A single crystal ESR study of the Cu(C5H5N)42+ ion in Cd(C5H5N)4S2O8

J. inorg,nucl.Chem., 1969,Vol. 31, pp. 1983to 1992. PergamonPress. Printedin Great Britain

A SINGLE

CRYSTAL

C u ( C 5 H s N ) 4 2+ I O N

ESR IN

STUDY

OF

THE

Cd(C5H5N)4SzOs*

H A R R Y G. H E C H T and R A L P H C. KERNS University of California, Los Alamos Scientific Laboratory, Los Alamos, N.M. 87544 and J O H N P. F R A Z I E R , III Sylvania Electric Products, Towanda, Penna. 18848

(First received 14 October 1968; in revised form 2 December 1968) A b s t r a c t - T h e ESR spectra of Cu(CsHsN)4 z+ in single crystals of Cd(CsHsN)4S20s are reported and analyzed. It is found that the ESR parameters are consistent with an essentially square planar configuration, and the orientation of the ions in the crystal is determined. Using powder reflectance techniques, the optical transition energies are inferred and used in a molecular orbital treatment. The assignment of optical transitions determined in this study is compared with that reported by other workers, and the m. o. parameters are derived.

INTRODUCTION

IN A eREVlOUSLY published note[l], we commented on the ESR parameters determined for polycrystalline Cu(CsH5N)4SzOs and Cu in C d ( C s H s N ) 4 S 2 O s o r Zn(CsHsNhS2Os. An attempt to interpret these parameters in terms of molecular orbitals has led to some inconsistencies which made the assignment of the powder data seem questionable. In particular, the low values of the perpendicular hyperfine coupling for copper seemed to be unreasonable. It has furthermore been pointed out by Kuska, D'ltri, and Popov [2] that the assignment previously made leads to an unusually low isotropic splitting factor for the nitrogen atoms. There are two ways in which ambiguities in the powder data may be resolved. The first of these involves the study of the spectra at two different microwave frequencies. The g-factors are field dependent but the hyperfine coupling terms are not, so that in principle a unique set of parameters may be found which simultaneously accounts for the observed features at the two frequencies. An attempt was made to apply this method in the present instance by observing the resonance at both K-band and X-band frequencies. The results were less than satisfying, however, because of two complicating factors which frequently occur in powder work. The first is due to the occurrence, in complex systems such as this, of near degeneracies in many of the transitions, the result of which is that the satellite lines become weak and indistinct so that one cannot be sure of the extent of overlap of the multiplet patterns. The second complication which is often encountered *Based in part on work performed under the auspices of the U.S. Atomic Energy Commission. 1. H. G. Hecht and J. P. Frazier, J. chem. Phys. 44, 1718 (1966). 2. H. A. Kuska, F. M. D'Itri and A. I. Popov, lnorg. Chem. 5, 1272 (1966). 1983

1984

H . G . H E C H T , R. C. KERNS and J. P. F R A Z I E R , III

is the appearance of the so-called extra absorptions which have been discussed by Neiman and Kivelson [3] and by Gersmann and Swalen [4]. These extra absorptions, which occur for certain combinations of the ESR parameters, can also contribute to the complexity of the powder spectra. Because of these considerations, we have made no further attempt to interpret the powder data in this case. The second and most powerful method of removing the ambiguity involves single crystal work, the result of which is reported here. It has been shown that in [Cu/Cd(CsHsN)4] S2Oa mixtures, a single crystalline phase is formed for all compositions [5], which indicates that the Cu(C5H5N)4 z+ ion can be incorporated by isomorphic substitution into a Cd(C5HsN)4S208 matrix to achieve a magnetic dilution. The ESR parameters we have determined are consistent with this hypothesis.

