Inorg. Chem. 1980, 19, 1079-1080
1079
be generally difficult to predict accurate values of ASo,, for could well experience an increase in polarization in going to mixed-ligand couples containing aquo ligands. the Ru(I1) state since the added tpelectron will be significantly Since a large number of complexes employed in redox kidelocalized around the aromatic rings, acting to increase their netics contain anionic as well as neutral ligands, it is of interest net charge density. This latter effect would yield a negative to examine the stepwise changes in ASo,, that occur as anions contribution to ASo,, which will be roughly proportional to are substituted into the coordination sphere. Substantial and the number of pyridine rings. Therefore the stepwise reeven qualitative changes in ASo, are generally expected from placement of ammine by bipyridine or pyridine ligands would the Born model as well as from local solvation effects due to result in an approximately linear decrease in ASo,,, as obthe variations in the overall charges of the complexes. Unserved. For R ~ ( b p y ) , ~ + / ~these + , two effects presumably fortunately, suitable data are extremely sparse. Table I cancel, yielding ASo,,= 0. The small (or slightly negative) contains data for three Fe(III/II) couples containing bipyridine values of ASo, observed for Ru(III/II), Fe(III/II), and and/or cyanide ligands. Again it is seen that the estimated Os(III/II) polypyridine couples have been previously ascribed values AS',)"^ for the mixed-ligand couple Fe(CN)4bpy-/2to the efficient shielding of the metal center from the solvent is in reasonable agreement with the experimental value of by these ligands.1° This latter explanation seems less reaASo,,,whereas the Born model fails to provide adequate essonable since the polypyridine ligands will allow some solvent timates of ASo,,for all three couples. molecules to approach close to the metal center along the open It therefore appears that the assumption that the reaction channels formed by the planar aromatic rings. Also, the entropies for mixed-ligand couples arise from simple linear markedly larger values of ASo (22 eu) observed for both additive contributions from each ligand can provide a useful, C ~ ( b p y ) ~ ~and + / ~Co(phen),’:f2+ + can be more easily unalbeit approximate, means of estimating ASo, under some derstood on the basis of the present model. These Co(II1) circumstances. However, often one or both ASo,,values for Co(I1) reactions involve the electronic conversion t22 t2,5e,2 the corresponding pure ligand couples are unavailable. Inwhich should minimize the extent of electron delocalization asmuch as the values of ASo,,are often expected to depend in the reduced state and therefore discourage any increase in chiefly on the ligands and the charge type of the redox couple? solvent polarization in the vicinity of the pyridine rings, This the required reaction entropies could be inferred from those change in electronic configuration will also yield a marked for other couples containing the same ligands. George et al.4 expansion of the cobalt center13 and hence an especially large have given empirical parameters for various ligands obtained decrease in the polarization of nearby water molecule^.^ from ionic entropy data that allow estimates of ASo,,to be We have obtained further evidence that favors the present obtained for couples containing these ligands. We have pointed interpretation from the observation that the ferrocinium/ out that this approach has limitations since in some cases AS’, ferrocene couple has a distinctly negative value of ASo, in can be significantly dependent on the nature as well as the aqeous media (-5 eu, Table I). Since this couple is of the charges of the central metal ion^.