7431 (13) S. R. Langhoff. E. R. Davidson, and C. W. Kern, J. Chem. Phys., 63,4800 (1975). (14) K. Lonsdale, H. J. Milledge, and K. V. K. Rao, Proc. R. SOC.London, Ser. A, 255, 82 (1960). (15) The x-ray observations are, of course, for the ground state but we expect a similar arrangement of axes in T2211 as the structure is still comparatively rigid. The 6" deviation mentioned in ref 4 will only have a 1-2% effect. See also H. Hope, J. Bernstein, and K. N. Trueblood, Acta Crystallogf.,Sect 6,26, 1733 (1972). (16) Address correspondence to this author at the Chemical Research Center, Allied Chemical, Morristown, N.J. 07964.
E. Wasserman,*I6 R. S. Hutton Bell Laboratories Murray Hill, New Jersey 07974
Figure 1. An ORTEP drawing of (CH3NC)6((C6H5)3P}2Pdj2+showing the 50% thermal ellipsoids.
onance a t T 6.71 and a phenyl resonance a t 2.45, with a 3:5 intensity ratioe7 In order to obtain an accurate and detailed description of Brooklyn College the structure of this unprecedented species, an x-ray structural Brooklyn, New York I 1 21 0 investigation was undertaken. W e choose to work with 2 beReceived November 24, I975 cause of its greater stability. Air-stable, red-yellow dichroic crystals of 2 were obtained by slow diffusion in a nitrogen atmosphere of diethyl ether into an acetone solution of the The Preparation and Structure of the Linear complex containing excess methyl isocyanide and triphenylTripalladium Cations (CH3NC)8Pd32+and phosphine. Crystal data (85 K): space group P i ; reduced cell a = 12.094 (2), b = 12.127 (2), c = 10.660 (2) 8,; CY = 102.70 (CH3NC)6((C5H5)3PJ2Pd32+ (2), /3 = 112.92 (2), y = 75.49 (2)'; V = 1380.9 A3; pexptl298 Sir: = 1.60, pcalcdg5 = 1.66 g/cm3 for Z = 1; ~ M , , K=:~ 1 1.7 cm- I Reactions between metal ions in different formal oxidation Intensity data were collected a t 85 K on a Picker automated states can be used to form metal-metal bonds. Two examples four-circle diffractometer. Using M o KCYradiation, a total of using reaction partners with d8 and d6 electronic configurations 3568 reflections were collected by a 20-s o scan of the most are shown in reactions 1' and 2.2 intense part of the peak a t a scan speed of 0.25' min-l in the range 3.5' < 2%< 45'. A background curve was measured as Rh(CNC6Hi 114' a function of 2%. The structure was solved and refined by RhI2(CNC6Hi 1)4+ F! R ~ ~ I ~ ( C N ,C) g 2~+ H I(1) Patterson, Fourier, and least-squares methods to a final R index8 of 0.063 using 3339 reflections having I,,, > 2u(Inet). 17Pt(CN)42- 3PtC12(CN)42- 40K' The positions of the phenyl hydrogens were easily located from 6 0 H 2 0 ---* ~ K ~ o P ~ I o ( C N )* ~30H20 O C I ~ (2) a final difference Fourier while the methyl hydrogens were less well-resolved. In the first case a dimeric, formally Rh(I1) species, is formed The structure of the cation is illustrated in Figure 1. The while in the latter case the solid, known as Krogmann's salt, has a more complex but still well-defined stoichiometry and three palladium atoms are collinear with the two phosphorus atoms of triphenylphosphine groups, forming? five atom chain contains parallel, linear columns of platinum atoms which ideally extend for the full length of a crystal. Equations 3-5 (since the point group for the molecule is 1, the Pd-Pd-Pd angle is constrained to 180', the Pd(l)-Pd(2)-P angle is 173.7 (t-C4H9NC)lPdI2 (0.1)'). Each palladium atom is coordinated to two methyl (t-&HgNC)2Pd ( c - C ~ H ~ N C ) ~(3) P ~ ~ Iisocyanides ~ and exhibits approximate square planar geometry. The square planes are twisted