EXPERIMENTAL

TECHNIQUES

The materials used in the present study were all of reagent grade quality. Single crystals of Cd(CsHsN)4S208 and Cd(CsHsN)4SzOs containing approximately 0.5% Cu were grown from aqueous solutions at room temperature. The growth was quite rapid (5-6 hr), since further aging of the solutions resulted in rapid decomposition of the pyridine, presumably through oxidation by the S2Os ffi ion. Polycrystailine samples of Cd(CsHsN)4S208 doped with Cu were prepared in a manner similar to that described by Barbieri and Calzolari [6]. The latter material was used for optical study by powder reflectance techniques, for which a Unicam Sp. 500 spectrophotometer was used. The single crystals were oriented by optical reflection goniometry. The crystal studied by ESR techniques was glued to a small right-angle quartz holder which was attached to a quartz rod. The mounted crystal was coated with collodion to retard decomposition, which was sufficiently rapid in air to dull the crystal faces significantly within about 30-40 min. The ESR spectra were recorded at room temperature at 9530.0--_0.3 MHz using a superheterodyne detection system. A proton magnetometer was used for field calibration, and the klystron frequency was monitored by use of a transfer oscillator and electronic counter. Spectra were recorded at 18° intervals about each of three mutually perpendicular axes, which are defined in the next section.

CRYSTAL STRUCTURE

O F Cd(CsHsN)4SzOs

The Cd(CsHsN)4S2Os crystals grown from aqueous solution are in the form of elongated needles, most of which are twinned. Preliminary X-ray studies[7] have shown that they belong to the monoclinic system, the space group being either Cc or C 2/c. The unit cell dimensions are a ~ 15-09 ,~,, b --~ 11.04 ~., c --~ 16.02 A,/3 ~ 111.5 °. The a axis is the needle axis, and there are 4 Cd(CsH5N)4S2Os molecules per unit cell. A density of 1-62 g cm -3 was determined by the flotation method which gives 3.91 molecules per unit cell. A sore.ewhat low value is probably to be expected because of the twinning of the crystals. By close examination of a batch of crystals, several well-formed crystals could usually be found which had the appearance of single crystals and which gave reproducible results when measured by goniometric techniques. The (021) and (021) 3. 4. 5. 6. 7.

R. Neiman and D. Kivelson, J. chem. Phys. 35, 156 (1961). H. R. Gersmann andJ. D. Swalen, J. chem. Phys. 36, 3221 (1962). N. Perakis and L. Capatos, J. phys. Radium, Paris 9, 27 (1938). G . A . Barbieri and F. Calzolari, Z. anorg, allg Chem. 71,347 (1911). E. A. Meyers, Private communication.

A single crystal E SR study

1985

reflections were particularly well formed, the acute bisector of which was used to locate the b-axis. The doped crystals were mounted for rotation about (1) the a axis, (2) the b axis, and (3) an axis which is mutually perpendicular to the a and b axes, referred to subsequently as the c' axis. Because the drying glue tends to pull the crystals somewhat, the precision of alignment is probably no better than _+0.5 ° for the a zone alignment a n d _ 1° for the alignments on the b and c' axes. RESULTS AND DISCUSSION An abundance of superhyperfine structure due to the ligand nitrogen atoms was observed at all orientations of the crystal. However, at orientations where the four ligand nuclei are inequivalent, the spectrum is complex and no unambiguous assignment could be made. This is because the magnitude of the splitting due to the ligand nitrogens is small ( - 12-14 G) and its variation with orientation is even smaller ( - 1-2 G). The spin Hamiltonian we have taken thus includes only a Zeeman term and a copper hyperfine term, with no a priori assumptions being made concerning the symmetry of either interaction: -----f l H . g . S + S . A . I .

(1)

We did not feel justified in including quadrupole terms or in carrying the analysis beyond first order, due to the complexity caused by overlapping of the hyperfine components. Thus from each spectrum we extracted only an effective g-factor and an average spacing for the four copper hyperfine components. Within the limits of a first order analysis, these parameters (Q -= g,A) are the projections of the corresponding tensor along the direction of the applied field [8]:

,[(H'Q)211n Q

=

[-VH--~j

.

