SINGLE CRYSTAL SPECTRA OF PYRAZOLE COMPLEXES
561
solution of tryptophan containing CdC12, whatever the concentration of CdC12. With a CdClz-saturated frozen aqueous solution of tryptophan, the esr resonance is independent of the uv irradiation time. All the results reported here have been obtained in frozen solutions. However, flash-photolysis studies have already shown that photoionization is the main primary photochemical reaction of aromatic amino acids in aqueous solutions. From a biological point of view, this ejection process could be of great importance in the ultraviolet inactivation of enzymes. Photoejected electrons could react with amino acid residues which are far apart from the ejection center. Such reactions have already been demonstrated in frozen solutions; this includes breakage of disulfide bonds and deamination reactions.12 They have also been shown to occur in the direct electron irradiation of aqueous amino acid solutions.38
The results reported above show that the ejected electrons are easily trapped in metal ions such as Cd2+ Zn2+, etc.; these reactions could be important in the photoinactivation of metalloenzymes. More work is clearly needed to elucidate these points, but the methods presented here appear to be very convenient to understand some of the primary photochemical processes in biological molecules.
Acknowledgments. We wish to thank Professor C. Sadron for his interest in this work. We are indebted to Doctors F. Kieffer, A. Deroulede, and J. Roncin, (Laboratoire de Physico-chimie des Rayonnements de la Facult6 des Sciences d’Orsay) both for providing a y-ray source and helpful discussions. Helpful criticism by the referees is gratefully acknowledged. (38) R. Braama, Radiat. Res., 27,319 (1966).
The Single Crystal Spectra of Dichlorotetrapyrazolenickel(II), Dibromotetrapyrazolenickel(11) ,and
Hexapyrazolenickel(11) N’ itrate by Curt W. Reimann National Bureau of Standards, Washington, D. C . 20234 (Received June SO, 1969)
The single crystal spectra of hexapyrazolenickel(I1)nitrate, dichlorotetrapyrazolenickel(II), and dibromotetrapyrazolenickel(I1) from 6000 to 30:OOO em-‘ have been measured. The spectra of the halide complexes have been assigned on the basis of tetragonal molecular symmetry using the spectrum of hexapyrazolenickel(I1) nitrate as a comparison. The tetragonal splittings in the first octahedral bands are considerably larger than observed in related complexes. These large splittings are related to low values of the effective Dq of the halide ions. Detailed crystallographic data are presented which show that the low effectivefield of the halide ions arises through an internalhydrogen bond interaction with the coordinated pyrazole molecules.
Introduction The spectrochemical series has been widely used to interpret the spectra of octahedral complexes of transition metal ions. The spectra of ions in lower symmetry environments, however, have been less extensively studied and relatively few data are available to serve as a guide in the interpretation of the spectrum of a given ion in a variety of environments. One method for investigating the influence of changing environments utilizes crystallographic and spectroscopic data ob-
tained from octahedral species and their substitution products. The primary advantage of this method is that the energy levels of the substitution products can be traced to those of the parent octahedral species. Moreover, the most conspicuous spectroscopic changes which occur upon substitution can be interpreted in terms of descent in symmetry. I n an effort to obtain a series of complexes suitable for detailed study, two pyrazole (I) complexes of nickel(I1) were prepared-hexapyrazolenickel(I1) niVolume 74, Number d February 6 , 1970
562
CURTW. REIMANN
H
N-N.