^*^ Nevertheless, the method charge type +1/0, the simple dielectric polarization model could be usefully employed, albeit with caution, to estimate predicts a small positive value of ASo,. Although the added the changes in AS’,, resulting from minor alterations in ligand electron will be substantially delocalized around the cyclocomposition. pentadienyl rings in a manner similar to the polypyridine couples, the “sandwich” structure of the ferrocinium/ferrocene Acknowledgment. We are grateful to Dr. Gilbert Brown couple should largely prevent the close approach of solvent and Professor Larry Bennett for supplying some of the commolecules to the metal center. Consequently in this case the plexes used in this study. Professor Tom Meyer provided predominant contribution to ASo,,is anticipated to be the valuable information on the photochemical properties of Ruincreased polarization of water molecules adjacent to the (OH2)2(bpy)2+.E.L.Y. was partially supported by a summer aromatic rings in the lower oxidation state, in accordance with fellowship from funds made available by the General Electric the observed negative value of ils’,,. Negative values of Mor, Corp. This work is supported by the Air Force Office of have also been observed for some blue copper protein couples Scientific Research. having the charge type +1/0 and have been interpreted in Registry No. Ru, 7440-18-8; Fe, 7439-89-6; bpy, 366-18-7; phen, terms of hydrophobic effects in the vicinity of the copper redox 66-71-7; py, 110-86-1. center. l 4 The large values of ASo, typically observed for aquo couples have been attributed to field-assisted hydrogen bonding between the aquo ligands and surrounding water molecules.2 As noted previously,2 substitution of a single aquo ligand into R u ( N H ~ ) ~ ~ yields + / ~ +a proportionately larger increase in Contribution from the IBM Thomas J. Watson Research Center, ASo,,than those which result from subsequent aquo substiYorktown Heights, New York 10598 tutions (Table I). However, there is reasonable agreement between ASo,,and (ASorc)estd for C ~ ~ - R U ( N H ~ ) . , ( O H ~ ) ~ ’ +Formation /~+ of Extrinsic (Pt3+-Doped) Magnus’ Green Salt (Table I). On the other hand, substitution of one bipyridine in the Platinum(I1)-Catalyzed Substitution Reactions of in R ~ ( b p y ) ~ ~by + /two ~ + aquo ligands in either a cis or trans Platinum(1V) Complexes config~ration’~ yields somewhat smaller values of ASo, compared with (ASorc)estd (Table I), It is possible that the extent Geraldine M. Summa? and Bruce A. Scott* of hydrogen bonding involving aquo ligands is very sensitive Received August 27, I979 to the electrostatic and steric environment. It therefore may
--
H. C. Stynes and J. A. Ibers, Inorg. Chem., 10, 2304 (1971). N. Sailasuta, F. C. Anson and H. B. Gray, J . Am. Chem. Soc., 101,455 (1979). The “reaction entropies” tabulated in ref 14 are actually values of ILS’H~ so that 20.5 eu should be added to correct them to the “absolute scale’’ used in the present paper? B. Durham, S. R. Wilson, D. J. Hodgson, and T. J. Meyer, J . Am. Chem. SOC.,102, 600 (1980). Although the pK, for R U ( O H ~ ) ~ (is~unavailable, ~ ~ ) ~ ~ +for the closely similar complex Ru(bpy)2(py)OH2+,pK, = 0.85 ( p = 1) [B. A. Moyer and T. J. Meyer, J. Am. Chem. SOC.,100, 3601 (1978)l.
0020-1669/80/1319-1079$01.00/0
Magnus’ green salt (MGS), Pt(NH3),PtC14, has long been of interest as a quasi-one-dimensional semiconductor.1 ~ 2 This is because the alternate cation and anion stacking in the crystal structure3 permits strong overlap of the Pt(I1) dzz orbitals and t Marymount College, Tarrytown, N. Y . ,undergraduate research participant 1978-1979.
0 1980 American Chemical Society
1080 Inorganic Chemistry, Vol. 19, No. 4, 1980
Notes
-
heat hence highly anisotropic properties. The semiconducting Pt(NH3)4C12 K2PtCl6 properties of this salt, however, have been found to be impurity [Pt(NH3)2C12][Pt(NH,),Cl,] + 2KC1 (3) d~minated.~,’By selective doping, we recently showed that the extrinsic behavior can be attributed to Pt(II1)-like centers The mixed-valence Pt”/PtIv product is a red-brown solid (red introduced into the Pt(I1) chains by Pt(1V) impurities in the crystals) with orthorhombic lattice constants a = 7.79, b = nominally “pure” solutions used to prepare Magnus’ ~ a l t . ~ , ~1.31, and c = 10.54 A, based on complete X-ray powder The Pt(II1)-like “hole” states are charge-compensated by inpattern indexing. It is thus likely isomorphic with the analterstitial halide ions which bind them with an energy Eb = 0.46 ogous Pd”/PdIv and Pd”/PtIv salts.1° The mixed-valence eV,8 or about twice the activation energy for electrical concompound is composed of two neutral complexes which can d ~ c t i v i t y .On ~ ~the ~ basis of the observed Pt195superhyperfine also be viewed as forming from the initial reactants by ligand structure in the electron spin resonance spectrum, it has been exchange rather than redox reaction. Nevertheless, the actual surmised that the hole extends over several Pt sites but does course involves a redox reaction and the formation of MGS* not interact with the ligand^.