1’. J. STEPHENS, AND P. N. SCHAIZ 1614 A. J. LICCAFFERY, CONTRIBUTION FROX
lnorganic ClzemLsLvy
DEPARTMENT OF CHEMISTRY, U ~ I V E R YS IOF ~ VIRGIYIA,CHARLOTTESVILLE, VIRGINIA, THEISSTITUTE FOR THE STUDY O F METALS, UNIVERSITY O F CHICAGO, CHICAGO, ILLIhOIS
THE
AXD
The Magnetic Optical Activity of d + d Transitions. Octahedral Chromium(III), Cobalt(III), Cobalt(II), Nickel(II), and Manganese(I1) Complexes’ BY A. J. MCCAFFEKY,~ P. J. STEPHEKS,3 A N D P. N. SCH4TZ2
Received Mulch 24, 1967 The utility of magnetic optical activity in the study of the d --L d transitions of transition metal complexes is discussed. Magnetic circular dichroism (MCD) data for d -c d transitions in a range of octahedral complexes are presented and analyzed. Mn(HZO)a2+,Co(H20)a2+,and Ni(H20)e2- show large effects, while Co( SHs)e3-, CO(CS)s3-, and the spin-allowed transitions of Cr(NH3)e3+,Cr(H20)a3-, and Cr(CN)e3-, in contrast, exhibit very small MCD. The latter result is tentatively attributed to quenching of orbital angular momentum in the degenerate excited states by vibronic (Jahn-Teller) interactions. A similar diminution was observed in Co(r\”3)6C12+, cis-Co(~H,),(H,0)r3+,cis- and tvans-Co(en)zC1zf, Co(en)p3+, C O ( O X ) ~and ~ - ,the spin-allowed transitions of Cr(en)33-and Cr( OX)^^-, with the exception of the ‘A1 -+ ‘E transition a t 24,000 cm-’ in C O ( O X ) ~ ~Here, -. a term characteristic of excited-state Zeeman splitting is found and the ‘E magnetic moment obtained is in good agreement with that predicted by ligand-field theory. The spin-forbidden transitions of Cr(II1) coniplexes show huge effects: in some cases greater than the much more intense spin-allowed transitions, which can greatly facilitate the assignment of these transitions.
Introduction
optical rotatory dispersion and circular dichroism Experimental studies of natural optical activity. results for a variety of d 4 d transitions have been reported by Margerie,22Briat,23p24 ShashouajZ5Djerassi,2G Foss, 27 Denning,28Yoshiwara and K e a r n ~ and , ~ ~ their respective co-workers. Theoretical interest has also been stimulated by the growth of ligand-field theory30 and by the recent d e ~ e l o p m e n t31-33 ~ ~ , and applicat i o n ~ ~ of ~ -the ~ *theory of the Faraday effect in regions of absorption. However, so far there have been few theoretical treatments of the experimental data for d --t d transitions39 and, in particular, no broad
The dispersion of the Faraday effect (magnetic optical activity) through regions of absorption has been studied sporadically for many years.4 Much of this work has involved the d --.t d transitions of transition metal complexes, since they occur in the most accessible spectral regions and can show large effects6 I n the early decades of the century attention was primarily directed a t the qualitative dispersion form since contradictory predictions were given by different applications of classical electron theory. The major work in this period was associated n-ith RobertsG,’and Cot(18) C. Djerassi, “Optical Rotatory Dispersion,” hIcGraw-Hill Book Around 1930 the early quantum mechanical Co., Inc., New York, X . Y., 1960. theories of the Faraday effect were d e v e l ~ p e d ~ ~ - ~(19) ~ S.F. Mason, Q X Q Y ~Rev. (London), 17, 20 (1963). (20) L. Velluz, h.1. Legrand, and M. Grosjean, “Optical Circular I)iand, following this, some attempts were made to classify chroism,” Academic Press Inc., New York, N. Y., 1965. 