D. A. MORTON-BLAKE
1508
the reaction of cyclopropane over dry deuterated catalyst in an attempt to measure the concentration of active sites and to learn more about the initial reaction step of cyclopropane with the catalyst.
Acknowledgments. The nmr spectra were recorded by G. Bigam, Chemistry Department, University of Alberta. Excellent technical assistance was provided by J. J. Mendiuk.
Electron Paramagnetic Resonance of Nickel Acetate. Irradiation-Induced Spin Pairing by D. A. Morton.Blakel Division of Molecular Science, National Physical Laboratory, Teddington, Middlesex, England
(Received July 14, 1060)
Single crystals of X-irradiated nickel acetate tetrahydrate were investigated by epr at 77’K. The spectra of several8 = species were observed, indicating that one of the electron spins in the Niz+ion had paired with a free spin created in one of the ligands. Using a single mounting position of the crystal and assuming cylindrical 8-tensor symmetry, a least-squares computation gave the components 911 = 2.463 ;t 0.002, g l = 2,076 += 0.002 for the main species formed. However, there is some evidence for the presence of an appreciable rhombic component which would introduce an uncertainty of 0.02 in gii.
Introduction Recently it was shown2 that X-irradiation of cupric acetate monohydrate [(CH3C0J2Cu.H2Ol2 crystals results in partial spin pairing within the S = 1 magnetic triplet species. This was evidenced by the presence of an S = species whose spin was centered on one of the two copper atoms, as revealed by the epr spectrum. The inference drawn was that the irradiation had removed an H atom from one of the four acetate ligands, and that the resulting radical had spin-paired with the nearest cupric ion, leaving a free spin on the remaining cupric ion. This result suggested the possibility of partial spin pairing in other paramagnetic species for which S > and this paper describes a similar investigation of irradiated nickel acetate tetrahydrate, (CHaC02)zNi.4Hz0. Niz+ has a 3d* shell, and normally forms a ground-state S = 1 triplet. The crystal structure3 shows that the Ni2+ion is surrounded by six 0 atoms in the form of a slightly distorted octahedron. Four of the oxygens, which are in HzO ligands, form an approximate s uare in an (‘equatorial plane” at *2.06 and & 2.11 from the Ni2+ion a t the origin, while the other two oxygens, which are in two different acetate anions, are a t zk2.12 A along the “polar” directions. An irradiation mechanism similar to that supposed to occur in cupric acetate would be expected to eliminate an H atom in one acetate ligand, converting it to the radical anion, CH2C02-. Describing this acetate
B
The Journal of Physical Chemistry
+
radiolysis by the reaction A- +a’- H, the irradiation of nickel acetate can, in principle, create any of the species represented schematically on the right-hand side of the reactions
r* 2 f
Tt
A--Ni(ds)-A-
,I -H
--f
2+ Tt
T
A--Ki (d8)-a’+
(la>
t
A--Ni(d9)-& a+ t t tJ A--Ni(d7)-&2-
(lb) (1c>
If there were no metal-ligand spin interaction, the epr spectrum of species (la) would be merely the S = ‘/2 spectrum of a’- superposed upon that of nickel acetate (S = 1). Interaction of the spins would produce an S = 3 / z quartet or S = l/z doublet, and spin delocalization on metal or ligand would produce the species (lb) or (le) in which the nickel ion is isoelectronic, respectively, with CuZf(d9) or with Co2+(d7),and should possess very similar paramagnetic properties (g values and relaxation times). Unlike cupric acetate which has an S = 0 ground state, the only appreciably populated state in nickel acetate is the triplet. Therefore any paramagnetic species created by the irradiation is subject to spin (1) Chemistry Department, Trinity College, Dublin 2, Ireland. (2) D. A. Morton-Blake, J . Phys. Chem., 73, 2964 (1969). (3) J. N. van Niekerk and F. R. L. Sohoening, Acta Cryst., 6, 609 (1953).
