494
J. Phys. Chem. 1982, 86, 494-500
is consistent with calculated H,O dehydrogenation energy barriers of 5-11 kcal/mol, in the -1- to 1-V range. A potential-dependent experimental study would be interesting: We would predict a barrier decrease in the anodic direction and, at some point, also in the cathodic direction (Table 111). At some point in the anodic range it will be energetically favorable for a hydrogen atom which is released from the HzO dehydrogenation reaction to lose an electron, which flows through the circuit, and become H+, which leaves the surface to become solvated. The second step, leading to surface oxide or 0 and subsequent 0,formation, requires loss of H from OH and a second oxidation to form H+(aq). This is a more difficult step, for the activation energy for OH dissociation increases with potential (Table VI). At -1.2 V (vs. the normal hydrogen electrode) this reaction finally occurs and will be driven by several probable causes, dependent on alternative H+ abstraction mechanisms, OH coverage, H+(aq)and O2stability, and entropy considerations. These must counteract our predicted decrease in stability gains for OH 0 + H in the anodic range (Table VII). H30+is a stable species by 5 eV in our calculations with respect to H+ H,O. This stability will drive the reaction when the proper potential is reached. The high stability of OH in the anodic range is consistent with discussions in the passivation literature,k6 which generally postulates OH or OH+ formation as an initial step in air-passive or anodic-passivefilm formation. Our calculations produce an OH charge of 1 at 1 V and 0 at -0.5 V. On our model uncharged surface the OH charge is 0.2 because of oxygen p orbital delocalizationover the neighboring iron atoms. It is unlikely that an OH+ species can exist for long on the electrode surface, and it is probable that an [FeOH+], complex forms above the electrode. This has been postulated in the passive film literature.5 We see that dehydrogenation energies for both HzO and H30+ are small in the cathodic range, and the stability gains are almost identical for the products (Tables IV and X), as are the initial molecular adsorption energies (Tables
-
+
+
I1 and VIII). The OH + H and H20 H reaction products have neutral net charges at --1 V. It would appear that Hz evolution could stem from a reduction mechanism involving either HzO or H30+ a t a bare metal electrode surface. Will the surface be bare or covered with OH or even 0 in the hydrogen evolution range? Coverage by 0 is unlikely because, even though our calculated OH dissociation energy barrier is small (Table VI), the products at -1 V have unrealistic charges (Table VII). 0 + H are neutral at around 1 V, where surface oxidation and 0, evolution are likely. OH has a small negative charge at -1 V, so that it may be stable. Can OH act as an intermediary in transferring an electron to H30+in the solution? Its orbitals lie probably too low in energy for that. Therefore it is probable that H, evolution occurs at active bare surface regions by reduction of H 2 0 or H30+.
Concluding Comments We have, on the basis of surface species charges, restricted our discussion of water reactions on an iron electrode to a range of about 2 V. This compares well with the passive film range, with 0,evolution at the anodic end, and H, evolution at the cathodic end. Our results support most of the recent hypotheses put forth in the literature concerning surface species, reactions, and passive film formation in the beginning stages. The technique of shifting metal valence band levels to mimic electrode surface charging and electrochemical potential changes appears to be useful. The simplicity of the one-electron calculations will allow future studies of these and related systems with better models, including larger surface clusters, various coverages of OH and other species, and solvation of ions before and after surface redox reactions. Acknowledgment. This study was supported by a Select Research Opportunities Grant from the Office of Naval Research. We acknowlege Professors E. Yeager and B. Cahan and Dr. A. Nazri of the Case Laboratories for Electrochemical Studies for helpful discussions and encouragement.
Photoreactivity Patterns in Nickel( I I ) Compounds L. 0. Vanqulckenborne,' A. Ceulemans, D. Beyens, Department of Chemistry, University of Leuven, Celestijnenlaan 200F, E3030 Heverlw, Belgium
and J. J. McGarvey' Department of Chemistry, Queen's University of Belfast, Belfast BT9 5AG, Northern Ireland (Received: April 14, 1981: I n Final Form: September 21, 1981)
It is fiist shown that the ligand field parameters, describing a given Ni(II)-ligand interaction, depend significantly on whether the complex is high spin ( S = 1) or low spin ( S = 0). The low-spin u and T parameters are approximately 40% larger than the high-spin parameters. Using this result, it is possible to analyze a number of photoreactions of Ni(I1) compounds. A rationalization is offered for singlet == triplet conversions along photoisomerization, photoassociation, and photodissociation reaction paths. Introduction In solution Ni(I1) complexes are characterized by various equilibria featuring a variety of coordination structures. It has been shown, using relaxation techniques, that these equilibria can be perturbed upon irradiation. Apparently 0022-3654/82/2086-0494$01.25/0
very rapid photochemical pathways can convert these structures into each other. The reactivity pattern is therefore significantly different from the overall chemistry of stable d3 (Cr(II1)) or strong-field d6 (Co(III), Rh(III), Ir(II1)) complexes, which 0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982 495
Photoreactivity Patterns in Nickel(1I) Compounds
is characterized by one unique coordination form: the octahedron. Irradiation of the latter complexes can provoke photosubstitution processes in a stereospecific manner,' but ultimately photoproducts show the same favored geometry. Ligand field models2p3have contributed to an understanding of photosubstitution processes. In the next sections we investigate how this model approach can be applied to the present problem, where no dominant geometry can be imposed.