away from each other such that (CH3NC)4Pd2+ (CH3NC),Pd the angle between the two five-atom least-squares planes is (CH3NC)6Pd22+ (4) 74.5'. The most striking aspects of the structure are the short Pd-Pd bond at 2.5921 (5) 8, and the displacement of the (CH3NC)4Pt 2+ (CH3NC), Pd equatorial isocyanide towards the center of the molecule. The -P (CH3NC)6PdPt2+ ( 5 ) palladium-palladium bond in (CH3NC)6Pd2"+, which is the give examples of reactions between complexes with d8 and d10 shortest reported palladium-palladium bond, is just slightly electronic configurations that also produce metal-metal shorter at 2.5310 (9) bond^.^-^ We now report a new aspect of this chemistry which The displacement of the equatorial isocyanides towards the has led to the preparation and structure of a linear tripalladium center was also observed in (CH3NC)6Pd22+ (av Pd-Pd-C complex. angle = 85.0 (9)') but occurs to a greater extent in 2 (av In contrast to reactions 1 and 2 where a single, unique Pd-Pd-C = 80.0 (2)"). Although the steric bulk of the axial product forms regardless of the reactant stoichiometry, varitriphenylphosphine groups could contribute to this large disation of the reaction stoichiometry in the case of reaction 4 placement in 2, it is probably not the sole cause, since normal alters the product. Addition of 2 mol of (CH3NC),Pd6 to nonbonded distances between the isocyanides and the phenyl (CH3NC)4Pd2+ or addition of 1 mol of (CH3NC),Pd to groups are found. The acute Pd-Pd-C angle brings the triply (CH3NC)6Pd22+ in acetone solution produces (CH3bonded carbon atom of the equatorial isocyanide to within van NC)8Pdj2+, 1 which has been isolated as the crystalline hexder Waals distance of the adjacent palladium atom.I0 For afluorophosphate salt. The infrared spectrum of this solid inexample, the Pd(1)- -C(2) distance is 2.921 A. By contrast, dicates that only terminal isocyanide ligands are present (YCN the bonded Pd(2)-C(2) distance is 1.989 (8) A. We therefore 2221,2213,2202 cm-I). The 'H N M R spectrum of this ion feel it likely that the inward bend of the equatorial isocyanides in acetonitriled3 consists of a single resonance a t 7 6.59.7 is in part an electronic effect arising from interaction between Addition of triphenylphosphine to 1 produces the disubstitution filled d orbitals on palladium with empty A* orbitals on the product [(CH~NC)~((C~H~)~P~ZP~~](PF~)~, 2. Like 1, 2 isocyanide ligands of the adjacent metal. An analogous effect contains only terminal isocyanide ligands (VCN 22 16, 21 86 for carbonyl ligands has been termed "semibridging".' I In the cm-I). Its 'HNMR spectrum consists of a single methyl reselectron-precise metal carbonyl dimers the M-M-C angles
F. B. Bramwell
I
+
+
+
+
+
+
-
+
+
Communications to the Editor
7432 range from 86.2 to 88.8O,I2 and the possibility of such an effect Tris(2,2'-bipyridine)chromium(II), Cr(bpy)j3+, appears has been noted13 previously. Considerably smaller angles are to be an ideal candidate for investigations on bimolecular exfound in the 16-electron (CH3NC)6Pd22+ and in 2 as well as cited-state processes. The lowest excited state of this complex, K4[Ni2(CN)6]14 and [i-Pr4N]2[Pt2Cl4(CO)2] l 5 which have Le., the metal-centered doublet state, (2MC)Cr(bpy)33+,is angles of 7 6 O and 82.7', respectively. From these values we remarkably long lived in fluid solutions (T = 53 /AS in aqueous conclude that semibridging is more important in unsaturated deaerated solution a t 25 "C) and can be conveniently monimetal-metal bonded species than in their electron-precise