J. CHEM. SOC. DALTON TRANS. 1994 2809Preparation and Aqueous Solution Properties of theHeterometallic Cuboidal Complex [W,CuS,( H,0),J5+ tMohamed Nasreldin, Carol A. Routledge and A. Geoffrey Sykes"Department of Chemistry, University of Newcastle, Newcastle upon Tyne NEI 7RU, UKThe first heterometallic derivative of the trinuclear W'", incomplete cuboidal cluster [W3S,( H,0),]4+ hasbeen prepared by reaction with ( i ) Cu metal, and (ii) Cu+ or CuCl(so1id). In both cases the product is thecuboidal [W3CuS,( H,0)lo]5+ ion. The corresponding reactions of [Mo3S,( H20)J4+ yield [Mo,Cu-S,( H,0)lo]4+ and [ Mo,CuS,( H20)10]5+ respectively. The observation that the reaction of [W3S,( H20)J4+with copper yields the 5+ and not the 4+ product suggests that the 4+cluster is less stable in the tungstencase.Rate constants (25°C) for the reaction of Cu+ with [W3S4(H20)9]4+ (1.82 x lo3 M-' s-l) and[ Mo,S,( H20)9]4+ (980 M-l s-l) suggest a similar addition process. The 1 : 1 stoichiometries forthe oxidation of [W3CuS,( H,0)lo]5+ with [Co(dipic),] - (dipic = pyridine-2.6-dicarboxylate) and[Fe(H,0),I3+ giving [W3S,(H,0)g]4' and Cu2+ as products, further support the 5+ overall chargeassignment. Rate constants have been determined and mechanisms assigned to these reactions. At 25 "C,/ = 2.00 M (LiCIO,), the [Co(dipic),] - reaction is outer-sphere with a rate constant of 17.7 x 1 O3 M-l s-.',while outer-sphere [Fe(H,0),I3+ (2.68 x lo2 M-l s-') and inner-sphere [Fe(H,O),(OH)IZ+ (5.0 x lo5M-' s-l) reactions are observed for the Fell' oxidant.Rate constants are of the same order of magnitude tothose for the corresponding reactions of [ Mo3CuS,( H,O),o]S+, suggesting that a common electron-transferprocess involving the copper centre may be relevant in both cases.Recent studies on the preparation, structure and reactivity ofheterometallic M o , M S ~ ~ + and related cuboidal complexes,formed by reacting the trinuclear Mo"~ incomplete cuboidalcomplex [Mo,S,(H,O)~]~+ with metals,' or metal ions inthe presence of BH4-,2 equations (1) and (2), have resulted inM o , S ~ ~ + + M - Mo,MS,~+ (1)a rapidly developing area of chalcogenide cluster chemistry.To date clusters with M = Cr,4 Fe,5, C O , ~ Ni,7 Pd,* C U , ~ , ~ 'Hg," In,12.13 Sn', or Sb,15 have been reported. Structuresidentified are of three main types: (a) single cubes (Cr, Fe, Ni,Pd, In or Sn), (b) edge-linked double cubes (Co, Pd or Cu) and(c) corner-shared double cubes (Hg, In, Sn or Sb), Fig.1. Whilein the case of the single cubes early transition metals such asMo and Cr are octahedrally co-~rdinated,~ the later transitionmetals Fe, Ni, Pd and Cu have tetrahedral geometries as shown.Evidence for the existence of a series of Mo3MSe4 hetero-metallic clusters from [ M O ~ S ~ , ( H ~ O ) ~ ] ~ + has also beenpresented.I6 There are, however, no known examples ofheterometallic derivatives from [ M o ~ O ~ ( H , O ) ~ ] ~ + . Moresignificantly no heterometallic derivatives of the now wellcharacterised trinuclear w3S4(H20)9]4+ Group 6 analogue of[Mo3S4(H2O)J4+ have been rep~rted,'~-'~,$ a topic we nowaddress.In the context of reactions (1) and (2) the conversionunder investigation can be represented by equation (3).f Non-SIunit employed: M = mol dm-3.1 Note added at proof: Since this paper was submitted the preparationand characterisation of a W,SnS, cube has been described; A. Miiller,V. P. Fedin, E. Diemann, H. Bogge, E. Krickemeyer, D. Solter, A. M.Guiliani, R. Barbieri and P. Adler, Inorg. Chem., 1994,33, 2243.