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MULTIPLE SCATTERING EFFECTS IN THE COBALT K-EDGE EXAFS OF METAL CARBONYL
COMPLEXES
N. Binsted, S. Cook, J. Evans, R. Price, G. Greaves
To cite this version:
N. Binsted, S. Cook, J. Evans, R. Price, G. Greaves. MULTIPLE SCATTERING EFFECTS IN
THE COBALT K-EDGE EXAFS OF METAL CARBONYL COMPLEXES. Journal de Physique
Colloques, 1986, 47 (C8), pp.C8-589-C8-592. �10.1051/jphyscol:19868110�. �jpa-00226007�
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment au n o
12,Tome 47, dkcembre 1986
MULTIPLE SCATTERING EFFECTS
INTHE COBALT K-EDGE EXAFS OF METAL CARBONYL COMPLEXES
N. BINSTED, S.L. COOK,
J.EVANS, R.J. PRICE and G.N. GREAVES*
D e p a r t m e n t
of
Chemistry, Universityof
Southampton, GB-Southampton S O 9 5NH, Great-Britain"SERC
Daresbury
Laboratory, Daresbury, GB-Warrington WA4 4AD, Great-Britainsum;?.
Les donnges EXAFS des complexes carbonyles du cobalt ont &t&analyskes avec la mgthode des ondes sph6riques. On a utilisg les dgplacements de phases thgoriques et on a pris en considsration le scattering multiple au troisi&ine ordre. Les distances lnteratomique ont etg determinees (avec une prgcision de 0.03
8)
et les valeurs angulaires moyennes des lialsons Co-C-0 sur un ordinateur en utilisant des donnges mesurges a 14 eV en dussus du seuil d'absorption. Le scattering multiple intraligand gtait dominant dans ces complexes. Les critere statistiques pour 1iml;ter le nombre des paramztres variables dans yn systkme complexe sont presentes pour rgduire les difficult& qui sont causees par la corrglation des paramLters des couches adjacentes.Abstract. EXAFS data of cobalt carbonyls have been analysed by a spherical wave method using ab initio phase shifts and including multiple scattering to third order. Distances were determined to within 0.03
2
using data from 14 eV above the absorption edge, and the mean Co-C-? bond angle could be estimated. Intra-ligand multiple scattering is dominant in these complexes. Statistical criteria for limiting the number of refinable parameters in a multi-shell system are presented to reduce the difficulties caused by correlation of parameters of adjacent shells.1. Introduction
Metal carbonyls present a signrficant problem in EXAFS analysis due to the strong multiple scattering effects of the colinear M-C-0 array, as observed in Mo(COI6 [I]. We report calculations using the spherical wave formalism [21 in an algorithm suitable for polycrystalline samples [3] and allowing for multiple scattering. Most previous studies of multiple scattering has utilised a plane wave theory [4]-[61, but this has resulted in errors in the phase of the multiple scattering contributions [7]. We have examined three model compounds to test the application of these calculations viz [PPN] [CO(CO)~] (1) PPN = N(PPh3) 2,
C03 (C0j9 (CH) ( 2 ) and Co4 (CO1 (PPh12 (3) (Figure 1).
FIGURE 1. Structure of the cobalt species in PPN[CO(CO)~]
1,
CO~(CO)~CH2
and C O ~ ( C O ) ~ ~ ( P P ~ ) ~2.
Cobalt, oxygen and carbon atoms are shown as filled, hatched and open circles respectively.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868110
JOURNAL DE PHYSIQUE
2. Experimental
The Co K-edge X-ray absorption spectra were recorded in transmission mode on Station 7.1 of the SRS at the SERC Daresbury Laboratory. The procedures used for the analyses by spherical wave methods have been described previously [ 7 1 . These were performed on Fourier filtered data which included all significant structure
(0.9
-
3.88 ) .
13 shifts were used in calculating single, double and the forward scattering term of the triple scattering and 10 for the third order back scattering term. Phase shifts for each element were calculated by the method of Loucks 181, the atomic wave functions being calculated self-consistently using a relativistic Hartree-Fock programme with Slater exchange [9]. The central atom phaseshifts Included the effects of the core hole [lo]; an excited atom wavefunction with a Is core hole and the outer atoms relaxed to its presence was employed, as appropriate to the situation at low photoelectron energies [Ill. The results, however, were not strongly dependent on the prescription used for the central atom.3. Results and Discussion
The relative importance of the different orders of scattering is illustrated for [PPN][CO(CO)~] (1) in Figure 2. The double and triple scattering terms are more important than the oxygen shell single scattering and therefore must be taken into account. Higher order terms, however, are negligible except wlthin 10 eV of the absorption edge. It appears that multiple scattering to third order is adequate to describe the spectra of most molecular species. In carbonyl complexes it seems that inter-ligand paths are relatively unimportant as is evident even in this highly symmetric structure.