tored by means of the moderately efficient emission centered counterparts. at 727 nm." The complex is appreciably photostable in acidic Further investigations of these compounds are in progress. solutions.I2 A recent communication by Bolletta et has Attempts to make longer metal center chains by reactions reshown that the doublet state of C r ( ~ b y ) 3 ~can + be efficiently lated to eq 3-5 are in progress. Complexes 1 and 2 are coorquenched by Ru(bpy)32+. Since the energies of (*MC) dinatively unsaturated and preliminary investigations indicate Cr(bpy)j3+ and (3CT)Ru(bpy)32+ are 13 800 and 17 100 that they are more reactive than the surprisingly r o b ~ s t ~ ,cm-I, ~ ~ respectively, energy transfer is forbidden in this system. (CH3NC)6Pd22+. For example, recrystallization of 1 and 2 can Thus, the assumption was made that the quenching takes place only be accomplished in the presence of excess ligands. via the thermodynamically allowed electron transfer from the quencher to the excited state of C r ( b ~ y ) 3 ~Here + . ~ ~we present Acknowledgment, This work has been supported by the the results of some flash photolysis experiments which provide National Science Foundation. direct evidence for the occurrence of electron transfer in the quenching of the C r ( b ~ y ) 3 ~doublet + state. References and Notes Flash photolysis14 of aqueous p H 3 solutions of C r ( b ~ y ) 3 ~ + (1)A. L. Balch and M. M. Olmstead, J. Am. Chem. SOC.,98,2354 (1976). gives rise to a transient absorption with maxima a t 390 and 445 (2)For reviews dealing with Krogmann's salt see H. Krogmann, Angew. Chem., lnt. Ed. fngl., 8,35 (1969),and J. S.Miller and A. J. Epstein, Prog. lnorg. nm.The absorption decays by first-order kinetics with a lifeChem., 20, l(1976). time of 47 p s , a value which compares well with the lifetime (3)S. Otsuka, Y. Tatsuno, and K. Ataka, J. Am. Chem. SOC., 93, 6705 of the phosphorescent emission. The transient absorption can (1971). (4)J. R. Boehm, D. J. Doonan, and A. L. Balch, J. Am. Chem. Soc., 96,4845 be assigned to a transition from the (2MC)Cr(bpy)33+state (1976). to upper doublet states.I2 (5)M. F. Rettig. E. A. Kirk, and P. M. Maitlis, J. Ofganomet Chem., Ill, 113 (1976). The Fe(H*0)62+ ion is an efficient quencher of the (6)Pd(0) isocyanide complexes frequently occur as polymers of low solubility: C r ( b ~ y ) 3 ~phosphorescence + (k4 = 4.1 X lo7 M-I s-l at /.t = L. Malatesta, J. Chem. Soc., 3924 (1955):E. 0. Fischer and H. Werner, 1). When p H 3 solutions containing 5 X M Cr(b~y)3~+ Chem. Ber., 95, 703 (1962).Treatment of tris(dibenzy1ideneacetone) dipalladium (T Ukai, H. Kawazura, Y. Ishii, J. J. Bonnet, and J. A. Ibers, J. and 2 X M FeS04 are flashed, the expected quenching Ofganomet. Chem., 65,253(1974)) with methyl isocyanide in acetone and of the doublet absorption is observed and a new transient abother solvents produces stable yellow solutions which from their chemical sorption is formed with maxima a t 470 and 560 nm. The reactivity appear to contain a Pd(0) complex here referred to as Pd(CNCH3),. spectrum of the new transient absorption matches closely the (7)The lack of resolution of the 'H NMR methyl resonance between different reported spectrum of C r ( b p ~ ) ~ ~The + . formation ]~ of the remethyl isocyanide environments is not surprising. The dimer (CH3NC)6Pd2*+ undergoes both rapid exchange with free methyl isocyanide and an induced complex upon quenching of the doublet state is a clear traionic fluxional process which exchanges axial and equatorial isocyanide proof of