(4Fig. 1 Structures of Mo,MS, and related cubes where M is aheteroatom: (a) single cubes with tetrahedral and octahedral hetero-atoms respectively, (b) edge-linked double cubes and (c) corner-shareddouble cubesIn only one case, that of [ M O ~ C U S ~ ( H ~ O ) ~ ~ ] ~ + and[Mo,C~S,(H,O),,]~ +, have two different oxidation states ofthe same heterometallic cluster been identified.While th2810 J. CHEM. SOC. DALTON TRANS. 1994toluene-p-sulfonate (pts-) salt of [Mo,CuS,(H,O), 0]4 + hasan edge-linked double cube struct~re,~ solutions from a Dowexcation-exchange column are eluted before [Mo,S4(H20),l4 +indicating the presence of the 4+ single cube structure. In allother studies the corresponding 5+ state is formed as atransient only in the 2:l oxidation of Mo,MS,,+ to[Mo,S,(H~O),]~+ and the heterometal as the 2+ aqua ion,equation (4).Mo,MS,,+ - 2e- --+ M0,S,4+ + M2+ (4)The assignment of oxidation states to individual metal atomsin the heterometallic cubes is not straightforward. For exampleit is not immediately clear from the chemistry observed, whetherin Mo,MS,~+ (M = Ni or Pd), the heteroatom approximatesto the TI or zero state.The oxidation state assigned determinesany adjustments which have to be made to the charge of theMo, part of the cluster.8 Although identical structures areobserved for a number of MV2 and MIv, (M = Mo or W)complexes, the tungsten complexes are generally much morereadily oxidised.20 It is also more difficult to reduce tungsten tothe lower oxidation states. Formation of W3MS4,+ by BH,-reduction of [W,S,(H20),]4+ in the presence of M2+ maytherefore be difficult, and if the existence of W,MS, clustersrequires tungsten to have oxidation states <IV it is under-standable that such cubes may be less common than in themolybdenum case.In the present studies it has been found that a newheterometallic Cu-containing cluster can be prepared from[W3S4(H20),I4+, and we report herein the characterisationand solution properties of this product.ExperimentalPreparation of m3S4(H20)9]4+ .-The purple trinuclearWIV3 cluster [W3S4(H20)9]4f was prepared by a procedureinvolving reduction of [NH,],[WS,] with sodium tetrahydro-borate, NaBH, (Aldrich), in 0.5 M HCl as previously de-scribed.'8,'9,21 The product was purified by Dowex 50W-X2cation-exchange chromatography.After loading onto thecolumn and washing, elution was with 2.0 M HCl, HClO, orHpts as required. Characterisation of w3S4(H,0)9]4+ was byUVjVIS spectrophotometry; a peak was observed at 563 nm( E = 446 M-' cm-' per W,) in 2 M HClO,.Care is required atthis stage to exclude other [W30xS4-,(H20)9]4+ products.Preparation of [W3CuS,(H20)lo]5 +.-Although moststudies were carried out in 2 M HCIO, other acids, 2 M HClor 2 M Hpts, can also be used. In the first method Cu+(aq),prepared by quantitative [Cr(H20),12 + reduction of [cu-(H20),I2+ (see below), was added to [W3S4(H20)9]4+ in 2 Macid. The reaction gives a purple to green colour change, Fig. 2.A modified procedure involving addition of [W,S,(H20),]4+(20 cm3, 2 mM) to insoluble copper(1) chloride (0.1 g, Aldrich)was also used, equation (5).In a second method m3S,(H20)9]4f (20 mM) in 2 M acidwas added to copper turnings (= 2 g, BDH), which had beenactivated by prior immersion in concentrated NN 12 M HC104for 15 min.The reaction was allowed to proceed for = 1 h.Formation of Cu+ by action of the acid on the metal, orH+/H20 oxidation of incipient ~ , C U S , ( H ~ O ) ~ ~ ] ~ ' is pre-sumably occurring.The same 5+ product is obtained on reacting w3S4-(H20),I4+ with BH,- in the presence of Cu2+. Since onaddition of BH,- to Cu2+ the latter is decolourised, theintermediate formation of Cu+ is able to explain the course ofthis reaction. The product is air sensitive and rigorous air-freeI \' ' I . I l\ \ \ \I I I250 450 650VnmFig. 