Survey of Experimental Data In order to place the later discussion of the ligand field model in perspective, we first present a synopsis of the photochemical reactivity patterns which have been observed for Ni(II) complexes participating in confiiational equilibria in various solvent media. Because these equilibria are established very rapidly, Q-switched laser techniques were necessary to effect sufficiently rapid photochemical Schemes 1-111 summarize the photochemical observations for a number of complexes. In each case, the central point is that the photoinduced changes take place between square-planar (SP) and nonplanar skeletons and are accompanied by spin changes. The nonplanar skeletons comprise several structures, i.e., tetrahedra (TD), trigonal bipyramids (TBP), square pyramids (SPY), and elongated octahedra (EOH). In each scheme, the parent point groups, the high- or low-spin character (hs. or 1s) of the complexes, and the irradiation wavelengths used are indicated. The actual (lower) symmetries of the species concerned are discussed more fully later, in the course of the theoretical analysis. Ligand abbreviations are listed at the end of the paper. Scheme I planar + tetrahedral isomerization
where X = C1 and Br, and the solvents are CH2C12or CHC13.
Scheme I1 5 coordinate * 4 coordinate
SP-Ni(Et4dien)C1++ C1- (i) (D4h; 1s) where the solvent is acetonitrile and (1 denotes a solventseparated ion pair.
-
SPY-Ni(dacoda)(H20) (c4"; hs)
1060 nm
SP-Ni(dacoda) (D4h; 1s)
+ H20 (ii)
(1) Zinato, E. In Adamson, A. W.; Fleischauer, P. D. "Concepts of Inorganic Photochemistry"; Wiley-Interscience: New York, 1975; Chapter
4.
(2) (a) Zmk, J. I. J. Am. Chem. SOC.1972,94,8039 (b) Wrighton, M.; Gray, H. B.; Hammond, G. S. Mol. Photochem. 1973,5, 164. (3) (a) Vanquickenborne, L. G.; Ceulemans, A. J. Am. Chem. SOC. 1977, 99, 2208. (b) Vanquickenborne, L. G.; Ceulemans, A. Ibid. 1978, loo, 475. (4) McGarvey, J. J.; Wilson, J. J. Am. Chem. SOC. 1975, 97, 2531. (5) Campbell, L.; McGarvey, J. J. J. Am. Chem. SOC.1977,99, 5809. (6) Campbell, L.; McGarvey, J. J.; Samman, N. G. Inorg. Chem. 1978, 17, 3378.
Scheme I11
-
6 coordinate + 4 coordinate 1080 nm
EOH-Ni(2,3,2-tet)(H20),2+ ( o h ; hs)
SP-Ni(2,3,2-tet)2++ 2H20 (D4h; 1s) The solvent in eq ii of Scheme I1 and in Scheme I11 is water. Scheme I and eq i in Scheme I1 show the common feature that the photochemical perturbation can be effected by irradiation (at the appropriate wavelength) of either the hs or the 1s participants in the equilibria, the key experimental point here being the invariance with irradiation wavelength of the thermal relaxation times of the perturbed equilibria. In eq i of Scheme 11, the formation of Ni(Et4dien)C12by irradiation at 530 nm of the SP ion pair (known from thermodynamic data' to be the principal absorbing species at this wavelength) is an example of photoassociation. Indirect but significant evidence for this conclusion has been provided by recent spectrophotometric and conductometric monitoring over a wide temperature range (+25 to 4 2 "C) of the relaxation of the system after irradiation. The existence of at least two coupled relaxation processes following 530-nm perturbation has been confirmed.& Recent studies of the planar + tetrahedral equilibrium in Scheme I using picosecond laser techniques indicate that the photochemical perturbation, in this case at least, is essentially complete within 60 ps after the laser pulse.8b
Electronic Structure of Reactants and Products As a preliminary, a ligand field description of the electronic structure of all species involved is required. I t turns out that parameters for the same ligand-Ni(II) interaction differ significantly whether the ligand participates in a high- or low-spin complex. We can classify all parameters into two classes. Class A parameters are to be used for square-planar complexes, characterized by a strong ligand field and consequently of the low-spin type. Field strengths are comparable to d3 and d6 octahedral complex values. Class B parameters cover all other nonplanar structures. They are of the weak-field type with triplet-spin ground states. Largely similar ligand field parameters can be used throughout this series. We now discuss these two classes in some detail. Class A. The absorption spectra of square-planar Ni(I1) complexes are almost structureless and consist of one broad band, corresponding to a spin-allowed transition from a cluster of orbitals: (dz2,dxy,dxz,dyz) -,dXLy2 The presence of the dz2 orbital