Electron Paramagnetic Resonance Studies of Iron( 111), Ruthenium( 111), and Osmium( 111) with Sulfur-Donor Ligands’ Richard E. DeSimone Contributionf r o m the Department of Chemistry, Wayne State University, Detroit, Michigan 48202. Received April 2.5, 1973
Abstract: Results of a detailed magnetic resonance investigation of iron group complexes of the form MI*I(S-S)p(M = Fe, Ru, Os; S-S = bidentate sulfur donor ligand) are reported. The apparently near-isotropic g values of these complexes, in contrast to those of most other t2,5 complexes, are explained in terms of a consistent model of the bonding which requires a large low symmetry distortion both geometric and electronic in origin. This conclusion, contrary to that reached in previous epr studies, is supported by analysis of the hyperfine coupling arising from 1°IRu. It is found that the extensive metal-ligand covalent interaction reduces the spin-orbit coupling constant of Ru(II1) to about 40z of its free-ion value and that the hyperfine coupling originates from both the usual indirect or polarization mechanism as well as a direct mechanism. Difficulties which arise from a purely 2T2 model are considered, and their effect on interpretation of the data is discussed. It is concluded that, when properly ap-
plied, the simple crystal field model will qualitatively yield the correct results, but, in the limit of large distortion from octahedral symmetry, the quantitative results of this treatment are of dubious value.
S
1
ments is essentially the same, it is not clear how such ince the original paramagnetic resonance work on drastically different results are obtained. In view of the Fe”’(CN)B”,? a large number of d5(bg5)complexes ambiguities which exist in this area and in view of the has been studied by electron spin resonance; this has continuing interest in sulfur coordination in a number been especially true during the last few yearsU3-” One of metalloenzyme systems1j-20 as well as in the numerfinds in general that the low-spin d5 configuration is a ous models proposed for these biological systems, it was good probe of molecular structure and bonding since felt that a detailed epr study of these complexes was the observed g values vary widely and are sensitive to warranted. It is hoped that the conclusions reached small changes in structure and to the covalent interacherein will shed some light on the nature and origins of tion with the ligands of the complex. the electronic distortions induced in metal complexes of In contrast to this “normal” behavior, one comes sulfur bonded ligands. across an apparent anomaly when considering the trisThe approach taken involves the study of the entire bidentate chelate complexes with sulfur donor ligands, iron group, Fe(III), Ru(III), and Os(III), all of which Fe(S-S)3“- (n = 0 or 3); here one finds the g values are form similar low-spin complexes with the ligands of markedly closer to lgi = 2.0 and apparently insensitive to interest. There are a number of advantages in studythe differences among the various ligands. These reing the complexes of the heavier members of the series. sults have in general been interpreted as arising from First, for comparative purposes, there are many more very small low-symmetry distortions,6,9 a conclusion to be found here; a comparawhich is surprising, since the results of M o s ~ b a u e r * ~low-spin ~ ~ ~ ~ ~complexes ~~ tively small number of Fe(I1I) complexes are low spin and magnetic s u s ~ e p t i b i l i t y ~experiments ~~4 as well as while this behavior is universal for Ru(II1) and Os(II1). structural investigations8 often d o not support this conSecond, the approximations inherent in the model6I2l clusion. Since the model invoked in all of these experiby which the epr results are interpreted are much less important where free-ion term and primary Crystal (1) Presented in part at the 165th National Meeting of the American Chemical Society, Dallas, Texas, April 1973. field splittings are large as in the second- and thirdney, and I g,, g, and g,, g,, g, all negative. Throughout the remainder of this paper, solutions 1 and 2 will refer to these two sets of solutions as defined here.