18$ experimental results on the basis of Serber’s (21) P. Crabbh, “Optical Rotatory Dispersion and Circular Dichroism ~ I I Organic Chemistry,” Holden-llay, San Francisco, Calif., 1965. However, a t that time neither the instrumentation for (22) J. hlargerie, Contpt. R e n d . , 257, 2634 (1963). accurate experimental work nor the molecular theories (23) B. Briat, hl. Billardon, and J. Badoz, ibid., 266, 3440 (1963). (24) (a) B. Briat, ibid., 259, 2408 (1964); (bj B. Briat, Thesis, C‘nivernecessary for its detailed interpretation existed, and sity of Paris, 1966. interest was transferred to the low-temperature studies (26) V. E . Shashoua, J . A m . Cheni. Soc., 86, 2109 (1964). (26) D. A. Schooley, E. Bunnenberg, and C. Djerassi, PYOC. Kall. A c a d . a t a single wavelength being made by Becquerel, de Sci. C.S . , 53, 579 (1965). Haas, and van den Handel.4b,17 Very recently the (27) D. S. Martin, J. G. Foss, bl. E. McCarville, M. A . Tucker, and A. J. Kassman, Iriorg. Chem., 6 , 490 (1966). pendulum has swung back, following the resurgence of (1) This work was supported in part by t h e Kational Science Foundation. (2) University of Virginia. (3) University of Chicago. (4) For reviews see: (a) W. Schutz, “Magnetooptik, Handbuch der Exgerimentalphysik,” Vol. 16, Akademische Verlagsgesellschaft, Leipzig, 1936; ( b ) A. D. Buckingham a n d P. J. Stephens, Ann. Rev. P h y s . C h o n . , 17, 39Y (1966). ( 5 ) Transition metal complexes are often paramagnetic, too, and i t has frequently been assumed (wrongly) t h a t a direct relation between paramagnetism a n d t h e Faraday effect exists. (6) R . W. Roberts, Phil. .Ilng., 9 [ i ] 361 , (19301, and preceding papers. (7) R. W. Roberts and S.F. Adams, ibid., 28 (71,601 (1939). (8) A. Cotton, Conzpl. Rend., 196, 01.5 (1932). (9) b1. Schiiel-, ibid., 196, 950 (1932). (10) M . SchPrcr and K. Cordonniei, ibid., 196, 1724 (1933). (11) K. Cordonnier, ibid., 206, 313 (1937). 112) L Rosenfeld, Z . P h y s i k , 67, 836 (1929). (13) IT, .4 Kiamer>, Ko?cinkl. . i - e , / , .4ktid, l V ~ L r ~ ~ i / z Pioc ~ i / ~,. 33, ‘.lfi!l (l!4iLIj, (11) II, molar ellipticity per unit magnetic field, for C O ( H ~ O ) ~ solid ~ + : line, experimental data; dashed lirie, gaussiaii best fit;peak-to-peak noise level negligible. E is the molar extinction coefficient.
[el
MO I -
-.oo - . O o 0I O l
.. .. .. . . ..* ... . . * .
00
F r e q u e n c y ( c TI-') Figure 3.--[8] AI for xi(&O)a2+: solid line, experimental data; dashed line, gaussiaii best fit; peak-to-peak iioisc level negligible.
1617 MAGNETIC OPTICALACTIVITYOF d + d TRANSITIONS
Vol. 6 , No. 9 , September 1967
... ... .e..
*
.
..*. *.. .. :.' *...*
100-
C i ~
..e*
...... F r e q u e n c y (crn-l)
Figure 4.-[8] M for Cr(NH3)o3+;vertical bar shows peak-to-peak noise level. M C D unmeasurable in 20,000-30,000-~m-~ region.
... .. ...
I 30,000
40,000
F r e q u e n c y (ern-') Figure ;I.-[e]11 for Co(CN)sa-: solid line, experimental data; dashed line, DOM best fit; vertical bar shows peak-to-peak noise level. MCD unmeasurable above 39,000 cm-'.
/. .. .*.. .... . .* . . . . I
7-
. .. .. . . ..* * .' .
.a*
6 '6j
ts...
.5
.4-
Figure 5.-[8] AI for Cr(H30)e3+; vertical bar shows peak-to-peak noise level. MCD unmeasurable above 24,000 ern-'. T'ertical arrow indicates shoulder due to spin-forbidden transition.
-60
.
Z'..
.
*.
-60
*.
a
-40 -20
left scale
.e*'
right scale
*..e
1
'
1
'
I
I
)OO
Frequency (cm-') for Cr(CN)63-; vertical bar shows peak-to-peak Figure8.-[8] noise level. M C D unmeasurable between 20,000 and 35,000 cm-1.
-
HMo: -.0010-
.00002
--.00207
I
€
Frequency
(crn-')
Figure 6.-[0] ar for Co(NH8)c3+:solid line, experimental data; dashed line, gaussian best fit; vertical bar shows peak-to-peak noise level.