in the cluster can only be explained by a considerable stabilization of this orbital, very probably attributable to substantial 4s-3d mixing. A detailed analysis of this effect in the electronic structure of SP complexes of Pt(I1) and Pd(I1) is presented elsewhere.g From ref 9, as well as from spectral data of SPCUC~,~-,'~ the magnitude of the z2 stabilization can be (7) Hirohara, H.; Ivin, K. J.: McGarvev, J. J.; Wilson, J. J. Am. Chem. SOC. 1974,96,4453. (8) (a) Dawson, K.; McGarvey, J. J., unpublished results. (b) McGarvev, J. J.: Lockwood,. G.:. Devonshire.. R... to be submitted for publication. (9) Vanquickenborne, L. G.; Ceulemans, A. Inorg. Chem. 1981,20,796. (10) Hitchman, M.; Cassidy, P. Inorg. Chem. 1979, 18, 1745. (11) Van Hecke, G. R.; Horrocks, W. Dew. Inorg. Chem. 1966,5,1968. (12) Dori, 2.;Gray, H. B. J. Am. Chem. SOC.1966,88, 1394. (13) Bosnick, B.; Gillard, R. D.; McKenzie, E. D.; Webb, G. A. J. Chem. SOC.A 1966, 1331. '
496
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Vanquickenborne et ai.
TABLE I : Cluster Transition Energy and Ligand Field Parameters for Square-Planar Ni(I1) Complexes"
SP complex
G, pm-1
parameters, pm-l
2.13b
o(C1) = 0.6
1.88c
u(P) = 1 . 1 3 o(C1) = 0 . 6 o ( N ) = 0.8
Ni(2,3,2-tet)** 2.22d
o ( N ) = 0.86
Ni(ddp)Cl, Ni(Et,dien)Cl+
u(Cl)/n(Cl) = 6 u(P)/n(P)= 10 o(Cl)/n(Cl) = 6 n(N) = 0 n(N) = 0
In all cases 4s-3d mixing was put equal to the average u interaction. Reference 11. Reference 1 2 . References 1 3 and 14; similar results were obtained for N i ( d a c o ) , * +(ref 15). a
estimated to be of the same order of magnitude as one entire u interaction. In this approximation, the average transition energy for the cluster is expressed as G = 35 - 2% - 6/2B - C (1) ti and iirepresenting mean values for the relevant ligand field parameters. Using typical Ni(I1) values, C = 0.35 pm-l, B = 0.08 pm-l, hv = 2.0 pm-l, the strong ligand field contributions in these class A complexes are evident. Table I lists representative ligand field parameters for the present compounds. They were obtained by using eq 1, while, for the u / a ratio, the Cr3+values3*were taken. As far as we know, no information on the position of phosphine in a two-dimensional spectrochemical series is available to date. We assumed that aryl phosphines could show moderate n-donor interactions, in agreement with their coordination chemistry.16 Class B. Several high-spin complexes with varying coordination numbers can satisfactorily be described by a unique set of ligand field parameters, thus satisfying the transferability condition. We therefore consider them to belong to a single class. Tetrahedral Skeleton. Ni(dpp)Cl,. Tetrahedral NiC142has been studied thoroughly. A reliable lODq(Cl-) = 30 - 477 of approximately 0.88 pm-l is available." Ni(dpp)C12 shows a typical absorption1' around 1.22 pm-', which can be attributed to a 3T1 transition (in Td symmetry notation). From this absorption lODq(P) can be estimated. n/a ratios were transferred from class A compounds. Parameters thus obtained were checked in a fit of singlet crystal datals of Ni(PPh3)2C12,exhibiting a distorted tetrahedral structure. The calculation was performed for the given angular positions and assumed similarity in the coordinating properties of triphenyl phosphine and dpp. Results are shown in Table 11. Trigonal Bipyramid. Ni(Et4dien)C12.We transferred C1 values from NiCld2-and Ni(dpp)C12 and calibrated u(NH& accordingly, using the parameter ratios established in C P complexes. Good agreement with spectral data was obtained (Table 111). It should be noted that the electronic spectrum is not very sensitive to the specific coordination site of both Cl- ligands. The Co(I1) analogue was characterized in solid state as a distorted TBP with one Cl- in apical and one in equatorial position.22 We consider it likely however that, for a de system, strong ligands preferentially coordinate at apical sites. In solution, a
-
(14) Jmgensen, C. K. Acta Chem. Scand. 1957,11, 399. (15) Hitchman, M. A.; Bremner,J. B. Inorg. Chim. Acta 1978,27, L61. (16) Que, L.; Pignolet, L. Inorg. Chem. 1973,12, 156. (17) Lever, A. B. P. "Inorganic Electronic Spectroscopy"; Elsevier: Amsterdam, 1968. (18) Fereday, R. J.; Hathaway, B. J.; Dudley, R. J. J. Chem. SOC. A 1970, 571. (19) Ciampolini, M.; Nardi, N. Inorg. Chem. 1967, 6, 445. (20) Wood, J. S. h o g . Inorg. Chem. 1972, 16, 227. (21) Di Varia, M. Inorg. Chim. Acta 1980, 38, 21. (22) Dori, Z.; Eisenberg, R.; Gray, H. B. Inorg. Chem. 1967, 6, 483.