EPROF NICKELACETATE
1509
exchange with neighboring triplet species, with consequent line-broadening. Irradiated nickel acetate crystals gave epr line widths of 35 to 40 G. I n order to obtain narrow lines and therefore better resolution, nickel-doped crystals of magnesium acetate tetrahydrate were used.
v
P
Experimental Section Crystals of magnesium acetate tetrahydrate (monoclinic space group P21/c) containing 1% nickel acetate tetrahydrate were grown from dilute acetic acid. A crystal irradiated with l5O-kV X-rays to a dosage of 5 Rlrads was mounted in an HI01 rectangular cavity with the (011) face on the narrow vertical wall, so that the b axis was in the plane of the magnetic field. Measurements were made at 77"K, and since there was evidence that some of the radiogenic species were unstable a single crystal setting was used as described previously,2 the spectra being recorded every 10" over the magnet rotation range x = 0 to 170". The spectra were calibrated by a variable-frequency proton resonance probe. The frequency of the X-band superheat spectrometer used in the investigation was 8859.7 MHz.
Calculation The results indicate that all the observed paramagnetic species created by the irradiation are described by formula l b . This may be thought of as a pseudocupric complex consisting of a 3d9 ion at the center of a distorted octahedron of oxygen atoms. Distortions in octahedral complexes may have various causes, but the most important one in d9complexes is that due to a Jahn-Teller effect which lifts the orbital degeneracy of the eg orbital (with which the spin is associated) by removing some of the symmetry elements of the octahedron.4 The result of this distortion is either (i) (usually) to increase the distances of both ligands on one of the three Ca axes of the octahedron, so as to produce tetragonal (&J symmetry, or (ii) (occasionally) to compress the ligands about one of the four CS axes so as to produce trigonal ( b a d ) symmetry. In either case a unique symmetry axis is imposed on the complex formed from one of the seven possible directions described. This axis, which we shall refer to as the Jahn-Teller (or J-T) axis, becomes a principal axis for susceptibility, g, and hyperfine tensors. We assume that all our radiogenic species contain such a unique axis of cylindrical symmetry, and so we can use the least-squares procedure described in ref 2 to evaluate the spectral parameters
G
(g1l2
- gL2)sin2e
(W
and
r = g12
(2b) The quantities 911 and gl have their usual meanings for a complex with cylindrical symmetry, and e is the angle
Figure 1. Symmetry (J-T) axis OP and its projection onto the plane of rotation of the magnetic field which includes the direction of the b axis.
made between the axis of the complex and the vertical direction (i.e., that about which the magnetic field is rotated). e is known approximately from the crystal setting, and Figure 1 shows that it is related to the two other angles a and q i a is the angle between the b axis and the seven possible J-T axes OP described above, and cb is the angle between b and the projection of OP onto the plane of rotation of the magnetic field. Clearly the relationship is cos a = sin 0 cos 9
(3)
Since the b axis is in the plane of rotation of the field @ can be found directly from the spectra, the g value is a maximum (or in some cases a minimum) along the projection OQ of the J-T axis; 4 is therefore the angle between such a position and the b axis. The seven possible angles a can be easily calculated from the crystal structure. A computer was programmed to calculate values of 4 for each of the seven angles a,and for each of a set of 0 values within a reasonable range. By comparing calculated and observed values of I$ the J-T axes and their directions 0 with the vertical could thus be assigned to the six species. In many of the species there were considerable shifts (-20") in the directions of the J-T axes from those of the parent axes as a result of the radiolysis. Since the crystallographic values of a used related to the unirradiated complex, the e calculated via (3) were subject to some uncertainty, and in such cases the calculation of gll was not attempted. For the main species, however, the J-T axis apparently lay quite close to one of the symmetry axes of the nickel acetate (4) A. D. Liehr and C. J. Ballhausen, Ann. Phys., 3, 304 (1958). Volume 74, ivumber 7 April 8 , 1970
1510
D. A. MORTON-BLAKE i
I
f-
I
2700
2800
I
2900
I
3000
I
3100 Gauss
Figure 2. Epr spectrum of X-irradiated nickel acetate at 77°K. Field along b axis.
complex, and so the value of 0 obtained by this procedure was used t o calculate gli via the relation g1I2 = r G/sin2 0.
