden bands, 4A2,+ (2Eg,2T1,) (G) and 4A2g+ 2T2g (G). During the course of other studies concerned with this complex ion, an additional weak and previously unreported band was observed. Therefore, we thought i t of interest to study the complete spectrum, since a knowledge of these bands would enable one to calculate the Racah parameters, B and C, of electrostatic interactions between the d electrons in the complex and permit a chemical interpretation of the “ n e p h e l a ~ x e t i c ’ ) ~ ~ * ~ ~ ratios, p35and p 5 5 , with respect t o the ‘covalent’ tendency of the cyanide ligands in the complex. Using concentrated solutions, we detected a t the foot of a strongly increasing absorption a distinct shoulder of very low intensity located a t Amax = 530
-
(1) R. Finkelstein and J. H.Van Vleck, J . Chem. P h y s . , 8 , 790 (1940). (2) C. K. JZrgensen, Reports t o t h e Tenth Solvay Conference, 1956, p. 362. (3) C. E. Schaffer, J . Inorg. Nucl. Chem., 8, 149 (1958). (4) H . L. Schlafer, Z . Physik. Chem. (Frankfurt), 11, 65 (1957). (5) T. M. Dunn, “Modern Coordination Chemistry,” J. Lewis a n d R . G. Wilkins, Ed., Interscience Publishers, New York and London, 1960. (6) L. E. Orgel, “An Introduction t o Transition-Metal Chemistry: Ligand-Field Theory,” Methuen, London, 1961, p. 97. (7) J. S . Griffith, “The Theory of Transition Metal Ions,” Cambridge University Press, Cambridge, 1961, p. 301. (8) C. J. Ballhausen, “Introduction t o Ligand Field Theory,” McGrawHill Book Co., New York, N. Y., 1962, pp. 235-239. (9) H. Kuroya and R. J. Tsuchida, J . Chem. SOL.Japan., 61, 597 (1940). (10) G . Racah, P h y s . Rev., 62, 438 (1942); 63,367 (1943). (11) E. U. Condon and G. H. Shortley, “The Theory of Atomic Spectra,” Cambridge University Press, London, 1959. The Racah parameters, B ( = Fz - 5Fa after Slater, Condon, and Shortley) and C ( = 35Fa), describe the energy differences between t h e Russell-Saunders states in t h e free ion. (12) C. E. Schaffer and C . K. J#rgensen, Symposium on Coordination Compounds, Rome, 1957; Ricevca Sci., 28, 143 (1958). (13) C. K. J@rgensen, “Absorption Spectra and Chemical Bonding in Complexes,” Pergamon Press, New York and London, 1962, pp. 134-145. “Nephelauxetic” is derived from the Greek words meaning “cloud expanding.” T h e charge cloud of the d electrons expands, leading t o t h e decrease of t h e interelectronic repulsion parameters in going from t h e free ion to t h e complex, a n d corresponds t o t h e expanded radial function of t h e partly filled ion. For a d-shell. T h e nephelauxetic ratio, 8, is equal t o Boompier/Bfree given metal ion, the more common ligands have been arranged in t h e following series in order of decreasing values of 8: F- > H20 > NHs > en > 0x2- > S C N - > C1- > CN-.
2
(‘29
I ; i
. ___
)(ed[
.4 T 9
4F
4F 2G *G
4F Fig. 1.-Energy
level diagram for octahedral chromium(II1) complexes.
mp, with a molar extinction coefficient of 0.44 (Fig.
3a). It is not surprising that this weak shoulder was not reported previously, since i t is hidden in the tail of the much more intense first spin-allowed band (Fig. 2 ) . Because of its intensity and position compared to similar bands of other Cr(II1) complexes, we assign this weak band to the ground 4A2g+ 2T2, (G) transition. The method of “subtraction of extrapolated background” was used to locate and separate the band.14 The differences in optical densities of the shoulder and the extrapolated background are plotted against the wave length in m p in the region of interest and show a parabolic shape characteristic of a n absorption band (Fig. 3b). The third spin-allowed band of Cr(II1) complexes, corresponding to the transition 4A2,+ 4T1g (P), is predicted to lie far in the ultraviolet and could (14) Reference 13, p. 101.