/
C
/
\
C
C
I
n I trate [Ni(pyrazole),(NO,)~1and dichlorotetrapyrazole nickel(I1) [Ni(pyrazole)rCIZ]. Preliminary spectroscopic measurements show that the band system in Ni(pyrazole)~(NO3)~ contains three broad bands while that of Ni(pyrazole)&lz has five broad bands which are distributed over a wider energy region. A comparison of these spectra indicates that large low symmetry splittings exist in the spectrum of Ni(pyrazole)&lZ. The structures of the above complexes were then determined to provide a structural basis for an analysis of their spectra. This analysis shows that the spectrum of the chloride complex can be interpreted on the basis of tetragonal symmetry but that the magnitude of the tetragonal splittings can be accounted for only if the spectrochemical Dq for the C1- ion is greatly reduced from its typical value. ks the Dq values for C1- and Br- in the related pyridine complexes [Ni(pyridine)rC ~ and Z Ni(pyridine)aBrz] were found to be normal,' the complex Ni(pyrazole)aBrz was prepared for comparison. The structure of Ni(pyrazole)rBrz has recently been completed. I n this paper the single crystal spectra of Ni(pyrazole)nClzand Ni(pyrazole)rBre are reported and assignments are made on the basis of tetragonal symmetry with the aid of the Ni(pyrazole)G(NOt)z comparison spectrum. An analysis of these spectra is presented which shows that the magnitude of the t e tragonality is considerably larger than could have been anticipated from spectrochemical series data. Finally, the unexpectedly large tetragonality is interpreted in terms of specific structural features of the pyrazole complexes.
Crystal Data Crystals of Ni(pyrazole)s(N03)z2 are hexagonal (space group P3) with one molecule per unit cell of dimensions a = 9.958 and c = 7.278 A. The structure of the Ni(pyrazole)2+ cation viewed along the threefold axis is shown in Figure 1. The coordinating nitrogen atoms in this cation describe a polyhedron which is very nearly a regular octahedron. Figure 1 shows that the deviation from a regular octahedral configuration is small and that the observed polyhedron may be described as a trigonally compressed octahedron. Crystals of Ni(pyrazole)rClza and Ni(pyrazo1e)rBrz' are monociinic (space group C2/c with four molecules The Journal of Phyaicol Chemistry
Figure 1. Structure of the Ni(pyrazole)$+ cation.
per unit cell of dimensions a = 13.876, b = 9.263, c = 14.451 A, @ = 116.83" for the chloride complex and a = 14.127, b = 9.334, c = 14.702 A, p = 118.62" for the bromide complex. The molecular structure of Ni(pyrazole)rClz is shown in Figure 2 and that of Ni(pyrazo1e)rBrZ is shown in Figure 3. I n both of these structures the planar pyrazole molecules are virtually normal to the plane defined by the coordinating nitrogen atoms. The only rigorous symmetry element in these molecules is a center of symmetry (T site symmetry a t the nickel ion). If the geometry of only the coordination polyhedra is considered, however, the symmetry is approximately tetragonal (Dah). The details of the coordination polyhedron in the chloride complex are shown in Figure 4 and the projection of these polyhedra down the crystallographic b axis is shown in Figure 5.
Experimental Section Crystals of Ni(pyra~ole)~(NO~)~, Ni(pyrazole)rCla, and Ni(pyrazole)rBrp were grown from stoichiometric aqueous solutions of pyrazole and nickel(I1) nitrate, nickel(I1) chloride, and nickel(I1) bromide, respectively. For spectroscopic measurements crystals -3 X 3 X 1 mm were polished to a thickness of about 0.5 mm. Crystal thicknesses for measurements reported here were determined with a micrometer. Spectroscopic measurements were carried out on a recording spectrophotometer ( C a y 14). Glan-Thompson polarizers (1) D. A. Rowley and R. S.Drago. Inmg. Chem., 6 , 1002 (1967). (2) C. W. Reimann. A. Santoro. and A. D. Mishell. A& Cry&. in
preen. (3) C.
W.Reimann. A. D. Mishd. and F. A. Msuer, ibid., 23. 185
(1967).
(4) A. D. Mishell, C. W. Reimsnn. and A. Santoro, &bid., B2S. 596 (1069).
SINGLECRYSTAL SPECTRA OF PYRAZOLE COMPLEXES
563
Figure 4. Detailed geometry of the coordination sphere in Ni(pyraaole),CL.
Figure 2. Molecular structure of Ni(pyrazole)&b.