^^^ as an intermediate. The presence of Pt(1V) complexes during the preparation Confirmation of the extrinsic nature of the Magnus salt in of Pt(NH3),PtC14 leads to complex redox processes which (1) was obtained from EPR measurements, which showed a apparently become “trapped” in the solid ~ t a t e . ~In. ~our powder spectrum with gll = 1.96 and g , = 2.50. These paoriginal experiments, K2Pt(CN)4Br2, K2Pt(CN)4C12, and rameters are identical with those obtained in detailed powder K2PtC16were used as oxidizing agents to introduce the imand single-crystal studies.6 Although it is difficult to establish purity states into MGS powders. In those studies’ aqueous a precise value, we estimate a maximum Pt(II1)-doping level Pt(NH3)&12 was permitted to react with K2PtC14solutions of 0.5 mol % in our samples from the EPR signal intensity. containing Pt(1V) anionic oxidizing agents. This approach Reaction 2a is clearly an example of a Pt(I1)-catalyzed was chosen because PtCld2-reacts very slowly compared to Pt(1V) substitution reaction.’.” Mechanistically, we can write Pt(NH3)42+with anionic Pt(1V). We have now investigated the redox doping reaction in greater detail and have isolated Pt(NH3)4’+ C1[Pt(NH3)4--Cl]+ both the intermediate and final reaction products. In particular, we have examined the Pt(NH3)42+/PtCl,” couple more [Cl-PtC14-C1l2- + [Pt(NH3)4...Cl]+ e fully and have identified the reaction leading to the incorpo[Cl-PtCl4.*.Cl-Pt(NH3)4-C1]- e ration of Pt(II1) hole states in MGS. C1- PtC142- Pt(NH3)4C122+(4) The rapid addition, with stirring, of 0.01 M aqueous solutions of Pt(NH3)4C12and K2PtC16resulted in the immediate Reaction 4 involves a two-electron transfer from Pt(1I) to precipitation of Magnus’ green salt. The salt was filtered and Pt(1V) with the transinsertion of two C1- ligands into the the solution evaporated to dryness to crystallize the remaining coordination sphere of the tetraammineplatinum. Although products. Analysis of the products and stoichiometry indicated this step is rapid, it is not likely to be faster than the precipthat the overall reaction for equimolar concentrations of itation reaction of Pt(NH3)2+with PtC14’-, for MGS* would reactants can be written not form otherwise. Hence (2a) and (2b) compete for the tetraammineplatinurn( 11) ion, resulting in the incorporation 2Pt(NH3)4C12 2K2PtC16 --* of “trapped intermediates” in MGS*. As previously proposed, MGS* [Pt(NH3)4C12]C12 + KzPtC16 + 2KC1 (1) these are Pt3+ centers with associated charge-compensating halide ions occupying interstitial sites.@ Trapped Pt4+centers where the Magnus salt product is denoted MGS* to indicate are unstable relative to two Pt3+, so that the holes must be its extrinsic (Pt3+-doped) nature. We explicitly show in (1) mostly singly ~ h a r g e d . ~ . ~ that only half the K2PtC16is consumed because MGS* must The very slow rate of reaction 3 at rmm temperature makes be formed in the reaction of the tetrachloroplatinate(I1) reit highly unlikely that ligand exchange can provide a facile duction product with tetraammineplatinum(I1): route for the incorporation of charge-compensatinghalide ions into MGS*. This is consistent with our model based upon EPR Pt(NH3):+ PtC16’Pt(NH3)4C122++ PtC1:(2a) experiments6 and Coulomb energy arguments8 which rule against charge compensators occupying platinum coordination Pt(NH3)42+ PtC142- MGS* (2b) sites (e.g., Pt(NH3)3C12+). Finally, since PtC1,Z- is generated in the redox reaction, its For simplicity, the reactions illustrated in eq 2 are considered presence is unnecessary in the preparation of MGS*, unless separately, but in fact if (2a) and (2b) were to occur conother Pt(1V) complexes are used as dopants (Le., PtC12- serves secutively, as subsequently discussed, MGS would not be exas both an oxidizing agent and a PtC1:This trinsic. suggests the possibility of producing even more unusual mixOn warming of the reaction mixture ( l ) , MGS* is found ed-valence chain salts by combining other Pt(II)/Pt(IV) to rapidly dissolve, and as shown in (3), evaporation of the couples with favorable redox potentials. resulting solution leads to the isolation of only two products:
+
+
+
+
+ +
+
-
+
-
J. R. Miller, J. Chem. SOC.,713 (1965). L. Atkinson, P. Day, and R. J. P. Williams, Nature (London), 218, 668 (1968); P. S. Gomm, T. W. Thomas, and A. E. Underhill, J . Chem. Soc., 2154 (1971).