Figure 9.-[e]
for Mn(H20)s2+;vertical bar shows peak-to-peak noise lcvcl.
r
I
r i g h t scale
€
left
right iscale
scale
20.000
40,600
30.000
F r e q u e n c y (crn-I) Figure 13.-[H] AI for Co( SH3);Cl2-: solid line, experimental ddta, dashed line, gaussian best fit; vertical bar shows peak-topeak noise level MCD was unreliable above 40,000 cm-I.
I
IO04
50
O i -.001
,
. . I
,...*.'....,*,,
bl,
1
-.003-
€
*
0
,
,~,,.,**.'*.'
~
......(. .
50
+'
l6.000
20,000
24,000
-
* . .. .. :e.
iI
28,000
..... ...: .. . .. ... . .. .. *.. ... .,. .* .. .: .... ... ..
., .. ... ... a
.
Frequency (ern-') Figure ll.--IH1\, fol- c-is-Co(en),C12: solid liiic, cxperitnciltal data; dashed line, IIOM bwt fit; vertical lxir S I I O W , ~ pcak-twpeak iioise level.
.
9
.
a.
.
I
.,. 1
I
15,000
!
25,000
I
35,000
F r e q u e n c y (crn-l) ry
Figure 14.-/8],1 for c i s - C 0 ( S H ~ ) ~ ( H ~ 0 )solid ~ 3 + line, : experimental data; dashed line, gaussian best fit; vertical bar shows peak-to-peak noise level.
_ _ - _ _ - - - ---.0010
:-1600
il
€
Frequency
(cm-')
Figure 12.-[e] 31 for trans-Co(en)2Clzf : solid line, experimental data; dashed line, gaussian best fit; vertical bar shows peak-topeak noise level. a Eactur
-
2 . 1)ipole strerlg'th values (/)(a j ) =: ( ' / & ) , Z ! ( a J r n J j ) l ? have ) been obtained either by iiumeric;il integration or by gaussian fitting of absorption data. In some cases where . I terms were expected but not observed, we iriclude upper limits (if
IAI?
....
'A,
&4 A.-.
._ 1 ,- D,
LYVD5 oh
, Oh
Cr
corn) Figure 15.-Splitting
of
01, d7Lstates
om
of Co(II1) and Cr(II1) in trigonal field.
:L
I
.a
Frequency
(cm-')
....
Figure 16.-/@] 31 for Co(en)a3+: solid line, experimental data; dashed line, gaussian best fit; vertical bar shows peak-to-peak noise level.
16.000
15,000
2aooo 24000
F r e q u e n c y (crn-1) Figure lS.-[e] y for C ~ - ( e n ) ~ ~vertical +: bar shows pedk-topeak noise level; solid line, experimental data; dashed line, DOM best fit. MCD unmeasurable above 25,000 cm-l.
.Ol-J&
T
I
,002
i
crystal
. .*. .a'.*
. I
.
.
.. ... .e*.
e
15,000
20,000
25.000
30C
0
F r e q u e n c y (crn-I) Figure 1 7 . - [ 0 ] ~ for C O ( O X ) ~ solution ~and Coa+-NaMgAl(oX)3.9H& crystal: solid line, experimental data; dashed line, gaussian best fit; vertical bar shows peak-to-peak noise level. The ordinate scales for the crystal are arbitrary.
for A I D . 111 other cases, the A-type terms observed iiiay, iii fact, arise from overlap of C terms of opposite sign (see below); the A values quoted are then to be construed as "effective" parameters. The accuracy of the MCD results varies considerably arid dc-
IJ,. . .
......
15,600
...... ...
.....
20,boo
.
I
. . . .'...... 25,600
F r e q u e n c y (cm-') Figure 19.-[e] AI for Cr(ox)a3- solution and Cr3+-NaMgAl(ox)a.QHtO crystal; vertical bar shows peak-to-peak noise level; vertical arrow indicates spin-forbidden absorption. The ordinate scales for the crystal arc arbitrary.
1620 A . J. M C ~ A F F E RP. UJ. , STEPHENS, A N D P.N . SCIIATZ
Inougaiiic Chemistry
TABLE la PARAMETERS FOR SPIS-.~LLOIVED TRAXSITIONS AND Aln( H20)6*' vnvaxr [eIar,""x /
,
cm-'
19,600 14,000; 13,500 25,300 21,600 2 8 , 500 17,200
24,300 21,000 29,500 32,100 16,00042,000 19,000 27,800 16,200 22,000 26,000 17,000 18,700 26,000
2
x x
10-J 10-4
2
x
10-4
x x 1x