'\ CI-P
t) I)
c- '1 C2VITdl
1060 n m h
y
P
'2viD4h'
I
530 n m
Figure 1. Correlation diagram for the TD-SP equilibrium in N(dpp)C12 Symmetry labels refer to point groups indicated (parent group labels between brackets). Excitationwavelengths are Indicated, and proposed photochemical relaxation pathways are marked by arrows. The u, plane of C 2 , bisects the P-Ni-P angle.
fluxional structure can be expected. Six Coordination. Ni(2,3,2-tet)(H20)22+. Table IV lists our results for the detailed spectra of tr~ns-Ni(en)~(H~0),2' and tran~-Ni(~-Et~en)~(H~0)~~+~2C1--2H~O which can be considered as valid analogues for the complex under inv e ~ t i g a t i o n . ~The ~ ~ amine ~ ~ parameter corresponds approximately to the TBP value. It is noteworthy that the axial ligand field (H20 parameters) in these tetragonal complexes is exceptionally weak, indicating a considerable elongation of the axial bond^.^^^^^ With the exception of the axial sites in the six-coordinated complexes, one consistent set of ligand field parameters is offered for three different coordination geometries. In comparison of class B parameters with SP values, the large reduction (-40%) is apparent. We concluded that nonplanar coordination forms are realized at the expense of reduced metal-ligand interactions, probably balanced by more favorable ligand-ligand interactions and/or increasing coordination number. The fact that all d orbitals are at least partially occupied in high-spin de complexes may be accompanied by an increase of the metal-ligand distances. Results Figures 1-3 show correlation diagrams for typical equilibria. They are constructed from the following principles: (i) Initial and final low-spin and high-spin (23) Farago, M. E.; James, J. M. J. Chem. SOC., Chem. Commun. 1965, 470. (24) Lever, A. B. P.; London, G.; McCarthy, P. J. Can. J.Chem. 1977, 55, 3172. (25) Burdett, J. K. Inorg. Chem. 1975, 14, 931. (26) In Table N u(H2O) must be considered as an effective u strength, incorporating the energy lowering of the d,z orbital as a result of s-d mixing. The structural aspects of six coordination in Ni(I1) complexes are treated more fully in a separate paper: Ceulemans, A.; Beyens, D.; Vanquickenborne, L. G., submitted for publication. (27) Lever, A. B. P.; Walker, I. M.; McCarthy, P. J. Inorg. Chim. Acta 1980, 44, L143.
The Journal of Physical Chemlstty, Vol. 86, No. 4, 7982 497
Photoreactivity Patterns in Nickel( I I) Compounds
TABLE 11: Comparison of Observed a n d Calculated Transitions in Dichlorobis(triphenylphosphine)nickel(II) Using Estimated Values for Ni(dpp)Cl," obsd"
calcd
0.48 0.83 1.02 1.12 1.70 1.79
0.34 0.52 0.82 1.20 1.67 1.80
"
assignmentC 3A,(3T1) 3A1(3T,) 3B,(3T,) 'Aa( 3A21 3Aa(3T,(P)) 3B2(3T1(P))
parameters
B = 0.08 C = 0.35 u(C1) = 0.377 u(P) = 0.61
u(Cl)/n(Cl) = 6.2 u(P)/n(P) = 10
See text. Polarized single crystal spectrum f r o m ref 18. This work. Labels refer t o C, symmetry and Td parentage as defined in ref 18. Ground state: 3B,(3T,). Our assignment agrees with observed polarizations. TABLE 111: Observed and Calculated Ligand Field Transitions for Ni( Et,dien)Cl, obsd"
calcd
assignment"
parameted
'A2( )E1') B = 0.075 3B1(3E") C = 0.32 1.25 3B1(3A2") o(C1) = 0.377 o(Cl)/n(Cl) = 6.2 3A,(3A,") u ( N ) = 0.485 n(N) = 0 1.32 3B,(3A2') 1.88 1.90 3B1(""(P)) 3A2(3E':(P)) 2.21 2.21 3B,(3A2 (P)) a Reference 1 2 . This work. C, symmetry labels (see also Figure 2). D* parentage is given between parentheses. The In a recent study of ground state is 3A,(3E'). T h e relative ordering of these Dah levels is consistent with ref 19 and 20. the Co(I1) analogue, Di Variazl used very similar parameters, except for u(C1) F= 0.2 pm-'.