Figure 5. Projection of the coordination polyhedra in Ni-@ymole)d& down the crystallographic b ads.
e I,
8,
E
n
Figure 3. Molecular structure of Ni(pyrazole),Bre.
were used to obtain polarized light. Liquid nitrogen temperature spectra were recorded with the crystal in contact with a brass cold finger inside a simple silica dewar. Extinction coefficients were calculated from the observed optical densities and measured crystal thicknesses. The single crystal spectrum of Ni(pyrazole)6(NO& a t room temperature and at liquid nitrogen temperature is shown in Figure 6. The principal changes which accompany the reduction in temperature are the blue shift in the three main bands, the appearance of a weak narrow band near 22,500 om-', and the appearance of structure in the weak band near 13,400 om-'.
The single crystal spectra of Ni(pyrazole)&18 at liquid nitrogen temperature with light incident upon the (010) face are shown in Figure 7. Spectra were measured with light polarized with the electric vector approximately parallel and perpendicular to the 1101] direction. The absorption near 13,300 om-' consists of a broad band with narrower bands superimposed upon it. This feature is particularly evident along the [loll direction. For purposes of comparison, the mom temperature unpolarized spectrum was also recorded. The single crystal spectrum of Ni(pyrazole)rBr2 is shown in Figure 8. A summary of the bands observed in the room temperature spectra of Ni(pyrazole)s (NO&, Ni(pyraZOle)&b, and Ni(pymole)8r8 is given in Table I. Volume '74. Number 3 Fe6ruary 6.1970
CURTW. REIMANN
564 I
l
l
l
IO-
l
I-\
I
/
1
1
1
'
1
1
1
',,
I
1
I
I
1
1
l
l
1
- - - ROOM TEMPERATURE -N, TEMPERATURE
1
1
14
-
12 O I
e 8-
WAVENUMBER x 10-3
Figure 6. The electronic spectrum of Ni(pyrltzolea(NO& from 8000 to 30,000 cm-'.
WAVENUMBER
x 10-3
Figure 8. Single crystal spectrum of Ni(pyrazole),Bra at room temperature.
WAVENUMBER
x IO-'
Figure 7. Polarized electronic spectra of Ni(pyrazo1e)rCls a t liquid nitrogen temperature. Spectrum I was measured in the (010) face with light polarized along one extinction direction (approximately [loll). Spectrum I1 was measured 90" to the [loll direction.
Table I : Absorption Bands (in cm-1) Observed in Three Pyrazole Complexes at Boom Temperature Ni(pyrazole)e(NOs)a
Ni(pyraeole)&b
Ni(pyrazo1e)rBrr
10,650 13,500 17,100 27,500
8,000 10,950 13,100 13,500 16,400
7,240 10,900 12,000 15,900 25,800
26,500
Assignments of Spectra 1. Ni(pyru~ole)~(NO~)~. The structural data given above show that the coordination sphere in the Ni(pyrazole)a2+ cation is a very slightly distorted octahedron. The three most prominent bands in the spectrum of N i ( p y r a z ~ l e ) ~ ( N O ) ~ temperature show at~room no evidence for band splitting, nor does any evidence The Journal of Physical Chemistry
for such splittings appear in the liquid nitrogen temperature spectrum. Consequently, the approximation of octahedral symmetry should provide a valid basis to make the assignments of the bands in the spectrum of Ni(pyra~ole)~(N03)2. The nickel(I1) ion has eight 3d electrons which permit the free ion terms 3F, 3P, 'D, 'G, and 1s. I n crystalline fields of octahedral symmetry the sevenfold degeneracy of the 3F free ion ground term is partially removed, and the 3Azg(F),3T~g(F),and 3Tlg(F) crystal field states arise. The 3Pfree ion term remains unsplit in an octahedral field (3Tlg(P)). The remaining free ion terms, 'D, 'G, and 'S, become the crystal field states 'Eg(D), 'T2g(D), 'Alg(G) 'TdG) %g(G) IT2g(G),and 'Alg(S), respectively. The three broad bands observed in the room temperature spectrum of Ni(pyraz0le)6(NOa)~at 10,650, 17,100, and 27,500 em-1 are readily identified with the three spin-allowed transitions. The 10,650-cm-' band corresponds to the 3A2g(F) -+ 3T2g(F)transition while the 17,100- and 27,500-cm-' bands are assigned to the 3Azg(F)+ 3Tlg(F) and 3A2g(F) 3Tlg(P) transitions, respectively, The separation between the aA2g(F) and 3T2g(F) states is equal to 10 Dq, where Dq is the spectrochemical series parameter which is a measure of the ligand field strength. Thus, Dq for Ni(pyrazo1e)o(NO& is 1065 cm-1. The interelectronic repulsion parameter, B , can be computed6 from the energies of the 3Azg(F) 4 3Tlg(F) and 3A2g(F)--.+ 3Tlg(P) transitions using this value of Dq. The computed value of B for this case, 735 cm-l, represents -25% reduction from the free ion value.