M. Atoji, J. W. Richardson, and R. E. Rundle, J . Am. Chem. Soc., 79, 3017 (1957).
L. V. Interrante, J . Chem. SOC.,Chem. Commun., 6, 302 (1972). E. Fishman and L. V. Interrante, Inorg. Chem., 11, 1722 (1972). F. Mehran and B. A. Scott, Phys. Reu. Lett., 31, 99 (1973). B. A. Scott, F. Mehran, B. D. Silverman, and M. A. Ratner, ACS Symp. Ser., No.5 , 331-49 (1974). B. D. Silverman and B. A. Scott, J. Chem. Phys., 63, 518 (1975). F. Mehran and B. A. Scott, Physica B + C (Amsterdam), 83, 179 (1976).
Acknowledgment. We wish to thank Dr. F. Mehran for providing the EPR results. Chemical analyses were carried out in these laboratories by B. L. Olson and B. J. Gilbert. G.M.S. also wishes to thank Dr. Phyllis Didado for her advice and guidance. Registry No. Pt(NH3)4C12, 13933-32-9; KZPtCl,, 16921-30-5; MGS, 13820-46-7; [Pt(NH3)&12] [Pt(NH3)ZC14], 72845-24-0.
(10) J. WallCn, C. Brosset, and N.-G. Vannerberg, Ark. Kemi, 18, 541 (1962). ( 1 1 ) W. R. Mason, Coord. Chem. Reu., 7 , 241 (1972).
Inorg. Chem. 1980,19, 1081-1082
1081
Contribution from the Organisch-Chemisches Institut der Technischen Hochschule Darmstadt, D-6100 Darmstadt, West Germany, the Organisch-Chemisches Institut der Universitiit Heidelberg, D-6900 Heidelberg, West Germany, and the Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21 21 8
Photoelectron Spectra of Biferrocenylene and Biferrocene Michael C. Bohm,'* Rolf Gleiter,*Ib Federico Delgado-Pena,Ic and Dwaine 0. CowaniC
Received October 19, I979
Biferrocenylene (l), biferrocene (2), and the various charged
I
I
7 k
1
H I
e
2
ions derived from 1 and 2 have found increasing interest in recent years from both experimental2 and theoretical3 points of view. The experimental data available indicate a difference between 1 and 2 insofar as in '1 there is a considerable interaction between the two iron sites while in 2+ both Fe atoms have integral oxidation states leading to a formal FeI1,Fe1I1 system with trapped valences? In order to contribute to the discussion concerning the bonding in 1 and 2,we have recorded the He I photoelectron (PE) spectra of 1 and 2. This measurement provides direct insight into the various hole states of 1+ and 2+.5 Due to large relaxation effects in transition-metal compounds6 (breakdown of Koopmans' theorem'), we performed calculations on the various hole states of 1+ and 2' by using the ASCF8 and the TOM9 procedures'O on the basis of a (1) (a) Organisch-Chemisches Institut der Technischen Hochschule
Darmstadt. (b) Organisch-Chemisches Institut der Universitat Heidelberg. (c) The Johns Hopkins University. (2) D. 0. Cowan, C. LeVanda, J. Park, and F, Kaufman, Acc. Chem. Res., 6, 1 (1973); G. LeVanda, K. Bechgaard, D. 0. Cowan, and M. D. Rausch, J . Am. Chem. Soc., 99, 2964 (1977); C. LeVanda, K. Bechgaard, and D. 0. Cowan, J . Org. Chem., 41, 2700 (1976); C. LeVanda, K. Bechgaard, D. 0. Cowan, u. T. Mueller-Westerhoff, P. Eilbracht, G. A. Candela, and R. L. Collins, J. Am. Chem. Soc., 98, 3181 (1976). (3) R. F. Kirchner, G. H.Loew, and U. T. Mueller-Westerhoff, Inorg. Chem., 15, 2665 (1976). (4) N. S.Hush, Prog. Inorg. Chem., 8, 391 (1967); G. C. Allen and N. S. Hush, ibid., 8, 357 (1967). (5) M. Bbhm, part of dissertation at Technische Hochschule Darmstadt. (6) M.