1.00
0.78 0.90 1.25
"
f' N-CI
c2v(D3h)
-
1060 n m
530 n m
Figure 2. Correlation diagram for the TBP-SP equilibrium In Ni(Et,dlen)CI,. Chloride loss from TBP and equatorial rearrangement occur simultaneously. An ion pair SP((CTis formed. The u, plane of C2, contains the tridentate.
complexes were described by class A and class B parameters, respectively, as outlined in the previous section. (ii) Parameters for the calculation of state energies were linearly varied along the reaction coordinate. The s-d mixing effect was gradually introduced upon the formation of the SP complex. (iii) In view of the nontransferability of parameters from class A to class B, the model cannot be expected to offer a comparison of thermodynamic stabilities of class A and class B ground states. Experimental AHo values were used to determine the relative position of the energy levels at the left- and right-hand sides of the diagram^.^.'.^^
'Lh
1060 n in
DL h
530 n m
Flgure 3. Correlation diagram for the EOH-SP equilibrium in transN1(2,3,2-tet)(H20),2+.
The figures also show irradiation wavelengths and proposed photochemical pathways. Assignments refer to real symmetries. The reaction pathways follow least-motion coordinates. A tetrahedron is carried over into a square plane by the well-known twist m e c h a n i ~ m . ~Examples ~ are the reactions of Ni(dpp)C12 and Ni(d~p)Br,.~O (28) Creutz, C.; Sutin, N. J. Am. Chem. SOC.1973,95,7177. (29) (a) Whiteaides, T.H.J. Am. Chem. SOC.1969,91,2395. (b) Lob, L. L. Ibid. 1978, 100, 1093. (30) Campbell, L.; McGarvey, J. J. J. Chem. SOC.,Chem. Commun. 1976, 749.
498
The Journal of Physical Chemistfy, Vol. 86,No. 4, 1982
Vanquickenborne et al.
TABLE IV: Observed and Calculated Ligand Field Transitions for truns-Ni(en),(H20),2* and trane-Ni(s-Et,en),(H,O)~+ obsd U
b
calcd
assignmentC
0.98 1.26
0.98 1.26 1.33 1.37 1.59 1.82 2.70 2.90
1.006 1.193 1.307 1.38 1.68 1.87 2.77 2.88
3Eg(3T2g) lAlg(lEg) 'B ,g( IEg) 3B*()TZg) 3A (3T )
1.37 1.84 2.90
parametersd
B = 0.08 C = 0.35 u(H,O) = 0.28 u(H,O)/n(H,O) = 7 u ( N ) = 0.46
n(N)= 0
~EJ~TJ 3Azg("T,,(P)) 3Eg( 'T lg( P 1)
Solid-state spectrum of truns-Ni(en),(H,O),(ClO.,),, ref 23. Polarized crystal spectra at 10 K of trans-Ni(s-Et,en),(H,0)~z+~2Cl~~2H,0, ref 24. Ground state )Big( 3Alg). Dd labels and their Oh parentage. Our assign. ment of the 3B2g(3Tzg).bandis at variance with ref 24. trans-Ni(us-Me,en),(trichloroacetate),,ref 27.
Parameters agree well with those determined for
TABLE V : Relevant Energy Expressions for t h e Orbitals Shown in Figure 4 TD-Ni(dpp)Cl,
TBP-Ni(Et,dien)Cl,
EOHNi( 2,3,2-tet)(H,0),1'
A TBP (Figure 2) looses preferentially an equatorial ligand, concerted with a rearrangement of the remaining two ligands in the equator of the TBP. Ni(Et4dien)C12can be accomodated in this s ~ h e m e . ~ ,The ~ * octahedron is converted into a square plane by simple continued tetragonal elongation. An example is Ni(2,3,2-tet)(H20),2+.31 It should be noted that the SPY SP conversion, observed for Ni(dacoda)(H20),6can also be described by Figure 3, the square pyramid corresponding to the center of the abscissa in the diagram. The high-spin nature of this complex is evident from Figure 3. As a general result one simple electronic mechanism prevails: the lowest singlet state of class B complexes correlates in all cases with the square-planar ground state. We propose that this state is responsible for the very rapid reaction of the class B complexes, resulting in the SP geometry. In the reverse direction, several pathways, either Flgure 4. Orbital schemes of class B compounds. The occupatlon singlet or triplet, are apparently possible. Anyway, the by eight electrons corresponds to the triplet ground state. Dominant lowest excited triplet state of SP always correlates favorOrMtal change in the first triplet-single absorption is Indicated by arrows. ably with the class B ground state. Symmetry labels are consistent with previous figures.