+
Results The spectra, one of which is shown in Figure 2, revealed the existence of several distinguishable paramagnetic species, seven of which were sufficiently resolvable to follow their resonances over the field-direction range. The resonant fields were all between 2670 and 3065 G, which fact strongly suggested that all the paramagnetic species observed were of the kind S = l/z and possessed effective g values in the range g = 2.06 t o 2.38 for this crystal setting. From this Cu2+like g value it would appear that the species were of type (lb) rather than (IC) since the Co2+ ion (d') has much larger and more anisotropic g values, and its epr spectra can only be well resolved at low temperatures (4-20°K) due to its short spin-lattice relaxation time. All the species gave rise to probably equal line widths of 4.5 to 6 G ; for some lines apparent widths of 10 G were observed, but at certain orientations there was evidence for the presence of two or more species with almostparallel J-T axes. All possessed roughly similar degrees of g-value anisotropy and showed site splittings which were consistent with monoclinic crystal symmetry, as seen from the H(x) resonant field curves in The Journal of Physical Chemistry
Figure 3. Resonant field curves for the seven observed paramagnetic species in X-irradiated nickel acetate.
Figure 3. These curves and the resonance lines in Figure 2 are numbered from I to VI1 in the order of decreasing concentration of paramagnetic species as estimated roughly from the relative intensities of the spectra. Table I shows the J-T axes assigned t o the seven paramagnetic species, their relative concentrations, and where possible the g-value components. It was not possible to evaluate gll accurately for these species which had a Ca J-T axis for two reasons. Firstly, for the crystal setting used, the angles QI and 4 were in the region 78 to 90" thus limiting the accuracy of (3) to calculate 13 (which itself turned out to be rather small for Cs(1) and so in its turn imposed an inaccuracy on 911 as calculated by (2)). Secondly, the HtO ligands in the Ni complex are involved in intramolecular hydrogen bonding with the nonligand oxygen atoms of the acetate groups and in intermolecular hydrogen bonding with such oxygen atoms of neighboring complexe~.~ As a result of the hydrogen bonding it is unlikely that a Jahn-Teller distortion would occur precisely along an octahedral Ca axis. In order to obtain the true directions of the distortion axes and the g-value components, two or three crystal setting positions would have to be used. The instability of some of the species precluded the use of this method here. If species I, V, and VI1 are indeed chemically identical and differ only by a shift in the directions of the
EPROF NIKCELACETATE
1511
Table I : J-T Axes and g-Value Components of the Radiogenic Species from Nickel Acetate Tetrahydrate Rel. Speoies ooncn
I V VI1
I1 I11 IV
VI
13 4 2 9
9 5 3
2.076 2.064 2.117 2.069 2.080 2.115 2.116
=t0.002 =k 0.002
0.002 0.002 i 0.002 =I=0.002 =k 0.002 z!= =k
J-T axis
811
Ll
1
2.463 f 0.002 (Ac)O-Niprobably the O(Ac) same
-
Cdl)
~2.34(?)