the occurrence of a mechanism involving electron ligands. With 1 and 2 the lack of resolution may be due to either insufficient transfer from the quencher to the excited complex (reaction chemical shift differences, a fluxional process, or exchange with adventitious traces of free isocyanide. Attempts to rule out the latter through the 1). addition of solid palladium chloride to scavenge free isocyanide have led only to SamDle decomposition. Z:IIFoI IFcI/ZIFol. (9)S. Z.Goldberg and R. Eisenberg, lnorg. Chem., 15,535 (1976). (IO) Van der Waals radii are estimated from values of L. Pauling," The Nature of the Chemical Bond", Cornell University Press, Ithaca, N.Y., 1960:-C=, 1.4A: - N S . 1.35A: the covalent radius for Pd is 1.38A. and the van der Waals radius is perhaps 0.5A longer. (11)F. A. Cotton, Prog. lnorg. Chem., 21, i(1976). (12)M. F. Bailey and L. F. Dahi, lnorg. Chem., 4, 1140 (1965);L. 8. Handy, J. K. Ruff, and L. F. Dahl, J. Am. Chem. SOC.,92,7312(1970);G.L. Simon, A . W. Adamson, and L. F. Dahl. J. Am. Chem. SOC.,94,7654(1972);F. A . Cotton, T. G. Dunne, and J. S. Wood, lnorg. Chem., 3, 1495 (1964). (13)R. A. Levenson and H. B. Gray, J. Am. Chem. SOC.,97,6042 (1975). (14)0.Jarchow, 2.Kristallogr., Kristallgeom.,Kristallphys., Kristallchem., 136, (8)
-
122 (1973). (15) A . Modinos and P. Woodward, J. Chem. SOC., Dalton Trans., 1516 (1975). (16)J . R. Boehm and A. L. Balch, J. Organomet. Chem., 112 C20 (1976).
Alan L. Balch,* John R. Boehm Hakon Hope, Marilyn M. Olmstead
Department of Chemistry, University of California Davis, California 9561 6 Received July 12, 1976
Bimolecular Electron Transfer Processes of Electronically Excited Tris(2,2'-bipyridine)chromium(III) Sir:
Recent reports on the facile electron transfer reactions of the lowest excited state of tris(2,2'-bipyridine)ruthenium(II), (3CT)R~(bpy)32+,1-10 have focused attention on the general problem of the redox properties of excited states of transition metal complexes. Journal of the American Chemical Society
/ 98:23 /
+
-
(2MC)Cr(bpy)33+ Fe(Hz0)62+ Cr(bpy)3*+
+ Fe(H20)63+
(1)
The C r ( b ~ y ) 3 ~absorption + disappears after the flash by fast second-orderI6 kinetics ( k z = 7.3 X lo8 M-I s-l a t /A = 0.2) due to the thermal back-electron-transfer reaction (reaction 2), which involves a free energy change of -23.5 kcal/mol. Cr(bpy)3*+
-
+ Fe(Hz0)63+
C r ( b ~ y ) 3 ~4- +Fe(H20)62+ (2)
A similar approach has been used with R ~ ( b p y ) 3 ~as+ a quencher of the C r ( b ~ y ) 3 ~doublet + state. When solutions containing 1 X M C r ( b ~ y ) 3 ~and + 3.3 X M Ru(bpy)3*+ (pH 3, /A = 0.2) are flashed, the characteristic transient absorption of C r ( b ~ y ) ~ is * +again observed. In order to interpret this result, however, the peculiar quenching behavior of the system is to be taken into account. Bolletta et al. have shown13 that Ru(bpy)3*+ is a good quencher of the C r ( b ~ y ) 3 ~2 M + C excited state (kq = 4.0 X IO8 M-' s-I a t CC = 0.2), but that a t the same time C r ( b ~ y ) 3 ~is+an efficient quencher of the 3CT excited state of R ~ ( b p y ) 3 ~(kq + = 3.3 X I O 9 M-' s - I a t /A = 0.2). The solutions used in the flash experiment are such that both components absorb a significant fraction of the exciting light and each of the complexes significantly quenches the excited state of the other one. Thus, the observed C r ( b ~ y ) 3 ~formation + could arise by several pathways, namely, reaction 3 followed by reaction 5 , reaction 4 followed by reaction 6, or reaction 4 followed by reaction 7 and reaction 5.
November 10, 1976