+
+
+
-+
(6) 0.Bostrup and (1967).
C. K. Jorgensen, Acta Chem.
Scand., 11, 1223
SINGLECRYSTAL SPECTRA OF PYRAZOLE COMPLEXES
565
The assignments of the spin-forbidden bacds, which are particularly evident in the liquid nitrogen temperature spectrum, can be made with the aid of the LiehrBallhausen6 energy level diagram for the 3d8 system. The low-intensity shoulder near 13,400 cm-l is assigned to the 3Azg(F) + lEg(D) transition. A relatively sharp, weak band a t 22,500 cm-l is assigned to the 3Azg(F)4 lAlg(G) transition, and a much broader weak band observed a t -24,000 cm-' is assigned to the 3Azg(F)+ lTZg(D)transition. The last two assignments are supported by the breadth of the bands as well as by their energies. The breadths differ as a consequence of the fact that the upper states have different dependencies upon Dq. Thus the 'Alg(G) level, which is less sensitive to changes in Dq, is considerably less vibrationally broadened. The remaining possible band in the spectral region studied, 3Azg(F)+ lTlg(G), is expected to be just below the more intense and broader aAzg(F) -t 3Tlg(P) band. I t was not observed. A summary of these assignments is presented in Table 11.
NICKEL D)
f I
I
f
Table 11: Assignments of Observed Bands in Ni(pyrazole)B(NO&. ,---Band, -300'K
10,650 13,500 17,100 27,500
cm-*-77'K
ll,l00 13,500 17,500 22,500 24 ,000 28,100
Assignment
'Azg(F)
'T2J.F) IE,(D) 4 'TI#') lAi,(G) ''X'ze(D) -+ 3T~,(P) -+
4
Oh
D,,
Figure 9. Schematic correlation diagram of the nickel(I1) energy levels from the terms of the free ion into fields of octahedral and tetragonal symmetry.