tha
Discussion Application of the preceding theory to the iron(II1) dithio chelates previously studied by epr has led to the conclusion that the ground-state Kramers’ doublet wave functions I) and I)’ are basically composed of the (dZ, f dy,) or t+ h ~ l e , ~implying -~ a very small distortion of only a few inverse centimeters from octahedral symmetry. Only in the case of the Fe(ttd)(dtt)z and Fe(dtt)(ttd)2 complexes (dtt = dithiop-toluate; ttd = trithioperoxy-p-toluate), did Rickards, Johnson, and Hi1112 propose a ground state consisting of a d,, hole and here more in order to obtain agreement with their Mossbauer than to fit the epr data. They were not able to reproduce g, well (this turns out to be the key to the whole Problem (29) A. Abragam and M. H. L. Pryce, Proc. Roy. Soc., Ser. A , 205, 135(1951). (30) A. J. Freeman and R. E. Watson in “Hyperfine Interactions.” Academic Press, New York, N. Y., 1967, Chapter 2, p p 53-94.
as we shall see) and in fact a much better fit is ob- I tained with solution 2. The Mossbauer study of Fe(~acsac)~by Reiff and Szymanski l 3 indicated a d,, hole, but their calculation based on reported7,* epr g values indicated an acceptable fit could only be achieved with a (dZP d,,) hole and small distortion. This is clearly inconsistent with not only the sign of the distortion but also with the large magnituie of AE,, = 1.9 mmjsec at 300”K.8~13They rejected this interpretation in favor of that indicated by their Mossbauer results. The previous epr studies then, indicate for all of these Fe-sulfur chelates a small distortion from octahedral symmetry. Many of these same complexes show large quadrupole ~ p l i t t i n g s , ls2~X-ray ~~ structures of F e ( e ~ a n ) ~Fe(dtt)3. ~ CHC13,32 and Fe(sacsa~)~g show considerable distortion from idealized geometry, and magnetic moments are generally low and rather temperature independent. This is not the kind of behavior which one would expect from slightly distorted molecules with T ground states. These should show little asymmetry in the electric field gradient at the Fe nucleus and should have large temperaturedependent second-order Zeeman contributions to the magnetic moments, which should thus be higher than are observed.33 The values of the orbital reduction factor for these complexes as determined from solution 2 are very similar, as in fact are all of the parameters, and range from 1.03-1 .08.6 There has recently been considerable discussion of the orbital reduction factor, its expected magnitude, its interpretation as a covalency parameter, and, in this context, as a “sink” into which all other unaccounted for effects are drawn.?$’> Griffith, 28 Th0rnley,3~and more recently Cotton3” have considered modification of these high values of k , and Cotton has shown that for F e ( d t ~ )and ~ Fe(e~an)~ k can be reduced from approximately 1.07-1.08 to 0.83-0.87 by considering t2*(3Tl)e and b4(lT,)e configurations mixed into t2,5(2Tz) by electrostatic interactions. Spin-orbit and charge transfer mixing have been assumed relatively unimportant. Steps of this sort are undoubtedly in the right direction, but cannot really be expected to attach any more significance to the value of k . The reasonableness of k must clearly not be taken as the sole criterion of acceptability for a solution, even though this is quite tempting. In view of the factors delineated above, a reinterpretation ofthe epr results warranted, The data in Tables I and I1 establish the essential similarity of Ru(II1) and Os(II1) complexes to their Fe(II1) counterparts. Fitting the results for Ru(l11) to the theoretical equations is a bit more illuminating, however. Table 111 presents the two acceptable fits for several Ru(II1) complexes, and Table IV compares the results for Fe(III), Ru(III), and Os(II1) with the same ligands, namely sacsac- and mnt?-. Examination of these tables shows that in fact for Ru(II1) and Os(I1I) there are two solutions which cannot be rejected out of hand. (As the results for Fe(sacsac)s and Fe(mnt)3:3-
I
*
(31) B. F. Hoskins and B. P. Kelly, Chem. ComJnlrjt.,45 (1970). (32) D. Coucouvanis and S. .I. Lippard, J . Amer. Chem. Soc., 91, 307(1969). ( 3 3 ) B. N. Figgis, Trans. Faraday Soc., 57, 198 (1961). (34) J. H. M.Thornlev.J.Phi,s. (Paris).1 . 1024(1965).