2 The UV/VIS absorption spectra of [W,CUS,(H,O),,]~+(-) and p3S,(H20)9]4f (- - -) in 2.0 M HCIO,(N2) conditions are required throughout. Concentrations of theproduct were determined at 570 nm ( E = 342 M--' cm-' percube).Other Reagents.-Solutions of Cu' (aq) were prepared by(H20)6]2+ was prepared by electrolytic reduction of [Cr-(H20)6]3+ at a mercury-po~lcathode,~~ and the [CU(H,O),]~'solution was made up from copper(I1) perchlorate [Cu(ClO,),-6H20, Aldrich] both in 2.0 M HClO,.Under rigorous air-free conditions >99% formation of Cu+(aq) is observed aspreviously described, equation (6).1 1 [Cr(H20)6]'+ reduction Of [CU(H20)6]2+.22 The [cr-Solutions of N N ~ mM Cu+(aq) are stable to disproportion-ation for only limited periods ( = 60 min). Equivalent amountsof [cr(H20),13 + formed in equation (6), absorption peaksh/nm (E/M-' cm-') at 408 (15.8) 574 (13.3),24 were present inall subsequent experiments.A literature method was used to prepare [NH,][Co(dipic),]-H20, absorption peak at 510 nm ( E = 630 M-' cm-'), dipic2-= pyridine-2,6-dicarboxylate. 2 5 Solutions of hexaaquairon(m),[Fe(H20)6I3 + , were purified by loading Fe(C10,),-6H20(Fluka) onto a Dowex 50W-X2 cation-exchange column andeluting with 1 .O M HClO,.Lithium perchlorate (Aldrich,Reagent Grade) was recrystallised twice from H20. Perchloricacid (70%, BDH AnalaR) was used as supplied.Analysis and Formula of Product.-The W : Cu ratio of a x 1mM solution of theW,CuS, cube in 2.0 M Hpts was determinedby inductively coupled plasma (ICP) emission spectroscopy atLaporte plc, Widnes, and found to be 3.06 : 1. A solid sample ofthe W,CuS, cube was prepared as the pts- salt by first loadingonto a Dowex 50W-X2 column.After washing with 0.50 MHpts (50 cm3) and then 1.0 M Hpts (50 cm'), the cube waseluted with 4.0 M Hpts. After several days at 4 "C under air-freeconditions a microcrystalline green solid was filtered off whichgave the following elemental analyses: Found (Calc.) C, 23.75(23.60); H, 2.65 (3.10%).A W,CUS,~ + cuboidal cluster has been characteriseJ. CHEM. SOC. DALTON TRANS. 1994 281 1previously by X-ray crystallography as the compoundw,CuS,(pO,CMe){ S,P(OEt),) ,I(py)] (py = pyridine).26The copper in this compound has a distorted-tetrahedralgeometry, and is attached to three core p,-sulfido and oneterminal iodo group. The structure is therefore as in the secondexample of Fig. 1 (a). No direct conversion to the corresponding5 + cluster with aqua terminal ligands as in this work has yetbeen achieved.In the case of the W,CUS,~+ aqua cube at concentrations inthe range (0.2-1.8) x lo-, M in 2.0 M HClO,, Beer's law wasfound to be applicable at 384 and 570 nm.Thus no W/VISspectrophotometric evidence was obtained for an equilibriuminvolving dimerisation to give an edge-linked double cube.Because of the increased charge to 5 + a double cube may havegreater difficulty in forming. Even with [Mo,CuS4(H,O), 0]4+the double cube does not appear to be the dominant form inaqueous acidic solutions. 'Stability and UV/ VlS Spectrum-Immediate colour changesare obtained on exposure of [W,CuS,(H,0),o]5+ to air, whenpurple [W3S4(H20)9]4+ is re-formed, Fig. 2. The changesobserved are consistent with equation (7).Since absorption4W3CuS45+ + 0, + 4H'-4W3S4,+ + 4Cu2+ + 2H,O (7)coefficients (E) for ~3S4(H20)g]4+ are known this reactionprovides the means of determining the E values for w3CuS4-(H,0)lo]5+. These were confirmed by addition of w3S,-(H,0)J4+ to a large (> 100-fold) excess of copper turnings.The reaction was allowed to proceed until no furtherabsorbance changes were observed (= 1 h), when the UV/VISspectrum was recorded. Peak positions h/nm (E/M-~ cm-' percluster) are 288 (5500), 384 (1940) and 570 (342). Absorptioncoefficients using the ICP analysis results were in goodagreement with the above values. The UV/VIS spectra of~,CuS,(H,0)lo]5+ and [Mo3C~S,(H,0),o]S~ exhibit simi-lar features, Fig.3.Stoichiometry of Oxidation Processes.