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PRESSURE DEPENDENCE AND SIGN OF THE
ELECTRIC FIELD GRADIENT AT Ir IN THE
LINEAR CHAIN STRUCTURE OF Ir(CO)2acac
A. Vasquez, F. Wagner, U. Klein, J. Moser, G. Wortmann, H. Keller, A. Bolz
To cite this version:
Abstract. — The 73 keV Mossbauer resonance of 193Ir has been used to study the linear chain
structure compound Ir(CO)2acac. From measurements with an absorber composed of aligned single crystals, the electric field gradient was found to be positive and, at least approximately, axially symmetric around the direction of the Ir-Ir bonds. The pressure dependence of the Moss-bauer parameters has been studied up to 39 kbar. The isomer shift turns out to depend only weakly on the pressure-induced change of the Ir-Ir bond length, but the electric quadrupole interaction decreases by a factor of three over the studied pressure range. These results are discussed quali-tatively in terms of the molecular orbital model.
1. Introduction. •— Square planar complexes of the efg at the Ir nuclei. We have performed such Ir and Pt that crystallize in linear chains with metal- experiments on acetylacetonatodicarbonyliridium (I) metal bonds have recently attracted considerable at pressures up to 39 kbar. This compound is well-attention. Mossbauer spectroscopy of the 73 keV suited for such an investigation, because its crystal gammaraysof 1 9 3Ir has been used to study compounds structure is known [5-8] and, moreover, the pressure
of this type with both mixed valence and pure dependence of the Ir-Ir bond length has already been Ir(I) (5d8) character [1-4]- These investigations indi- measured at pressures up to 66 kbar [7]. To facilitate
cate that the formation of the Ir-Ir bonds tends to the discussion of the pressure dependence of the elec-reduce both the electric field gradient (efg) and the trie quadrupole interaction, the sign and the asymme-electron density at the Ir nuclei. Such conclusions were try parameter of the efg have also been determined. To based on a comparison of different compounds with this end experiments with suitably oriented single crys-and without metal-metal bonds [1] or on the behaviour tals had to be performed since the 73 keV transition of the Mossbauer parameters in Ir(I) compounds takes place between the spins J+ for the excited and
that consist of identical molecular units but can be |+ for the groundstate, which renders it insensitive
made to crystallize in either a stacked or a monomeric to both the sign and the asymmetry parameter of the modification [2]. The application of pressure to sui- efg as long as only polycrystalline samples are studied, table compounds is a rather direct way of studying the Ir(CO)2acac is isostructural with its rhodium analogue
influence of the Ir-Ir bonds on the electron density and [6, 8] and forms triclinic crystals with two molecules per unit cell. The planar Ir(CO)2acac units are stacked
.„. „ . , , . . ,, _ , . „ . in such a way that the Ir atoms form linear chains along
(*) Work supported by the Deutsche Forschungsgemein- _, . „ , . . , T T ,. °
s c h a f t the crystallographic a-axis with an Ir-Ir distance of
(**) On leave from UFRGS-Brasil and under support from 3-2 3 A a t ambient pressure and of 2.90 A at 66 kbar [7].
COPERTIDE and CNPq-Brasil. The stacking axis is, to within a few degrees,
perpen-PRESSURE DEPENDENCE AND SIGN OF THE ELECTRIC FIELD
GRADIENT AT Ir IN THE LINEAR CHAIN STRUCTURE
OF Ir(CO)
2acac (*)
A. VASQUEZ (**), F. E. WAGNER, U. KLEIN, J. MOSER and G. WORTMANN Physik-Department, Technische Universitat Miinchen
D-8046 Garching, Germany and
H. J. KELLER and A. BOLZ
Anorganisch-Chemisches Institut, Universitat Heidelberg, D-6900 Heidelberg, Germany
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-519
Résumé. — En utilisant la résonance Môssbauer de 73 keV en 193Ir, on a étudié le composé
Ir(CO)2acac, dont les molécules planes cristallisent sous forme de chaînes linéaires avec des liaisons Ir-Ir. Comme résultat de mesures sur un absorbeur composé de monocristaux, le gradient de champ électrique s'est montré être positif et posséder une symétrie au moins approximativement axiale autour de la direction des liaisons Ir-Ir. La dépendance des paramètres Môssbauer de la pression appliquée a été étudiée jusqu'à 39 kbar. On trouve que le déplacement isomérique ne dépend que faiblement du changement de la longueur des liaisons Ir-Ir induit par la pression, mais l'interaction quadrupolaire diminue par un facteur de trois dans la région de pression étudiée. Les résultats sont discutés qualitativement dans le cadre du modèle d'orbitales moléculaires.