DeSimone 1 Epr Studies oj’Low-Spin d 5 Complexes
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I I
6242 Table 111. Comparison of the Results of the Two Possible “Fits” to the Epr Data Obtained for Ru(II1) Sulfur ChelatesQ Ru(~acsac)~ Ru(dtc), R U( dto) 3 3Ru(dtp), Ru(mnt), ?R u ( o x ) ’~ R u(acac)
No.
k
AI€‘
4Ee
A
B
C
1 2 1 2 1 2 1 2 1 2 1 2 1 2
0.96 1.03 0.70 1.06 0.26 1.01 0.46 1.03 0.41 1.03 0.44 1.02 0.60 1.03
32.72 -0.03 t o , 20 -0.05 11.05 -0.02 12.56 -0.03 11.94 -0.04 4.0 -0.146 2.4 -0.30
-5.36 +0.003 -0.53 $0.003 0.0 0.0 -0.71 $0 ,001 -1.69 $0.003 -0.48 +o. 01 -0.16 +o, 02
0.029 0.823 0.074 0.805 0.066 0.812 0.060 0.810 0.075 0.825 0.215 0.788 0.323 0.754
0,999 0.568 0.997 0.593 0.998 0.583 0.998 0.586 0.997 0.566 0.974 0.615 0.945 0.656
-0.013 0.004 -0.010 0.005 0.0 0.0 -0.009 0.003 -0.030 0.006 -0.067 0.028 -0.050 0.029
Rows labeled 1 and 2 refer to solutions 1 and 2 as defined in the text. In units of [. (i
c
R U ( O X ) ~and ~ - Ru(acac)3 have been included for comparison.
Table IV. Comparison of Acceptable “Fits” for Complexes of Fe(III), Ru(III), and Os(II1) with the Same Ligands”
No.
k
AI€h
e/€*
A Elh
AE2b
A
B
C
1 2 1 2 1 2 1 2 1 2 1
1.98 1.06 0.96 1.03 0.20 0.94 1.35 1.08 0.41 1.03 0.33 1.01
35.93 -0.042 32.72 -0.037 4.21 -0.123 17.75
-3.47 0.005 -5.36 0.002 -0.634 0.008 -1.85 0.007 -1.69 0.003 -0.463 0,011
25.54 1.48 15.68 1.48 2.33 1.46 12.22 I .48 6.92 1.48 2.96 1.46
46.37 1.52 48.82 1.52 6.36 1.55 23,39 I .52 17.12 1.52 6.01 1.54
0.022 0,824 0.029 0.823 0.219 0.838 0.045 0,827 0.075 0.825 0.191 0.834
0.999 0.566 0,999 0,568 0.971 0.546 0.999 0.560 0.997 0.566 0.980 0.550
-0.006 0,009 -0.013 0.004 -0.085 0.014 -0.014 0.012 -0.030 0.006 -0.053 0.019
Fe(sacsac)a Ru(sacsac)? Os(sacsac)? Fe(mnt)??R u( m nt )
: 0 P = -94 X 10-4cm-1 K = 0.161 (r3) = 10.75 au AIIIAl < 0 P = -24 X cm-1 K = 0.945 (r--3) = 2.71 au x = +3.84au Calculated free ion value of ( r - 3 ) = 6 . 5 au38 Calculated free ion value of x = - 8 . 5 81138
512, and the six A , , hyperfine components of the g , , resonance are nicely resolved. The six A , components of the g, resonance (the spectrum of this compound appears nearly axial so we use the designation of 1 1 and I rather than x , y , z ) are not all resolved; however, we can readily see the uniform spacing of four of the six lines. This spectrum will be further investigated to 4.2" to obtain more accurate results. Our hyperfine calculations will thus not be as precise as they could be, but simply knowing that A , , is nearly twice Ai is sufficient to provide all the information we need. With reasonable estimates of error. A , , = 38 i: 1 G, A , = 21 =I= 3 G. As with the g values, we do not know the signs of All and A L . We do know, however, that the sign of P must be negative since for lolRu pn = -0.69, and this is reflected in the sign of y. This simplifies things a bit since solutions to equations (7) which yield positive values of P may be re.jected. We are left with four solutions, two each for the two fits to the g values. These are given in Table V, along with the calculated parameters P , K , ( r - 9 , and x. Little is known of the nature of the hyperfine interaction in second-row transition metals, in large part because of the paucity of experimental data.4j37 Freeman and W a t ~ o n ~using ~ ~ ~ unrestricted * HartreeFock methods have calculated, for a great many ions, values of x and ( r - 9 , For first-row transition ions, x = - 3 au, increasing only slightly as one moves across the series. This was first noted by Abragam and Pryce and has since been d e m o n ~ t r a t e dfor ~~ a large number of complexes. Among the secondrow elements x and ( 1 - 9 are expected to be larger, but any such constancy in x has not been noted. The values calculated for Ru3+ by Freemen and Watson are ( ~ - ~ ) ~=d 6.5 au and x = -8.5 a u 3 * Covalent interactions are expected to decrease both of these quantities; x is less predictably affected than (r3-) due to the dependence on K . From the data in Table V, it is seen that three of the four possible solutions yield ( r - 9 much greater than the free ion value and may be discarded. One might be induced to allow a value close to or slightly greater than the free ion value if it appeared that weaknesses in the theory might be responsible. However, as we have seen, this is not the case for Ru(1II) and these values are clearly unreasonable, even in view of error associated with the Hartree-Fock calculation. The only accept(37) W. Low, J. Appl. Phqs., 39,1246 (1968). (38) A. J. Freeman and R. E. Watson, "Magnetism," Vol. IIA, G. T. Rad0 and H. Suhl, Ed., Academic Press, New York, N. Y . , 1965. (39) B. R. McGarvey, J . Phys. Chem., 71,51 (1967).
Solution 2 All/Al > 0 AllIAl
> A , A I , / A , < 0 of the configuration in which the hole resides in the d,, (or t o ) orbital. This conclusion is consistent with the large low-symmetry distortion expected on other grounds. The value of ( r - 9 = 2.7 au represents a reduction to 40z of the free-ion value, indicating considerable metal ligand interaction. Since the spin-orbit coupling constant depends , ~may ~ infer a similar reduction directly on ( ~ - 9 we in [ upon complex formation giving a value of teff = 480 cm-' based on [ = 1200 cm-'. This of course depends upon the correctness of the calculated value of ( r 3 ) + 3 for R u 3 t , but the results seem reasonable. Perhaps most interesting is the negative value of K which in turn produces a positive value of x = +3.84 au. A few comments are in order concerning the origin of x. For most transition metals in high symmetry environments, x is presumed to arise from polarization of filled inner s-orbital electrons by exchange interaction with unpaired electrons residing in the d orbitals,39 thus the term core polarization and the negative sign associated with x. It is possible, however, in certain instances, to have a direct interaction between s and d orbitals containing the unpaired spin. In this case the contribution to x would be positive and of course only a very small amount of direct admixture of s and d orbitals would be required to offset the relatively small effects of polarization. This situation is allowed in complexes of D 3 symmetry (and in most, but not all lower symmetries) and has been found to exist in complexes of vanadium, chromium, and molybdenum with dithio chelates. A discussion has been given by M ~ G a r v e y . ~The ~ results which we have found here are thus not unacceptable in any way, and it is felt that the problem is adequately resolved.
Conclusions Having established conclusively the situation which exists in Ru(detc)3, the remaining task is t o extend this to the other Ru(II1) and of course to the Fe(II1) complexes. The essential question is this: is it reasonable to assume that all of these complexes share the feature of a large low-symmetry distortion and a 2A ground term? As was discussed previously, there is independent evidence for large distortions in many of these complexes. Complexes with four-membered chelate rings obviously are subject to considerable strain due to the small S-M-S angle, often as small as 73 '. 3 2 , 3 6 Complexes with larger chelate rings would (40) M. Blume, A. J. Freeman, and R . E. Watson, Phys. Reu., 134, A320 (1964).
DeSimone
/ Epr Studies oJ'Low-Spin dJ Complexes