(2)

If we define the orientation of the magnetic field with respect to the crystallographic axes by a polar angle referred to the a axis, 0, and an azimuthal angle referred to the b axis, $, then the parameters determined by rotation about the three respective axes are given by Qa 2 = (Q2) 11 COS2 I~)+ (Q2)s2 s ins ~b+ 2 (Q2)12 sin ~bcos ~b Qb s = (Q2)22sin s 0 + (QS)aacos s 0 + 2(Q2ha sin 0 cos 0

(3)

Qc: = (Q~)11 sin s 0 + (Q2)a3 cos s 0 + 2(Q2)1~ sin 0 cos 0 Figures 1 and 2 show the g2 and A 2 values determined for rotations about the three axes, respectively. It will be observed that the Cu(CsHsN)4 2÷ ions occur in pairs, which become inequivalent only with H in the b-c' plane. The points were fit to equations (3) by a least squares procedure, the average values of the tensors describing the curves of Figs. 1 and 2 being 8. J. A. Weil, G. L. Goodman and H. G. Hecht, In Paramagnetic Resonance (Edited by W. Low), pp. 880-895. Academic Press, New York (1963).

1986

H. G. H E C H T , R. C. K E R N S and J. P. F R A Z I E R , IIl

_o.oo,,\ _+0"0080 4"5435 __+0.0066

gg =

-----0"0083 \/ --0.0173| ---0.0072 /

(4)

/

4.2156 / + 0.0053 / and

20723 _ 362

-- 12938 ± 496 12912 + 382

A2=

g

5"2

- 198 / _+379 -- 699 _+337 1228 __+244

2

4.8

4.4

l

I

5.2

l

l

(0.5 ~r, ~,)

I

I

4.8

• 4"0

I

I

I

(8,0.5

5-2

I

I

I

J 0'8

I I'0

7r)

4.8

4-0

I 0

J 0'2

I 0"4

I 0"6

(8, o.o ~r) Fig. 1. Values of g2 determined by rotation about the three axes, a, b, and c'. The solid lines are calculated from the 82 tensor shown as equation (4).

fS)

A single crystal E S R study

1987

A I

I

I

[

I

I

[

I

I

I

I

l

0-8

I'0

20000

1000(]

*

•

(o.s-.-, ~) .-. 2 0 0 0 0 N

a

I0000

o

{8,o.5 r)

20000

10000

0

0

0'2

0.4

0-6

(O, o.o-w) Fig. 2. Values of A 2 determined by rotation about the three axes, a, b, and c'. The solid lines are calculated from the A S tensor shown as equation (5).

The A2 tensor is in units of gauss 2 and the error limits represent standard deviations determined by the least squares analysis. Diagonalization of the g2 and A2 tensors gives the principal values and orientations of principal axes shown in Table 1. Although the standard deviations quoted are mathematically well defined, it does not appear to us that they are necessarily of physical significance, since they probably account only in part for systematic errors such as a slight misalignment of the crystal. It will be observed that the parameters are nearly axial, with the g values being the more precisely determined, as one would expect. It is of interest to note that the "parallel" or approximate symmetry axes for the two sites lie very nearly in directions perpendicular to the (021) and (021) planes. The differences between g2 and g3 and As and A3 are not thought to be particularly significant. The alignment errors are such as to have their largest effect on these parameters, as are

Aa

A2

A1

g3

g2

g~

---14-0

31"5

±7-6

56-2

+--.2-6

184.1

x 10-~m -I

x l O - ~ c m -~

× 1 0 - 4 c m -1

±0.0062

~

98 °

±4 °

126 °

+---22'

36 ° 38'

undet,

±4 °

--0.0081

2- 0561

117 °

±11-5'

2.0493

37°2.5 '

2-2642

Angle wrt b

_--+0.0033

Principal Value

---12 °

78 °

±7 °

39 °

±19'

53 ° 23'

undet.

±7 °

37 °

--+9'

52058 ,

A n g l e w r t c'

Site 1

---15 °

14 °

±15 °

104 °

±10'

90 ° 5 '

--11 °

41 °

undet.

-----7'

90 ° 4 '

Angle wrt a

±9 °

82 °

___4°

54 °

+---22'

36 ° 38'

undet,

±4 °

63 °

--11-5'

37 ° 2-5'

Angle wrt b

T a b l e 1. P r i n c i p a l v a l u e s a n d o r i e n t a t i o n o f p r i n c i p a l a x e s f o r t h e g a n d A t e n s o r s

±12 °

78 °

±7 °

39 °

±19'

126 ° 37'

undet.