--+(
2. Ni(pyraxo1e)dClz and Ni(pyraxo1e)dBrz. The structural data given above show that the halide complexes may be regarded as trans substitutior, products of the Ni(pyrazole)62+ cation. This interchange reduces the coordination symmetry about the nickel ion from octahedral ("oh) to tetragonal ( w D 4 h ) and each of the 3T octahedral states splits into two tetragonal states as shown in Figure 9. The degree of splitting and relative order of the two tetragonal states which arise from each 3T octahedral state depend on the nature of the tetragonal perturbation. This perturbation may be represented by two tetragonal splitting parameters Ds and Dt' which describe the splittings in all three octahedral 3T states. The deriveds splittings for the three octahedral bands are -85/1Dt, 6Ds - K/4Dt,and -3Ds 5Dt. The splittings in the second and third bands become 4.9Ds - 0.5Dt and - 1 . 9 ~ 9 ~4- 4.3502 when configuration interaction between the two 3Tlg octahcdral states is taken into account. Thus, the interpretation of the spectrum of a tetragonal niekel(I1) complex requires a band assignment and the description of the observed splittings in the three 3T octahedral levels in terms of Ds and Dt. In principle, the assign-
+
FREE I O N
ment could be carried out by determining the symmetry type of each level using the selection rules and the observed polarization properties of each transition, However, the halide complexes under investigation are rigorously centrosymmetric and the d-d transitions are Laporte forbidden. I n such cases the vibronico intensity mechanism must be invoked, and the symmetry of each coupling vibrational mode must be included. This analysis for the halide complexes (Table 111) shows that only the 3B1,-+ 3A2gand 3BIg-t 3B2, transitions have restrictive selection rules. These restrictions, though useful, do not provide a basis for an unambiguous assignment of the entire set of observed bands. In practice assignments will be made using the spectrum of Ni(pyrazole)~(NOa)tas ti comparison and reinforced by the available polarization data. To utilize the spectrum of Ni(pyra~ole)~(r\'O~)~ as a basis for specific assignments of the spectra of the derivative halide complexes, it is helpful to characterize the behavior of the octahedral ground state and first (6) A. D. Liehr and C. J. Ballhausen, Ann. Phys. (N. Y,), 6 , 134 (1959). (7) C. J. Ballhausen, "Introduction to Ligand Field Theory," McGraw-HillBook Co., Ino., New York, N. Y., 1962. (8) C . R. Hare and C. J. Ballhausen, J . Chem. Phys., 40, 792 (1964). (9) A. D. Liehr, Ann. Phys. (N. Y.), 1,221 (1967).
Volume 74,Number S February 6, 1970
566
CURTW. REIMANN
in the spectrum of Ni(pyrazole)~(NO&has been assigned to the spin-forbidden 3Azg(F) -+ lEg(D) transiand for Ni(pyrazole)& tion. The intensity near 13,500 cm-l in the halide complexes appears to be too large to attribute to differPolarization Coupling Transibion direction vibration ences in percentage triplet character. If this view is correct, the intensity a t 13,400 cm-l must be attributed ell 3B1,-'EE /I I wl, Pzu primarily to a spin-allowed transition as well as to the 'Big-'Azg i fU spin-forbidden transition. This view is supported by 3B1,---3Bzg L the appearance of narrower bands superimposed upon the broad band in the spectrum of Ni(pyrazole)4Clz. These narrower bands resemble those observed in the excited state under the action of a tetragonal perturbaspectrum of i%(pyra~ole)6(NO3)~ which have been tion. The tetragonal ground state (3Blg) is raised assigned to the aAzg(F) lEg(D) transition. Ac7Dt relative to the octahedral ground state (3A2g)by a cording to Figure 9 two possible assignments can be tetragonal potential. The splitting in the first octamade for the band which occurs at 13,100 cm-l in the hedral state gives a 3E2g state which is raised 7Dt and at 12,000 cm-l in the spectrum of Ni(pyraz01e)~Cl~ relative to 3Tzgand 3Eg which is 7/4Dtbelow 3Tzg. As spectrum of Ni(pyrazole)4Br2. If these bands are both 3Blgand 3Bzghave been raised 7Dt relative to their assigned to the 3B1, --t 3Eg transition, the computed parent octahedral states, the 3Blg 4 3B2gtransition values of Ds (assuming configuration interaction) would should occur a t approximately the same energy as the be (708), for C1 and (839) for Br. If these bands are 3Rzg(F) 4-3T2g(F)octahedral field transition. Moreassigned to the 3B1, -+ 3A2gtransition, Ds would be over, because the splitting in 3T2g(F)depends only on -640 cm-' in the chloride complex and -753 cm-I Dt, the identification of the split components of 3Tzg(F) in the bromide complex. The