CS(2)
J-T axes we should have to face the presence of an appreciable rhombic component at right angles to these axes. This is shown by the relatively large range of g values observed, namely from 2.064 to 2.117, and may be caused by differences in bond lengths and/or angles involving the two pairs of water ligands, Such a rhombic component would introduce some inaccuracy in our single-mounting-position theory, since the use of eq 2a is valid only for cylindrical symmetry. A correction for the rhombic component would lead to the value 911 = 2.47 f 0.01 for species I, V, VII, with g l = 2.06 to 2.12. Discussion An X-irradiated crystal of undoped magnesium acetate tetrahydrate shows two main multiplets which a t most orientations are recognizable as 1:2 : 1 triplets;6 this result therefore supports the identification of the radical a’-as CH2COO- which was made in the introduction. The results of the present experiment Indicate that the X-radiolysis of nickel acetate is analogous to that of cupric acetate. The free spin on the acetate radical a’- clearly finds an energy trough in the 3d shell of the nickel ion leaving a spin-paired, neutral group CH2C02, the complete paramagnetic species being represented by formula l b . The removal of the negative charge from one of the acetate anion ligands and of one of the positive charges on the nickel cation causes the electrostatic attraction between cation and the other acetate anion to be reduced to one-half its value in the parent complex, while that between Ni+ and a is considerably less (the actual amount depends on the degree of polarization of a by the Ni+ cation). This must result in an increase in the Ni-0 bond lengths in these directions, the Ni+-@ bond length being greater than that of A--Ni+. Similarly, the H20-Ni+ bond lengths will also be increased by amounts roughly equal to that of the A--Ni+ bond extension. The lengthening of the nickel cation radius as a result of its chemical reduction Ni2++ Ni+ by the irradiation
is about equal t o the lengthening in the step Ni3+-+ Ni2+which is 0.1 A.6 The lengthening of the ligand 0 atom in the A- -P @ step is probably about the same. Since the ground state of the unirradiated Niz+complex is already an orbital singlet (symmetry AI) a JahnTeller distortion does not occur and the complex approaches octahedral symmetry more closely than do spin-doublet species such as Cu2+or Ni+. However, the conversion of a Ni2+(3F)complex into a Ni+(2D) requires the selection of a distortion axis, and we are not surprised to find that the radiogenic species formed with highest concentration (species I) is one which has as its J-T axis the unique octahedral direction A-Ni+-a. The length of this axis, between the pair of “polar” oxygen ligands is now 4.54 8 as a result of the irradiation; the new “equatorial” axial lengths (between the two pairs of water ligands) are 4.32 and 4.44 8; we should therefore not expect these directions to form normal J-T axes. It,is not so obvious that some species would be created possessing a J-T axis of trigonal symmetry, but as has been suggested this could be caused by hydrogen bonding which does in fact make the octants of the octahedron inequivalent. However, the reason for the selection, by the complex, of the particular axes Cs(1) (which is directed between the ligands H20 (l), HzO (2) and O1 in the notntion of van Niekerk and Schoening3) and C3 (2) (which is directed between H2O (l), HzO (2) and 8,) is still not clear, and the calculation of crystal forces of distortion i s difficult. The striking similarity between the g components of species I in Table I and those of 2D-cupric acetate2 (911 = 2.458, gl = 2.075) indicates that in our 2D-nickel acetate complex too we have a high degree of spin population on the metal cation. If the values of X and AE in the expression g = g, - 8X/AE are similar to those for 2D-cupric acetate, the spin population is about 90-95%, which is measurably greater than in similar complexes involving formate ligands.’~~
Acknowledgment. My thanks are due to Mr. E. Bullock for preparing the crystals and to Miss R. H. Colton for identifying the crystal axes. This work forms part of the research program of the Division of Molecular Science of the National Physical Laboratory. (5) D. A. Morton-Blake, unpublished observations. (6).L. Pauling, “The Nature of the Chemical Bond,” Cornel1 University Press, Ithaoa, N. Y., 1960, p 518. (7) G. R. Wagner, R. T. Schumacher, and S. A. Friedberg, Phys.
Rev.,150, 226 (1966). (8) J.
M. Barbour, D. A. Morton-Blake, and A. L. Porte, J . Chem.
Soc., A, 878 (1968).
Volume 74, Number 7 April 8 , 1070