-
Discussion Simple orbital considerations can be helpful in providing an understanding of the results. Figure 4 shows the relevant orbital schemes and occupancies in the class B series. Also shown in Figure 4 are the predominant orbital jumps, corresponding to the first triplet single transition of the high-spin species. In the three cases under consideration, the orbital transitions are "downward". Therefore, the excitation process is characterized by a decrease in the one-electron orbital energies, but by a simultaneous-and more important-increase in interelectronic repulsion energy. Expressions for the corresponding orbital energies are included in Table V. We first discuss the excited singlet state of the high-spin complexes. In TD the spin flip from triplet to singlet produces a vacant b2 orbital, which stabilizes mainly the phosphine ligands, since this orbital is u antibonding along the Ni(11)-P axes, as is evident from its energy expression E(b2(h))in Table V. This orbital correlates directly with the LUMO of the SP complex, as is shown in Figure 5.
-
(31) Ivin, K.J.; Jamison, R.;McGarvey, J. J. J. Am. Chem. SOC. 1972, 94, 1763.
In EOH,the excited lAlg state is a mixture of two configurations:
*('Alg) = c1(b8,e;aTg) - cz(bHge;fb?,)
(2)
A detailed calculation yields c1 = 0.89 and c2 = 0.44, indicating that the blg orbital x2-y2 is essentially (-80%) vacant. Therefore, the axial sites are virtually nonbonding, resulting in facile ligand expulsion. We can analyze the bonding properties of this excited state with the I* methodology, as developed in ref 3a. ground state I(H20) = a(H20) N 0.28 pm-l I(N) = u(N) = 0.47 pm-l lAlg excited state
I*(H20) =
= 0.11 pm-'
I*(N) = (c12+ 0.5)a(N) = 0.61 pm-l (3) Equation 3 illustrates the decrease of axial bond strengths and the increase of amine bonding in going from ground to excited state.
The Journal of Physical Chemistry, Vol. 86, No. 4, 1982 499
Photoreactivity Patterns in Nickel(1I) Compounds
ligand enters the axial positions; for X = CN-, the three PMe3 ligands are found in the equatorial site. From the latter complex, only t r ~ n s - N i ( C N ) ~ ( P Mcan e ~be ) ~ formed, demonstrating the preferential loss of equatorial ligands in a stereoretentive path, in agreement with the mechanism of Figure 2. The same mechanism is also postulated for the thermal substitution reactions of Pt(I1)-SP complexes.343 We now focus our attention on the SP complexes; two photochemical modes can be anticipated from the orbital picture that was proposed in the discussion of Table I (see also ref 9): (i) The x z , yz x2-y2 transition would favor out-of-plane bending modes, leading to a TD complex (cf. Figure 1). (ii) The z2 x2-y2 excitation induces photoassociation along the z axis, forming SPY and EOH (Figure 3). In the presence of a-donor ligands in the SP, the first excited state corresponds to 3(22, yz &y2) and therefore enhances distortions, described in alternative i. a-acceptor ligands or strong u donors could promote the second pathway by lowering the 3(d,zd,2-,,2) excited state. Figure 3 suggests that photoassociation of Ni(2,3,2-tetJ2+would be an example in support of this proposal (530-nm irradiation is mainly absorbed by the hexacoordinated isomer, so that photoreactions of the SP could not be studied). The concerted path SP TBP in Figure 2 should be conceived as a sequence SP SPY TBP. In the first step photoassociation occurs along the z axis. The presence of a weak ligand in the equatorial plane of the SPY probably favors distortion to a TBP structure.
-
-
-- -
c2 C2”lTd)
Figure 5. Orbital correlation diagram for the TD-SP equilibrium in Ni(dpp)CI,, displayed in Figure 1. Orbital descriptions refer to the appropriate coordinate systems for TD and SP as shown. The u, plane of C Z vbisects the P-Ni-P angle.