-I
i0 50 100 150 200 250 300 350 0 50 KM 150 200 250 300 350
ENERGY A W E EDGE
(ev) -
FIGURE 2. Calculated contributions of the scattering paths for Co(C0)4 using the parameters in Table 1: (a) 4 x Co-0-Co (solid), sum of (a), (b) and (c) (broken); (b) 4 x Co-0-C-Co
+
4 x Co-C-0-Co; (c) 4 x Co-0-C-0-Co; (d) 12 x Co-C-C-CO; (e) 4 x Co-0-C-0-C-Co+
4 x Co-C-0-C-0-Co; (f) 4 xCO-C-0-C-0-C-Co; (9) 4 x Co-0-C-0-CO; (h) 4 x Co-C-Co.
The angle dependence of multiple scattering within a Co-C-0,unit is presented in.Figure 3. At a 90' angle the effect of multiple scattering is limited to a
slight amplitude reduction, this effect being lost at 150°. At the latter angle there is a large retardation of phase and a slight change in the apparent Co...O distance. Only in the linear case are a strong amplitude enhanement and changes in apparent Co...O distance (by 4 pm) and energy dependence of the amplitude noted.
(1)
XRAY EXAFS
R(% R (8) a (82)
(3)a
XRAY EXAFS
R(E) R(E) .(a2) 0.005 (0) C (T)
0.009 (0) C (B) P 0.009(1) Co (BR) 0.012(13) CO(NB) 0.008(0) 0 (T) 0.011 (0) 0 (B) 0.023 (3) C
Co (D)
aparentheses: T = terminal, B = bridging, BR = bridged bond, NB = non-bridged bond, D = diagonal Co...Co distance.
TABLE 1. Optimum parameters using all shells to 3.8
8.
a = 20 2.
WEVECTOR k i i -'I DISTANCE 6)
FIGURE 3. Oxygen shell contributions to k 3 $ ( k ) (upper) and the Fourier transform (lower) for a Co-C-0 unit. Parameters as in Table 1 excepting for the bond angle and Co-0 distance (as shown) ( - - - ) total contribution,
(-) single scattering, ( - - - -) double scattering and (- --) triple scattering.
Analysis of the EXAFS data of the three compounds by least squares refinement and including multiple scattering yielded the parameters in Table 1 and the fits in Figure 4. Good fits to ca 14 eV above the edge were obtained with all distances within 3 pm of the crystallographic values. In complex (1) the precision of the parameters is high. In contrast, the large number of shells in (2) and (3) cause
strong correlations between parameters of adjacent shells, with consequentially higher statistical errors. To limit the number of parameters either minor shells may be omitted or combined with other shells, or alternatively some parameters may be constrained to take common or related values. Here the number of shells was
C8-592 JOURNAL
DE
PHYSIQUEreduced by applying two criteria:
(1) shells whose k-space integral was less than 10% of the k3 weighted spectrum or
( 2 ) shells with correlations between parameters of adjacent shells of >0.8
were either excluded or arnzlganated.
FIGURE 4. Fits of the Co K-edge EXAFS shown (with Fourier transforms) for PPN [Co (C0) I+
I 1,
CoS (CO) gCH2
and Cot+ (CO) 1 0 (PPh) 22.
Applying these to (2) and (3) reduced the number of shells to 3 and 6 respectively [Co-C, Co-Co and Co
...
0 for (2) and Co-C, Co-P, 2Co-Co, Co...O and Co...Co for (3)l. The Debye Waller factors of the two bonded Co-Co distances in (3) were constrained to equal values.Having reduced the number of parameters, the Co-C-0 bond angles were included in the refinement. For (1) and (2), which only contain terminal carbonyl groups, a 180" bond angle was clearly preferred. The angle estimated for complex (3) was 166(21°, a value that is also within experimental error of the crystallographic mean for that complex (168'). Differentiating between bridging and terminal carbonyl groups was not possible on distance grounds, due to high correlations when these shells were each split into two components. However, the determination of the mean M-C-0 angle may provide a way of establishing their relative populations.
Acknowledgements
We wish to thank the SERC for support (NB, RJP and SLC) and Drs S J Gurman and P J Durham for helpful discussions.
References
Cramer, S.P., Hodgson, K.O., Stiefel, E.I., and Newton, W.E., J Am Chem Soc, 100. (1978). 2748.
Lee, P.A., and Pendry, J.B., Phys Rev B, 11, (1975), 2795.
Gurman, S.J., Binsted, N., and Ross, R., J Phys C, 17, (19841, 143.
Teo, B., in Teo, B., and Joy, D.C., Eds. "EXAFS Spectroscopy", Plenum (1981) Co, M.S., Hendrickson, W.A., Hodgson, K.O., and Doniach, S., J Am Chem Soc, 105, (1983), 1144.
Boland, J.J., and Baldeschwieler, J.D., J Chem Phys, 80, (1984), 3005.
Gurman, S.J., Binsted, N., and Ross, I., J Phys C, 19, (1986), 1845.
Loucks, T., "The Augmented Plane Wave Method", en jam in, New York, (1967)
.
Program by Desclaux, J.P., modified by Durham, P.J.
von Barth, U., and Grossman, G., Phys Rev B, 25, (1982), 5150.
Lee, P.A. and Beni, G., Phys Rev B, 15, (1977), 2862.