expression for the allows a determination of Dt. This value of Dt can, tetragonal splitting in the 3Azg(F)* 3T1,(P) band shows in turn, be used to interpret the rema,inder of the that a laage negative value of Ds would manifest itself spectrum. in a large splitting because negative Ds and positive According to the above analysis, both the 3Blg and Dt act together to split this band. No noticeable 3Bzglevels are raised 7Dt relative to their parent octasplitting occurs in the aA2g(F) 3T1g(P)transitions hedral states. Therefore, the 3Blg 4 3Blg transition which indicates that the negative values of Ds do not should occur with approximately the same energy as the lead to a consistent interpretation. Taking the positive first oclahedral band in Ni(pyrazolej~(N03j2 (10,650 valves of Ds,the predicted splitting in the 3A2g(F)-+ cm-*). On this basis the bands at 10,950 cm-l in 3T1,(P)bands are only 121 and 224 cm-l for the chloNi (pyrazole),C12 and 10,900 cm-l in Ni(pyraz01e)~Br~ ride and bromide complex, respectively. A summary of represent the most reasonable choices for the 3Blg + the assignments for the halide complexes is given in 3B2g transition. This suggests that the band at 8000 Table IV. cm-l in Ni(pyrazole)4Clz and 7240 cm-I in Ni(pyrazolej4Br2are the second components of the 3Tzg octahedral band and correspond to 3Blg 4 3Eg transitions. Table IV : Assignment of Spin-Allowed Transitions (cm-1) Table I11 shows that the vibronic selection rules are in Ni(pyrazole)&lp and Ni(pyrazole)rBrZat Room Temperature consistent with the assignments of the two lowest Transition Ni(pyrazo1e)rClo Ni(pyraaole)4Bm energy bands in the polarized spectrum of Xi(pyraxole)4Clz. More importantly, the observed polarizations 8Blg 4 $E, 8,000 7,240 10,950 10,900 -* a B ~ g rule out the choice of the lowest energy band as the 13,100 12 000 3Blg * 3B2gtransition. This conclusion follows from 3 3E, 16,400 15,900 the fact that the first tetragonal band has greater 26,500 25 800 4 'Em '-429 intensity parallel to the lourfold molecular axis than perpendicular to it. This should not be observed for a band with no allowed parallel component (3Big * Discussion ?Eizg). From these assignments Dt is found to be The foregoing analysis shows that the tetragonal 337 cm-l in the chloride complex and 418 cm-I in the splitting in. the first octahedral band (3A2,(F)-+ 3T2,(F)) bromide complex. is larger in Ni(pyrazole)dBrz than in Ni(pyrazole)4Clz. To assign the remaining bands in the halide complexes The lower component of the split 3Tz,(F)level is found and to determine Ds, it is once again helpful to compare to be particularly sensitive to the difference between the spectra of the halide complexes with that of NiC1 and Br. The upper component, however, is almost (pyraEOk!)@Oa)z. The most conspicuous difference identical in both complexes and lies -250 cm-' above in the region above 12,000 C M - ~ is the large intensity 3Tz,(F) transition in the octahedral comthe 3Az,(F) increase near 13,400 em-'. The band a t 13,500 cm-l Table I11 : Vibronic Selection Rules for r\Ti(pyrazole)rClz
-+
-+
~
~~~~~
~
-+
-+
The Journal of Physical Chemistry
SINGLE CRYSTAL SPECTRA OF PYRAZOLE COMPLEXES
567
plex Ni(pyrazole)s(NO&. As detailed structural data are available for all three complexes, the behavior of these related energy levels may be considered in terms of structural differences. Also, the qualitative and quantitative features of the tetragonal splittings may be compared with expectations based on spectrochemical series data. The tetragonality parameters are defined so that a positive sign corresponds to axial elongation or to a weaker ligand on the fourfold axis ( z ) than in the plane (z,y). Wentworth and Piperlo have given an expression (shown here as eq 1) which relates the splitting
in the tetragonal complexes is determined directly from the energies of the aBls aBzgtransitions, the large values of Dt must be associated with a reduction in the effective field of the halide ions. Using the observed values of Dt and of Dq(pyrazole), the effective Dq's for C1- and Br- are found to be 505 and 358 cm-l, respectively. The 26% reduction in Dq(Cl-) and 40% reduction in Dq(Br-) represent substantial deviations from the range of typical values for such parameters. For example, in the closely related pyridine1 complexes, Ni(pyridine)4Clz and Ni(pyridme)pBr,, the effective field of each halide ion agrees closely with the spectrochemical series value. Consequently, the behavior of the halide ions in the pyrazole complexes must be interpreted in terms of specific properties of these complexes.