In TBP, the triplet ground state is approximately characterized by the orbital occupation (e”)4(e’)3(ai)1.In the first singlet state the a; orbital (d,z) becomes vacant, thereby reinforcing the axial u bonds to full class A strength and labilizing the equatorial plane. ground state Z(ax) = u(N) = 0.485 pm-’ Z(Neq)= u(N) = 0.485 pm-l
+
Z(Cleq)= 7/16u(Cl) Y4a(Cl) = 0.211 pm-’ excited state Z*(ax) = 2u(N) = 0.970 pm-l Z*(N,)
= ‘/zu(N)= 0.243 pm-’
Z*(Cl,)
= f/2u(Cl) = 0.189 pm-’
(4)
In eq 4, ax and eq refer to axial and equatorial TBP sites, respectively. The direct correlation of this singlet with the SP ground state is also observed in the thermal equilibria of substituted low-spin NiX2(PMe3)3complexes32(X = Br-, C1-, CN-) TBP-NiX2(PMe3)3 SP-NiX2(PMe3)2+ PMe3 (D3h; 1s) (D4h; 1s) Apparently, alkyl phosphines are very strong ligands (possibly a acceptors) and the TBP structure remains a low-spin complex with class A parameters. The axial preference33for the strongest u donors is clearly demonstrated in this case: for X = I-, Br-, C1-, the phosphine (32) (a) Dawson, J. W.; McLennan, T. J.; Robinson, W.; Merle, A,; Darbguenave,M.; Dartiguenave,Y.; Gray, H. B. J. Am. Chem. SOC.1974, 96,4428. (b) Meier, P. F.; Merbach, A. E.; Dartiguenave, M.; Dartiguenave, Y. Znorg. Chem. 1979,18,610. (33) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365.
Concluding Remarks Several semiqualitative models are available on various features of coordination chemistry of d8 complexes. The approach by B ~ r d e t t and ~ ~ pby~Hoffmann ~ and co-worke r mainly ~ ~ concentrates ~ on problems relating to geometrical variables and site preferences. The context of reactivity patterns points to a different kind of problem. Consider a given SP complex. What type of class compounds, out of the different possibilities that we considered, will actually be formed preferentially? Although ligand field theory cannot compare the ground-state energies of these species in a quantitative way, it probably offers some principles for a rationale: (i) Very strong ligands such as CN- or PMe3 preclude any class B compound formation. In the presence of excess of free ligand, a low-spin TBP (Ni(PMe3)’S)32b or SPY (Ni(CN),%) product can be formed. (ii) Moderately strong ligands such as amines or aryl phosphines allow formation of high-spin complexes. If four a acceptors or u donors are present, a low-lying 3(z2 x2-y2) state could favor formation of five-coordinate or even elongated EOH structures. If a-donor ligands are present, there is a propensity for ~ , x2-y2) state formation of TD structures. A 3 ( ~ yz intervenes. In all of these cases, it has been shown that the induction of a spin change in the excited states can give rise to a spontaneous reaction leading to a ground-state product, characterized by a modified multiplicity.
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Acknowledgment. Financial support from the Belgian Government (Programmatie van het Wetenschapsbeleid) is gratefully acknowledged. A.C. thanks the Belgian National Science Foundation (NFWO) for a senior research assistantship. J.J.McG. thanks the Science Research Council for a grant in support of some of this work. (34) Vanquickenborne, L. G.; Vranckx, J.; Gorller-Walrand, C. J.Am. Chem. Soc. 1974, 96,4121. (35) Burdett, J. K. Znorg. Chem. 1977,16, 3013. (36) Burdett, J. K. h o g . Chem. 1976, 14, 375.
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J. Phys. Chem. 1982, 8 6 , 500-506
Abbreviations daco 1,5-diazacyclooctane as-Me2- asymmetric-N,”-dimethylethylenediamine Et4dien 1,1,7,7-tetraethyldiethylenetriamine en ethylenediamine 2,3,2-tet 1,4,8,11-tetraazaundecane
dpp PPh3 PMea s-Et2en as-
1,3-bis(diphenylphosphino)propane
triphenylphosphine trimethylphosphine symmetric-NB‘cdiethylethylenediamine asymmetric-N,N’-dimethylethylenediamine
Mezen
Metal Dispersions on Zirconium Phosphates. 1. Hydrogen Reduction of Copper-Exchanged a-Zirconium Phosphate7 Abraham Clearfleld,’ Deepak S. lhakur, and Hosea Cheung Department of Chemlstty, Texas A & M Unlverslty. College Station, Texas 77843 (Recetved:April 20, 1981; I n Final Form: September 16, 198 1)
The reduction of Cu(I1) by hydrogen in copper-exchangeda-zirconium phosphate, Z ~ C U ( P O was ~ ) ~ found , to proceed in two stages. Below about 150 torr the product was Z~CUH(PO~)~, which in turn reacted further with hydrogen above Hzpressures of 150 torr to yield copper metal and A-zirconium phosphate. The rates of both reactions were found to conform to an Elovich-type equation and to be strongly pressure dependent. These results were interpreted in terms of the sorption and diffusion of H2 to the metal as the rate-controlling step. Activation energies were low (7.1 and 5.7 kcal/mol) for both stages of the reduction, as expected for a diffusion-controlled reaction. The concept of active-sitegeneration is introduced to account for the observed induction period.