GO6 K. KRISI-INAMURTHY AND WARDU.SCIIAAP 0.9
I?aovgaizic Cheniistry
c
A, mp
Fig. 2.-Absorption
M solution of Cr(CS),3- a t 25’ with a 5-cm. cell.
spectrum of a 1.8 X
I
I
I
I
I
480
520
560
600
640
A. m Fig. 3.-(a)
-~-
p
Absorption spectrum of a 0.10 M solutioii of Cr(ChT)c3- a t 25’ with a 5-cm. cell. (b) Insert: An enlarged plot of the difference between observed spectrum and extrapolated background in the region 510-550 mp.
not be observed in our spectrum. I n addition, Schaffer15 has demonstrated that this band often deviates from the predicted position due to electron-transfer interactions. The other low energy transition, 4A2,+ (*E,, 2T1,)(G), could not be detected. Experimental “Electronic Grade,” yellow, crystalline potassium hexacyanochromate(111) was purchased from the City Chemical Corporation, New York, li. Y . There was no noticeable change in its absorption spectrum upon recrystallization from water. K C r (CN)e was synthesized in this Laboratory by a method described (15) Refereuce 12, p. 140.
previously.16 Its spectrum mas identical with that of the “Elcctrouic Grade” sample, both exhibiting the weak shoulder band a t 530 mp. The molar extinction coefficients of the 375 and 310 mp bands are in excellent agreement with those reported previou~ly.~ Near-infrared, visible, and ultraviolet absorption spectra were obtained with a Cary Model 14 recording spectrophotometer. The aqueous solutions were clear, yellow, and quite transparent. The spectra were immediately measured in matched silica cells of 5.0 cm. path length, against water as a reference, from i o 0 mp t o shorter wave lengths until intense absorption set in.
Calculations The strong field electrostatic matrix elements have (16) W. C. Ilernclius, I?$-YZ. Syiz., 2, 303 (104F).
VoZ. 2, No. 3, June, 1963
QUARTET-DOUBLET TRANSITION I N HEXACYANOCHROMATE (111) 607
Dq, B, and C to calculate the energy of the lower, spinforbidden transitions to the 2T1,and 2E, states predicts the absorption to be around 740 mp (13,500 cm.-l). The validity of the assignment to ZTz, can be justified further by comparison with experimental values for other Cr(II1) complexes. In the cases of C ~ ( O X ) ~ ~ - , C T ( H ~ O ) ~and ~ + , ruby (Cr3+, Alz03), whose spectra are well understood, the (2T16, 2Eg) lie a t 700 mp 4Aa, (F): -12Dq 'E, ( G ) : -1209 + 9B + 3C (14,350 cm.-'), 670 mp (15,000 cm.-l), and 692 mp 'TI, (G): -12Dq + 9B + 3C (14,450 cm.-'), respectively. It is very unlikely that 'Tz. ( G ) : -12Dp 3. 15B + 5C this transition would shift to 530 mp (18,870 cm.-l) The energies of these terms are not corrected for conin Cr(CN)6a-because, as can be seen from the Tanabefigurational interactions. Using the average value of Sugano diagram, the transitions between the ground C = 4B in the non-diagonal elements and applying the state and the 2T1,and 2E, states should be almost indesecond order perturbation theory, Schaff erl* has calpendent of the ligand field strength. Also, (9B culated from the Tanabe and Sugano determinants the 3C) would be larger than this quantity, and hence B > corrections due to the effects of intermixing of the 900 cm.-l and p 5 5 > 0.98 if the transition were 2TI,, other sub-shell configurations, The energy differences 2Eg. Actually, p 5 5 would be greater than 1 after the above the ground state of the doublet then correction for sub-shell configuration interactions in turn out to be this case and it would be unrealistic for p36 to be so much smaller than &. On the other hand, if the ob2EE,: 9B 4- 3C - 90B2/10Dg 'TI,: QB + 3C - 24B2/100q served weak band is fitted to the 2T2, energy level, 'Tz,: 15B f 5 C - 176B2/10Dg (15B 5C) gives a B value of 540 cm.-l, which after The energy of the first spin-allowed absorption band, correction for sub-shell configuration intermixing, observed a t 26,700 cm.-l, gives directly the value of -176B2/10Dq for 2Tz,,leads to 590 cm.-l for B and 1ODq. Using this Dq value and making the second 0.64 for p 6 5 , which are reasonable. spin-allowedband fit the eigenvaluesof the determinants The nephelauxetic effect has been correlated with assuming intermixing of the 4T1,states,21the Racah the reduction in term distance and with the covalency parameter, Boomplex, can be shown to be equal to 530 or polarizability of the ligands.12 In other words, the cm.