(1) Dt = %[D~z,Y) - D&)I parameter Dt to the spectrochemical series parameters for the in-plane and axial ligands. The Dq values for the ligands on the z axis [C1(680 em-'), Br (600 cm-l)I1l are both less than Dq(pyrszo1e) (1065 cm-I) and the positive signs for Dt determined from the spectra are qualitatively consistent with spectrochemical series data. Equation 1also shows that Dt should be greater in the bromide complex than in the chloride complex. Therefore, the observation that the separation between the 3Bl,7 aE,and 9B~,+ aBz.transitions increases with the substitution of C1 by Br is consistent with an increase in tetragonality which is proportional to the disparity between the field strengths associated with axial and in-plane ligands. The entire increase in splitting brought about by the substitution of C1 by Br occurs through the reduction in the 3B1g aE. level. In both halide complexes, however, part of the splitting in the first octahedral hand occurs through a -250 cm-l rise in the 'B1. 3Ba,relative to the 'AzApg -+ aT2,octahedral transition. I n making the assignment of the PB,, -+ 'Bo transitions it was mentioned above that these transitions should occur at the same energy as the first octahedral bond in Ni(pyrazole)~(NO& because the in-plane ligand is the same in all three complexes. This small (-250 cm-l) npward shift of aBlg- 8Bz.relative to 8A~,(F) 'Tz.(F) is undoubtedly a consequence of the fact that, in the halogen-substituted complexes, the four pyrazole molecules lie closer to the nickel ion than in the unsuhstituted complex (see Figures 1-3). The effective field of pyrazole increases by about 3% in the substituted complexes. Thus, the two tetragonal components of the split aTz,octahedral state behave in accord with expectations based upon reduction in symmetry. The absence of ambiguity in these assignments notwithstanding, the quantitative agreement between the values of Dt derived from the observed spectra and those calculated from the spectrochemical series values of Dq in verv Door. I n both cases. Dt derived from the suec~. tra is considerably larger than Dt computed using eq 1. This indica& that the disparity between Dq(pyrazo1e) and DqWogen) is larger than could be anticipated from spectrochemical series data. As Dq(pyrazo1e)
-
-
-
~~
"
-
d H
Figure 10. Detailed configuration of one pyrasole molecule in relation to adjacent Ni-Br group.
The specific property of the pyrazole complexes which is related directly to the large tetragonal splitting can be identified by comparing the structures of pyraBole complexes and those involving related ligands. Pyrazole and other ligands which contain > N and >N-H groups may utilize them in coordination ( S N ) and in hydrogen bonding (>N-H). I n pyrazole (I) these bonding groups are adjacent and the angle between the coordinate bond and the N-H direction is only -72". I n the structure of the tetragonal pyrazole complexes (Figures 2 and 3) each pyrszole ring is nearly coplannar with the CI-Ni-CI and Br-Ni-Br groups.
I
(10) R. A. D. Wentworth and T. S. Piper. Inoro. C h m . , 4, 709 (1965). lntersoimee (11) B, N. Figpis, 6aIntroduetianto Ligand publiehers, N~~ York, N. Y.,1966.