C6-520 A. VASQUEZ, F. E. WAGNER, U. KLEIN, J. MOSER, G. WORTMANN, J. KELLER AND A. BOLZ dicular to the planes of the Ir(CO),acac molecules [6,
S]. The molecular symmetry is essentially Cz,. The three principal axes of the efg tensor are therefore expected to coincide with very good accuracy with the stacking axis, the twofold axis bisecting the CO-Ir- CO angle in the molecular plane, and the axis perpen- dicular to these two.
2. Experimental and results.
-
Single crystals of Ir(CO),acac can easily be grown in the shape of flat needles that are about 10 mm long, 0.5 mm wide and 0.05 mm thick. The crystallographic a-axis, i. e. the Ir-Ir stackingaxis, coincides with the needle axis [5]. For the measurement of the sign of the efg, several of such crystals were aligned visually to within a few degrees and intercalated between a stack of 10 lucite discs of 0.5 mm thickness. The resulting absorber could be used for measurements with the direction of the gamma- rays at angles 5O<
6<
90° with respect to the needle axis, and it could also be rotated around the needle axis to arbitrary azimuths @ with respect to the normal on the plane of the needles. Some spectra of this type are shown in figure 1.- 4 -2 0 2 4 VELOCITY ( m m l s )
J?IG. 1. - Mossbauer spectra of a composite single crystal
absorber of Ir(C0)zacac taken at 4.2 K with a source of 1930s in 1920s metal. Due to the quadrupole splitting in the source [9],
each peak of the Ir(C0)z acac quadrupole pattern is actually a hardly resolved doublet. 6 is the angle between the Ir-Ir stacking axis and the direction of the gammarays ; the angle @ with respect to the normal on the planes of the needles has been zero
in all three cases.
The line intensities in these spectra show the 0- dependence expected for a field gradient that is, at least approximately, axially symmetric around the needle axis. This conclusion has been confirmed by several measurements at 0 = 900 but different azi- muths Q. These spectra all give intensity ratios close to that of the lower spectrum of figure 1. A quantitative interpretation of the intensity ratios must take the mixed E2/M1 character of the 73 keV transition into account. Using the formulae given by Prosser et al. [l01 and 6 = - 0.558(5) for the E2/M1 mixing para- meter [ll], one expects the intensity ratio
of the two components of the quadrupole doublet to be R = 0.375 and R = 1.62 for observation at angles of 50 and 900 with respect to the z-axis of an axially symmetric efg tensor. These values are in good agree- ment with the experimental ones, R(O = 50) = 0.42(6) and R(O = 900) = 1.66(4), if one assumes that the line at negative velocities corresponds to the
-+
+
; f3
transition. This assignment corresponds to a positive value of the electric quadrupole coupling constant EQ =+
e2 qQ and, since the quadrupole moment Q of the $+ groundstate of 931r is also positive 112, 131, to a positive value of the electric field gradient eq. The good agreement of the expected and observed inten- sity ratios R and the absence of a substantial @- dependence of R indicate that the asymmetry of the efg is too small to be observed. Detailed calculations show, however, that R is quite insensitive to the asym- metry parameter. Our data therefore set an upper limit of 0 G G 0.3 only.- 4 - 2 0 2 4 VELOCITY ( m m l s )
FIG. 2. - Comparison of Mossbauer spectra of Ir(CO)zacac
PRESSURE DEPENDENCE AND SIGN OF THE ELECTRIC FIELD GRADIENT C6-521 The pressure experiments were performed at 4.2 K in
the high pressure apparatus described by Klein et al.
[14]. The pressure was determined by a manometer making use of the pressure dependence of the super- conducting transition temperature of a lead strip mounted inside the pressure cell [15]. Our spectra (Fig. 2) show that there is a large pressure dependence of the electric quadrupole interaction. The observed pressure dependence of the isomer shift and quadrupole interaction is given in figure 3. The electric quadrupole
FIG. 3. -Dependence of the isomer shift with respect to Ir metal (S) and of electric quadrupole splitting (MQ = 112 ez qQ)
in Ir(C0)zacac on pressure and on the Ir-Ir distance.
interaction shows a strong and virtually linear decrease under pressure from AEQ =
+
2.30(2) mm/s at ambient pressure to AEQ =+
0.89(3) mm/s at 39.4(2.0) kbar. This corresponds to a markedly non- linear dependence of AEQ on the Ir-Ir distance a (Fig. 3), since the pressure dependence of the latter deviates considerably from linearity [7]. The pressure dependence of the isomer shift S is weak and seems to be slightly non-linear. The small increase of S under pressure (Fig. 3) corresponds to an increase of the electron density p(0) at the Ir nuclei, since A<
r 2>
is positive [g].3. Discussion.