-M. Coutitre, J. Demuynck, and A. Veillard, Theor. Chim.Acta, 27, 281 (1972); P. S.Bagus, U. I. Walgren, and J. Almlbf, J. Chem. Phys., 64,2324 (1976); M.-M. Rohmer and A. Veillard, J. Chem. Soc., Chem. Commun., 250 (1973); M.-M. Rohmer, J. Demuynck, and A. Veillard, Theor. Chim. Acta, 36,93 (1974); J. A. Connor, L. M. R. Derrick, M. (7) (8) (9)
(10)
B. Hall, I. H.Hillier, M. F. Guest, R. B. Higginson, and D. R. Lloyd, Mol. Phys., 28, 1193 (1974). T. Koopmans, Physica (Utrecht), 1, 104 (1934). P. S.Bagus, Phys. Reo. [Sect.] A, 139, 619 (1965). 0. Goscinski, B. T. Pickup, and G. Purvis, Chem. Phys. Lett., 22, 167 (1973); 0. Goscinski, M.Hehenberger, B. Roos, and P. Siegbahn, ibid., 33,427.(1975); M. Hehenberger, ibid., 46, 117 (1977); D. Firsht and B. T. Pickup, ibid., 56, 295 (1978). Both methods of calculation reproduce satisfactorily the PE bands of ferrocene.I2 Experimental: 6.88 (eo), 7.23 (al ), 8.72 (elu),9.38 (el& ASCF: 7.85 (aI,), 8.48 (ezg).9.27 (eiu), 10.41 (e,,). TOM: 6.27 (aig), 7.08 (e$, 8.88 (elu), 10.06 (el&
0020-1669/8U/1319-1081$01.00/0
I.P. (eV) Figure 1. PE spectra of 1 and 2.
recently developed INDO method for organometallic compound~.~ In Figure 1 the PE spectra of 1 and 2 are displayed." By comparison with the PE spectrum of ferrocene12it is evident that the first band in both spectra corresponds to ionization events from the six M O s with mainly 3d character (see Table I), mainly the g and u combinations of 3d,z, 3dX~-y~ and 3d, orbitals. Inspection of Table I shows the anticipated considerable electronic relaxation in the radical cation. The experimental values are only reproduced satisfactorily if the ASCF and TOM calculations were performed without the restriction that the Fe 3d M O s should be symmetry adapted. Both model calculations lead to localized Fe 3d hole states, meaning that the electron has been removed from one of the two iron sites of the molecule. Similar results have been found for the Is hole states of 02+ l 3 and N2+.I4 In all cases it is computed that the relaxation energy of a localized hole contributes twice as much as that of a delocalized one.l5 The
(11) The He I PE spectra were recorded on a PS18 spectrometer of Perkin-Elmer. Both samples were heated to a temperature of 140 OC. The spectra were calibrated with argon, and a resolution of about 25 meV (12)
on the argon line was obtained. S.Evans, A. F. Orchard, and D. W. Turner, In?. J. Mass. Spectrom. IonPhys., 7,261 (1971); J. W. Rabalais, L. 0. Werme, T. Bergmark, L. Karlson, M. Hussain, and K. Siegbahn, J . Chem. Phys., 57, 1185
(1972). (13) P. S.Bagus, and H. F. Schaefer 111, J. Chem. Phys., 56, 224 (1972). (14) R. L. Lozes, 0.Goscinski, and U. I. Wahlgren, Chem. Phys. Lett., 63, 77 (1979).
0 1980 American C h e m i c a l Society