±7 °

37 °

±9'

12702 ,

Angle wrt c'

Site 2

±15 °

14 °

___15°

104 °

±10'

89 ° 5 5 '

--11 °

41 °

undet.

-----7'

89 ° 56'

angle wrt a

t~

N

;>

.,.

e~

Z

©

.~

,~

m

C)

oo oo

A single crystal ESR study

1989

errors of analysis, since the overlapping of components is more significant in cases where the hyperfine coupling is smaller. The orientation of the "perpendicular" principal axes in the plane perpendicular to the symmetry axis of course cannot be determined at all in cases where the ion is exactly axial, so that very little confidence can be placed in the orientations listed in Table 1 for these components. In particular, g2 and ga, which are very nearly equal, showed large fluctuations for very small changes in certain of the matrix elements so that the direction cosines of the transformation to the principal axis system were essentially undetermined in several cases. Since a complete crystal structure determination of Cd(C5H5N)4S208 has not been made, we cannot be absolutely certain of the Coordination of the Cd(CsHsN)42÷ and Cu(CsHsN)42+ ions. It is assumed that they are alike since isomorphic substitution readily occurs. One would expect Cu(CsH5N)42÷ to be nearly square planar, and the parameters we have determined are certainly more representative of square planar bonding than of tetrahedral [9]. Thus we assume in the subsequent discussion that we are dealing with a square planar complex. A comparison of our results with those previously reported for the Cu(CsHsN)4 z+ ion is made in Table 2. The g tensors show quite good agreement, our values showing less anisotropy than is typical of those previously determined. This agrees with the observations of Schneider and Zelewsky[12], where no anisotropy was found in single crystal work. The "parallel" value of the copper hyperfine tensor agrees well with other measurements, but the "perpendicular" values are larger than previous determinations have indicated. We believe our data to be more reliable in these parameters, since their determination from powder spectra is difficult due to the amount of structure in the perpendicular region. Again, our values are in closest agreement with Schneider and Zelewsky's [12] single crystal data. Molecular orbital calculations have been made in a manner similar to that described by Gersmann and Swalen [4]. For this purpose, reference is made to the reflectance spectrum of [Cu/Cd(CsHsN)4]S2Os vs. Cd(CsH5N)4S2Os shown in Fig. 3. A great deal of controversy has existed in the literature for a number of years concerning the assignment of the optical data. Most people agree that the Bls state is the ground state, but the assignment of the excited states is subject to question. Piper and Belford[14] have assigned the states B,~, Es, Als all within the 14,000-18,000 cm -~ band, and this same assignment has been assumed by Schneider and Zelewsky [ 12]. On the other hand, Maki and McGarvey [ 15] have placed the Als and B~ states in the 14,000-18,000 cm -1 band, but placed the E~ level much higher. Gersmann and Swalen [4] assumed D2n symmetry and placed the Ag and Bag states within the lower band, assuming the B2~ state to lie at 31,400 cm -~. Since the optical data alone are less conclusive than one would wish[14], 9. 10. 11. 12. 13. 14. 15.

D. Forster and V. W. Weiss,J. phys. Chem. 72, 2669 (1968). J. A. McMiilan and B. Smaller, J. chem. Phys. 35, 763 (1961). K. Wiithrich, Heir. chim. Acta 49, 1400 (1966). W. Schneider and A. V. Zelewsky, Heir. chim. Acta 48, 1529 (1965). W. Schiibel and E. Lutze, Z. angew. Phys. 17, 332 (1964). T. S. Piper and R. L. Belford, Molec Phys. 5, 169 (1962). A. H. Maki and B. R. McGarvey, J. chem. Phys. 29, 31,35 ( 1958).

Frozen solution, 77°K

Single crystal, room temp.