-
The main experimental results calling for an explanation are (i) the small increase of the electron density at the Ir nuclei under pressure and (ii) the positive sign and surprisingly strong pres- sure dependence of the electric quadrupole interaction. Indeed, a linear extrapolation of the latter suggests that AEQ would pass through zero and become negative at a pressure of about 60 kbar. The behaviour of Ir(CO),acac may be compared with the change of the Mossbauer parameters of gold compounds under pressure [16, 171, and in particular with the behaviour of KAu(CN),, which is also a planar complex with a 5d8 electron configuration but without metal-metal bonds. In this compound, the isomer shift increases by about 0.3 mm/s when the pressure is raised to 60 kbar [17]. Considering that the A<
r 2>
values of the 1931r and 1 9 7 A ~ Mossbauer resonances are both positive and of similar magnitude [IS], one finds that Ir(CO),acac and KAu(CN), behave similarly as far as the change of p(0) under pressure is concerned.The electric field gradient at the Au nuclei in KAu(CN),, however, increases slightly under pressure, by about 4
%
between 0 and 60 kbar. It is tempting to attribute the completely different behaviour of the electric field gradient in Ir(CO),acac to the existence of metal-metal bonds in this compound.Neglecting the lattice contribution one can interpret the electric field gradient in terms of the 5d and 6p electron population on the transition metal. As has been suggested previously for the square planar Au(II1) complexes [19], the main contribution will be a nega- tive one (eqzz) arising from the doubly occupied non- bonding 5dz2 orbital and a positive one (eq,) that is due to the population of the 5dx2-y2, 6px and 6py atomic orbitals which take part in forming the c-bonding molecular orbitals in the molecular plane. The
5dx.
and 5dXz,,, orbitals are fully occupied by six electrons. In the absence of n-bonding they would, therefore, not contribute to the electric field gradient. In complexes with strongly back-bonding ligands like CO or CN-, electrons are delocalized out of these orbitals. One should expect, however, that th.e 5dxy and the 5dX,,, orbitals both take part in X-backbonding and that, consequently, their contributions to the electric field gradient, which are of opposite sign, will largely cancel and can therefore be neglected. With Q =+
0.7 b for the Ig31r groundstate quadrupole moment [12, 131 the experimental value for the electric field gradient in Ir(CO),acac at ambient pressure becomeseq,,, =
+
1.6 X 1018 V/cm2.
The contribution from a doubly occupied, nonbond- ing 5dz2 orbital is
C6-522 A. VASQUEZ, F. E. WAGNER, U. KLEIN, J. MOSER, G. WORTMANN, J. KELLER AND A. BOLZ (1
-
R) = 0.8 for the atomic Sternheimer shieldingfactor. To explain the positive experimental value of the electric field gradient, one has to assume that the contribution from the o-bonding orbitals is
eq, E
+
11 X 10'' V/cm2.The dominant role of the o-bonding orbitals indicates that the 6p electrons contribute significantly to the electric field gradient. Our simple estimates explain the observed electric field gradient as the relatively small difference of two large contributions of opposite sign. The strong pressure dependence of the quadrupole interaction in Ir(CO),acac can then be brought about by a relatively small increase (in absolute magnitude) of eq,, or by a similar decrease of eq,. The latter corresponds to a pressure-induced weakening of the o- bonds. The increase of the electric field gradient and the electron density at Au in KAu(CN), under pressure, on the other hand, has been attributed to an increased
a-bond covalency [16]. It is tempting to attribute the difference in the behaviour of Ir(CO),acac and KAu(CN), to the influence of the Ir-Ir bonds on eq,,. A compression of the 5dz2 wave functions on decreasing the Ir-Ir distance is quite conceivable and should, indeed, increase the magnitude of eq,,. Such a mecha- nism would explain not only the pressure dependence of the electric field gradient in Ir(CO),acac but also the general tendency noted previously [2] in compounds with Ir-Ir bonds to have lower electric field gradients than similar compounds without metal-metal bonds. The increase of the electron density at the Ir nuclei under pressure, however, cannot be explained in this way, since a compression of the 5dZ2 wave functions would rather increase their shielding power and thus reduce p(0). It is conceivable, however, that any such effect is overcompensated by a pressure-induced increase of the o-bond covalency or of the n-backbond- ing with the ligands in the molecular plane.
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