-', as Schaffer and Jqirgensen have shown.12 This effect is to delocalize the d electron cloud over the engives a value of 0.58 for p 3 6 . From the 4 A ~+ , 2T~, (G) tire complex, thus increasing the mean distance between transition, which occurs a t 18,870 cm.-l, the Racah the d electrons and thereby decreasing the interelecparameter, Bcomplex, turns out to be 590 cm.-l. This tronic repulsions. If both the sub-shells, t 2 g and eg, are leads to another nephelauxetic ratio, 0 5 5 , of 0.64. The expanded to the same extent, it may be assumed that value of 15B corresponding to the (4P - 4F) interval there is one nephelauxetic ratio.22 If 0 3 6 is significantly for the free Cr3+ion used for the calculation of /3 values lower than 666, the eg sub-shell, being u antibonding, is taken to be 13,770 cm.-'. 035 is a measure of the carries most of the nephelauxetic effect caused by interaction between one electron in each of the two 'symmetry-restricted' covalency, while the tzgsub-shell, sub-shells, eg (or 73) and tz, (or y6). 866 refers to the which alone can be affected by r-bonding, is closer to interaction between electrons in the lowest sub-shell, "electrostatic behavior ." In most chromium (II I) comtag,in which spin-forbidden transitions take place.20,22 plexes where n-bonding is much less important than u-bonding, &,6 shows a much less drastic variation than Discussion p 3 6 (Table I). However, in the case of Cr(CN)s3-, the value of If one uses the values of Dq and B derived from the p 6 5 is significantly different from the corresponding quartet-quartet transitions and assumes that B is values found for the other Cr(II1) complexes. We constant and that C = 4B, the calculated value for do not have a further independent check on this the 2T~, transition lies around 570 mp (17,600 cm.-l), which is close to the experimentally observed band a t TABLE I 530 mp (18,870 cm.-l), making this assignment reasonKEPHELAUXETIC RATIOS IN OCTAHEDRAL d3 CHROMIUM COMable. On the other hand, use of these same values of been calculated for dn systems in cubic field ( o h ) symmetry by Tanabe and Sugano.17 The diagonal elemeqts of energy of the lowest states springing from the sub-shell configuration (Tzg)3(Fig. l), in terms of Racah parameters, B and C,and the ligand field perturbation parameter, lODq (denoted also (E1 - Ez) or A), are
+
+
PLEXES
(17) Y. Tanabe and S. Sugano, J. Phys. SOC.Japan, 9, 766 (1954). (18) C.E. Schiiffer, unpublished results. (19) C. K. J#rgensen, private communications. (20) C. K. J#rgensen, "Orbitals in Atoms and Molecules," Academic Press, New York, N. Y., 1962, p. 57. (21) The only two states of the same symmetry designation arising from the different configurations (tpg)2(eg)1and (tzg)1(eg)2are PTig(F) and 4Tlg(P), respectively. They interact and give rise to the non-diagonal element, 6B in this symmetry, which perturbs the two levels, depressing the former and raising the latter by equal amounts of energy relative t o the non-interacting case. This interaction causes the hyperbolic shape in the Orgel or the Tanabe-Sugano diagrams. (22) C. K. Jgrgensen, Puogr. I i r o i g . Chem., 4, 73 (lQO2).
Complex
Environment (donor atoms)
p5sa
83jQ
Cr(HzO)s3+ 6 0 0.91 0.79 Cr(urea).?+ 6 0 .88 .72 Cr(ox)386 0 .88 .68 CrCI3(solid) 6 c1 .83 .56 Cr(dtp )3b 6s .80 .45 Cr( CN)e36C .64' .58 a 666 calculated from spin-forbidden and 6 7 5 from spin-aldtp- = diethyldithiophosphate anion. lowed transition^.^^.^^ o b 5 = 0.64 obtained from this work.
'
GO8 MANSOUR ABEDINIAND ALANG. ~L~ACDIARMID
parameter since the 4A2g+ (2Eg,2T1,)transition could not be detected in our work. The value of p55 is close to that of ,835. This demonstrates that the n*-antibonding delocalization is more or less of equal importance as c*-antibonding delocalization, presumably because the orbitals of the lower triplet have the correct symmetry to overlap with the empty n*-antibonding orbitals of the cyanide ligands. This results in a further depression of the triplet with respect to the upper doublet, which accounts for the relatively higher value
Inorganic Chemistry of lODq, and also argues against applying to cyano complexes the simple "electrostatic crystal field" approach in which n-bonding is ignored. Acknowledgments.-The authors wish to express their appreciation to the U. S. Atomic Energy Commission for a grant (AT 11-1-256) with which this work was supported. R. K. is particularly grateful to Dr. C. K. J@gensen for communicating preprints on related topics prior to their publication.