Volume 74,Number $ February 6,1070
I. RURAK,D. SHAPIRA, AND A. TREININ
568
H This orientation permits
/ the N-N group / / \
to co-
ordinate to the nickel ion through > N with the N-H groups pointing toward the coordinated halide ions. The steric relationship between one pyrazole ring and an Ni-Br group is shown in Figure 10. The same relative relationship exists in the zhloride complex with an Ni-CI bond distance of 2.507 A. In the corresponding dibromotetrapyridinenickel(I1)l2 and dichlorotetrapyridinenickel(I1) complexes the pyridine rings are inclined -45" away from the Br-Ni-Fr and C1-Ni-C1 directions, the Ni-Br distance is 2.58 A, and the Ni-C1 distance is 2.38 A. These data suggest that the pyridine rings assume positions which compromise the
mutual repulsion between the pyridine rings (which is a minimum when the rings are aligned with the Ni-X direction) and the repulsion between the halide ions and the pyridine rings. I n the pyrasole complexes, however, the interaction between pyrazole and the halogen ions is a net attraction which stabilizes the configuration shown in Figure 10. The hydrogen bond interactions (two interactions per halide ion) reduce the effective field of the halide ions, and this reduction manifests itself directly in the longer Ni-X bond lengths and indirectly in the unusually large tetragonal distortion in the ligand field of the Ni(I1) ion. (12) A. 8. Antsishkina and M. A. Porai-Koshits, Kristallografiya, 3, 684 (1968).
(13) M.A. Porai-Koshhits, Dokl. Akad. Nauk &WR, 10, 117 (1954).
The Photochemistry of N,- in Aqueous Solution at 2288 and 2139 by I. Burak, D. Shapira, and A. Treinin Department of Physical Chemistry, Hebrew University, Jerusalem, Israel
(Received June 2, 1969)
The photolysis of Ns- at 2288 hardly differsfrom that at 2537 A; both probably involve the n,n* state of Na-. On the other hand, new features are shown by the photolysis at 2139 A, which is within the charge-transfer-to-solvent (CTTS) absorption band. About 10% of the excited ions undergo ionization and -30% produce ",OH, probably by first being internally converted to the n,n* state. Ammonia is produced from the reduction of hydroxylamine by soivated electrons. This is indicated by the effect of electron scavengers The azide radical appears to abstract H atom from alcohols and, more readily, from acetone and NH20H. A full mechanism is proposed for the photolysis. The thermal and photochemical decomposition of heavy metal azides is usually considered to involve the formation of the azide radical as the primary chemical process.' For azides with relatively high exciton fevels (e.g., KN3 and Ba(N&) some internally excited states of Na- appear to play an important role.' This is also the case for Na- in solution, which has an n,r* (lA,2B2) state below a chargetransfer-to-solvent (CTTS) state (the latter may be considered as an exciton of low mobility) ;2 indeed, the photolysis of Ns- in aqueous solution is highly governed by the n,n* state. There is no indication of photoionization of N3- a t 2537 A, a wavelength lying well within the n+a* absorption band.3 On the other hand, recent flash photolysis studies have indicated that excitation within the CTTS band (below 230 mp) yields solvated electrons4 and azide radica1s;b still other processes may compete with ionization. The present work provides information on the efficiency of the photoionization process and its The Journal
f . 0
Phvsical Chemistry
subsequent chemical effects. It leads l o conclusions concerning the chemical properties of some important nitrogen containing radicals in aqueous solution, in particular N3 and "2. Experimental Section Light soumes. Irradiation a t 2139 A was conducted with a 25-W Philips zinc spectral lamp No. 93106. To reach stable intensity, the lamp was warmed 20 min before irradiation. The stability of the lamp was checked by measuring the azide depletion in a lods M NaN3 solution containing air, and this was carried out just before and after irradiation of the given sample. To minimize the effect of photolysis by light at wave(1) P. Gray, Quart. Rev.,(London), 17,441 (1963). (2) I. Burak and A. Treinin, Y. Chem. Phys., 39, 189 (1963). (3) I. Burak and A. Treinin, J. Amer. Chem. Soc., 87, 4031 (1966).
(4) D.Behar, D, Shapira, and A. Treinin, to be published. (5) A. Treinh and E. Kayon, J. Chem. Phys., 50,538 (1969).