C1-

$208 =

S~Os =

$208 = S20s =

$ 2 0 8 ffi

SO4 = SO4ffi NOaC~HTSOa ~ CI-

Experimental conditions

Powder, room temp. Powder, room temp. Powder, room temp. Powder, 77°I( Frozen solution, 77"1( Frozen solution, 77°1( Single crystal, room temp. Single crystal, room temp. Frozen solution, 77°K

Anion

Cd ~+

none Cd ~+ Z n ~+ Z n ~+ H~O H~O Pt ~+ Cd 2+ pyridine 60% pyridine [ 40% chloroform

Magnetic diluent

160, - 1, -- 1 184, 32, 56

2.264, 2.056, 2.049

--169, 13'9, 15.6 181, 13-5, 12-1 83, 14, 4 81, 13, 11 180, -, 192, 26, 26 162, 21, 21 158, 8, 8

2.241,2.079, 2.079

2.263, 2-080, 2.010 2.264, 2.071, 2.034 2.252, 2.059, 2.026 2-169, 2-056, 2.036 2.204, 2-061, 2.025 2.22, 2"04, 2-04 2-236, 2.050, 2.050 2.290, 2.050, 2.050 2.275, 2.057, 2.075

g Values

Copper hyperfine coupling constant (× 104 cm-t)

Table 2. Comparison of ESR parameters for the Cu(CsHsN)4 ~+ ion determined by various workers

This work

4

10 10 11 12 12 13

1

1 1

Reference

~v > N

.-o

Z

t~

m

m

1991

A single crystal E S R study 0.08

I

0.06

l

0.04 0.02 O"

15000

cm-I

25000

I

35000

Fig. 3. Powder reflectance spectrum if(R) = (1 - R)2/2R) of 0.5% Cu in Cd(CsHsN)4S2Os vs. Cd(CsHsN)4S2Os.

it appears to us that the best way to gain some assurance of a correct assignment is to rely primarily on the E S R data. Referring to Fig. 3, we assume bands at 17,200 cm -x and 26,000 cm -x and proceed as follows: W e average the values of g2 and ga and assume D4h symmetry. T h e appropriate molecular orbitals are then, ~I~Blg :

ad~,_y, -

½c~' [ - cr~ ") + o-u ~2~ + cr~ ~3) - o-u ")]

~ n ~ =/3 dxu -- ½(1 --/3 2)112[ put a~+ p~2) _ p11<3)_ pxt4)] ~ItAlg ---- Td3zi-r, - - ½( 1 - - T 2) 1/2 [o.x(1) .q_ O.11(2) __ O.X(3) -- O.11(4)]

WE°

(6)

= ~ S d x : - (1 - 8 2) 1,2[ p < l > _ p <3q/V' 2 t~id11,- ( 1 - - 82) 1/2 [ p z ( 2 ) _ _ p t 4 ) ] / ~ / 2 .

A good approximation of a 2 can be obtained independent of the optical assignment using the equation a 2 = --(AiJP) + (g~,-- 2) + ( 3 / 7 ) ( g ± - 2) + 0 . 0 4

(7)

given by Kivelson and Neiman[16], where P = 27fl/3N(dx,_u, lr-Zidx=_v2) = 0-036 cm -1. In our case we find a = 0.916 and assuming the group overlap integral for the Bly state to be,0.093, we have a '2 = 0.496. I f we then attempt to solve for/32 from the expression for gtt assumingAEB= = 26,000 cm -1, no solution can be found. H o w e v e r , taking AEB,, = 17,200 cm -1 gives the very reasonable value,/3 = 0.924. F r o m g± we get B = 0.847 using AEE° = 17,200 cm -1, whereas AEE, = 26,000 cm -1 yields the value B ~ 1.00. Since it is assumed that out-of-plane 7r bonding is of some importance in such complexes, we feel that the former assignment, corresponding to that of Piper and Belford[14], is the more likely. We do not feel that the deviations from axial s y m m e t r y observed are sufficiently large to invoke a 16. D. Kive|son and R. Neiman, J. chem. Phys. 35, 149 (1961).

1992

H . G . HECHT, R. C. KERNS and J. P. F R A Z I E R , III

splitting of the order of 10,000 cm -1 between the Bso and Bau orbitals of Dsh symmetry as assumed by Gersmann and Swalen [4], and conclude, therefore, that the assignment by Piper and Belford[14] of the 26,000 cm -1 band as ligand or AEA~is correct.