CONTRIBUTIOS FROM
THE JOIIN
HARRISON LABORATORY OF CHEMISTRY, 4, PENSSYLVANIA
GNIVERSITY O F PEKNSYLVANIA, PIIILADELPHIA
The Preparation and Properties of Some New Nitrogen and Fluorine Derivatives of Disilanel" BY M.4NSOUR ABEDIic'I
AND
ALAN G . M A c D I ~ ~ R M I D ' ~
Received Oclober 8, 1962 Disilanyl bromide, H,SiSiHaBr, has been found to react with ( C H J ) ~ N H to yield the new compounds H & ~ H ~ N ( C H and Z)~ H3SiSiH [N(CH3)2]2. Boron trifluoride underment reaction with (H3SiSiH2)& a t low temperatures to give the addition compound (H3SiSiH2)3S.BF,,which then decomposed to yield the new species, H3SiSiH2F. A number of physical properties of the above new compounds have been examined and the significance of certain properties of analogous silpl, disilanyl, methyl, and ethyl compounds are discussed.
As a continuation of a study of the Si-Si bond as reflected in the properties of compounds containing this l i r ~ k a g e , ~i -t ~was considered desirable to investigate further the properties of disilanylamines since only one nitrogen derivative of disilane, (H3SiSiH2)3N, had previously been described. Although the silylamines (H3Si)3N, (H3Si)?NCH3, and H3SiN CHI)^ have all been isolated,6-8 H3SiN(CH3)2is reported to be unstable thermally and i t decomposes a t a measurable rate a t 3-4'.* It was therefore of interest to attempt to prepare H3SiSiH2N(CH& to see what effect the substitution of an Si-H bond in H3SiN(CH3)2by a SiH3 group would have on the stability of the amine. A second purpose of the investigation was to ascertain whether BF3 would cleave the Si-N bond in (H3SiSiH2)3N to give H&3iSiHZF in an analogous reaction to that between BF3 and (H3Si)3N,S'10 vzz. (1) (a) This report is based on portions of a thesis t o be submitted by hfansoiir Abedini t o the Graduate School of t h e University of Pennsylvania in partial fulfillment of the requirements for the degree of Doctor of Philosophy. It is in part a contribution from the Laboratory for Research on t h e Structure of Matter, University of Pennsylvania, supported by the Advanced Research Projects Agency, Office of the Secretary of Defense. Reproduction in whole or in part is permitted for any purpose of t h e United States Government. (b) Alfred P. Sloan Research Fellow. ( 2 ) L. G. L. Ward and A. G. MacDiarmid, J . A m . Chem. SOC.,82, 2151 (1960). (3) I,. G. L. Ward and .4. G. MacDiarmid, J . Iizoug. Aruci. Chem., 21, 287 (1961). ( 4 ) L.G. L. Ward and A. G. MacDiarmid, ibid., 20, 345 (1961). (5) A. D. Craig, J. V. Urenovitch, and A . G. MacDiarmid, J . Chein. Soc., 548 (1962). (6) A. Stock and K. Somieski, B e r . , 64, 740 (1921). (7) H. J. Emelbus a n d N-. Miller, J . C h e w SOC.,819 (1939). (8) S.Sujishi and S . m'itz, J . A m . Chem. SOC., 76, 4631 (1954). (Y) S. Sujishi and S. Witz, ibid., 79, 2447 (1957). (10) -4. B. Burg a n d E. S. Kuljian, ibid., 72, 3103 (1950).
(H3Si)dS
+ BF3 -++H3SiF + (HdSi)2NBF2
(1)
or whether cleavage would occur preferentially a t the Si-Si bond. Results and Discussion Disilanyl bromide, H3SiSiH2Br,and (CH&NH were found to undergo reaction a t low temperatures t o yield the new compound N-disilanyldimethylamine,H3SiSiH*N(CH&. HaSiSiHzBr
+ 2( CH&NH -+
+
H$3iSiH,N( CH3)2 (CH3)2NH.HBr (2)
The presence of the H3SiSiH2group in the compound appeared, if anything, to make the material more stable thermally than the silyl analog, H3SiN(CH3)2, since 80% of a sample of H3SiSiH2N(CH3)*could be recovered unchanged after standing for 47 hr. a t 0". Also formed in the above reaction was the new compound, 1,1-bis-(dimethylamino)-disilane, H3SiSiH[N(CH8)?]2. I t appears that this may have been formed by the decomposition of some of the H3SiSiH2N(CH3)2 before it could be isolated and purified since a considerable quantity of S&H6also was found among the reaction products, viz.
+
2H3SiSiH2n'(CH3)2 -+HSiSiH[N( CH3)a]2 Si2H6 ( 3 )
The fact that CH3NH2, (CH3)&, and NH3 catalyze the decomposition of (H3Si)3NJ1lviz. 3(H3Si)dS --+ (SiH2SSiHd)3 4-3SiHa
(4)
(11) R . Schaeffer, L. Ross, M . Thompson, and R . Wells, Report No. 2, Office of Kava1 Research, Contract Nonr-908(14), Task N R 052-410, Aug. 31, 1960.