Introduction Metals dispersed on supports represent an important class of catalysts. They are generally prepared by impregnating the support with a salt solution or ion exchanging the required cation onto the surface followed by reduction with hydrogen at elevated temperatures.lP2 The nature of the resultant metal dispersions strongly depends upon the experimental conditions. In many instances, the mechanism of the reduction reaction is in doubt and the nature of the dispersed metal poorly characterized. This is especially true of cation reductions in zeolite^.^ Therefore, the study of a simpler ion-exchanger system, in which facile reduction to metals occurs, might prove to be useful in shedding light on these questions. In this connection, we have chosen to examine hydrogen reduction of certain cations in the ion exchanger, a-zirconium phosphate, Zr(HP04)2.H20,often referred to as a-ZrP. a-Zirconium phosphate has a layered structure with an interlayer distance of 7.55 Cations can be exchanged for monohydrogen phosphate protons, as illustrated for Cu2+in eq 1.6 Copper(I1)-exchangedzirconium phosphate A.475
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Zr(HP04)2.H20+ Cu2+(aq) Z ~ C U ( P O ~ ) ~+ . ~2H+(aq) H ~ O (1) was shown to exhibit high activity for the air oxidation of C07 and the oxidative dehydrogenation of cyclohexene.8 However, in the absence of oxygen, cyclohexene reduced Cu(I1) to Cu(0) and the copper metal formed a reddish brown coating on the surface of the zirconium phosphate. This reduction process was accompanied by a decrease in catalytic activity for oxidative dehydrogenation. No copper remained inside the exchanger, as X-ray powder patterns revealed the presence of only Zr(HP04)z,along with the ‘Presented in part at the 176th National Meeting of the American Chemical Society, Sept 10-15, 1979, Miami Beach, FL.
~ o p p e r . ~The zirconium phosphate phase was A-ZrP, which can also be produced by reaction 2.1° Zr(NaP04)2+ HCl(g) Z I ~ H P O , ) ~ 2NaCl(s) (2) When the freshly reduced solids were allowed to stand in air, reoxidation of the metal on the surface to Cu(II), followed by diffusion of the Cu2+ions back into the exchanger, took place. This was shown by the change in X-ray patterns from that of A-ZrP to ZrCu(PO& Water probably formed at the surface by the action of the displaced protons on the surface oxygen species. This reaction will be reported on separately. La Ginestra et a1.l1 reported that both Cu(0) and Cu(1) can be obtained by hydrogen reduction under different conditions. Similar oxidation-reduction reactions have been detailed for Cu(11)-exchangedzeolites and related supports.12-17 These
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(1)Bond, G. C. “Catalysis by Metals”; Academic Press: New York, 1962. (2) Moss, R. L. ‘Experimental Methods in Catalytic Research”; Anderson, R. B., D a m n , P. T., Eds.; Academic Press, New York; 1976;Vol. 2, p 43. ‘ (3)Jacobs, P. A. “Carboniogenic Activity of Zeolites”;Elsevier: Amsterdam, 1977. (4)Clearfield, A.; Smith, G. D. Inorg. Chem. 1969,8,431. (5)Troup, J. M.; Clearfield, A. Inorg. Chem. 1977,16, 3311. (6)Clearfield, A.; Kalnins, J. M. J.Inorg. Nucl. Chem. 1976,38,849. (7) Kalman, T. J.; Dudukovic, M.; Clearfield, A. Ado. Chem. Ser. 1974, 133,654. (8)Clearfield, A.; West, P. B., unpublished results. (9) Clearfield, A.; Pack, S. P. J. Catal. 1978,51,431. (10)Clearfield, A,; Pack, S. P. J. Inorg. Nucl. Chem. 1975,37,1283. (11)La Ginestra, A.;Ferragina, C.; Massucci, M. A.; Tomassini, N.; Tomlinson, A. A. G. Int. Conf. Thermal Anal., R o c . , 5th, 1977 1977,424. (12)Jacobs, P. A,; Tielen, M.; Linard, J.; Uytterhoeven, J. B.; Beyer, H. J. Chem. SOC.,Faraday Trans. I 1976,72,2793. (13)Herman, R. G.; Lunsford, J. H.; Beyer, H.; Jacobs, P. A.; Uytterhoeven, J. B. J. Phys. Chem. 1975,79,2388. (14)Naccache, C. M.; Ben Taarit, Y. J. Catal. 1971,22,171. (15)Misono, M.; Hall, W. K. J. Phys. Chem. 1973,77,191. (16)Maxwell, J. E.; Drent, E. J. Catal. 1976,41,412. (17)Tester, J.; Storme, D. H.; Herman, R. G.; Klier, K. J.Phys. Chem. 1977,81,333.
0 1982 American Chemical Society