-Two reactions with[Co(dipic),] - and [Fe(H20),l3 + were studied in detail. With[Co(dipic),] - as oxidant the stoichiometry was determinedby the addition of aliquots of ~ 3 C ~ S 4 ( H 2 0 ) l o ] 5 ~ to[Co(dipic),] - in 2.0 M HClO, in a spectrophotometer cell andmonitoring the decrease in absorbance at 510 nm ( E = 630 M-'cm.-'). Measurements indicated a 0.96 (k 0.1): 1 stoichiometry(average of three determinations), equation (8).W,CUS,~+ + Co"' + W3S4,+ + Cu" + Co" (8)With [Fe(H,0)J3+ as the oxidant the [Fe(H20)J2+produced was determined by two methods. The first involvedcomplexation of [Fe(H20),l2 + by 4,7-diphenyl-l,IO-phenan-throline (here abbreviated to dpphen, commercially known asbath~phen).,~ The pH of product solutions was adjusted to z 2using 10% ammonium dihydrogen phosphate and glacial aceticacid, followed by the addition of a large ( z 100 fold) excess ofligand.The complex [Fe(dpphen),12 + was extracted withisopentyl acetate, and determined at 535 nm ( E = 17 850 M-'cm-I). Measurements indicated a 1.04 ( f 0.09) : 1 stoichiometry(average of three determinations), equation (9).W,CUS,~+ + Fell1+ W3S44+ + Cu" + Fe" (9)An alternative procedure which involved mixing solutions ofFe"' and [W,CuS,(H,0)lo]5 + in different molar ratios andmonitoring changes in the UV/VIS absorbance spectra gavemeasurements consistent with the above.Kinetic Studies.-All runs were carried out at 25.0 k 0.1 "C,I = 2.00 f 0.01 M (LiClO,), and monitored on a DionexD-1 10 stopped-flow spectrophotometer.The reaction of w,S,-(H20)9]4' with Cu+(aq) was studied at 384 nm, with checkruns at 570 nm, [Cu'] in > 10-fold excess. The oxidations of~,CUS,(H,O),~]~+ with [Co(dipic),]- and [Fe(H,0),13+were monitored at 384 nm with the oxidant in > 10-fold excess.Treatment of Data.-Stopped-flow rate constants wereevaluated using exponential fitting programs supplied byOn-Line Instrument Systems (Bogart, Georgia). An unweightedleast-squares program was used to determine the rate constants(and standard deviations) from linear fits.ResultsAddition of Cu+ to w3S,(H,0)9]4+ .-A uniphasic processwas observed in studies at [H+ J = 1.90 M, I = 2.00 M. First-order rate constants, kobs Table 1, give a linear dependence on[Cu+(aq)], Fig.4, from which the second-order rate constantk,, = (1.82 k 0.02) x lo3 M-' S-' was obtained.Oxidation of ~3CuS,(H20),oJ5 + with [Co(dipic),j-.-First-order rate constants, kobs Table 2, exhibit a lineardependence on [Co(dipic),-1, Fig. 5, and no dependence on[H'] in the range 0.50-1.80 M. The second-order rate constant(25 "C) obtained is k,, = (17.7 k 0.2) x lo3 M-' s-'.Oxidation of [W3CuS4(H20),o]5 + with [Fe(H,O), J 3 + .-This reaction is also uniphasic yielding rate constants kobs,Table 3. A linear dependence on [Fe"'] is observed fromwhich the second-order rate constant kFe is obtained,equation (10).-dm,C~S,~+]/dt = ~ , , ~ , C U S , ~ + ] [ F ~ " ' ] (10)1250 400 550 700 850lilnmFig.3 Comparison of the UV/VIS absorption spectra for w,CuS,-(H,O),,]S+ (-) and [Mo,CUS,(H,O),,]~+ (-- -) in 2.0 M HClO,Table 1 The variation of first-order rate constants, kobs (26 "C), for thereaction of Cu+(aq) with w3S4(H,0)9]4+ (5 x lo-' M) at [Hf] =1.90 M, Z = 2.00 M (LiCIO,)1 O3 [Cu + ]/M0.53 0.850.78 1.401 .oo 1.831 S O 2.752.00 3.6kobs/s-28123 - 5 *r2-J. CHEM. SOC. DALTON TRANS. 1994Table 2 The dependence of first-order rate constants, kobs (25 "C),for the [Co(dipic),] - oxidation of [W,CUS,(H,O),,]~+, (0.50-1.00) x lo4 M, on oxidant concentration and [H'], I = 2.00 M(LiClO,)[H+]/M 1O3[Co"']/M kobs/S-l1.80 0.50 8.11.25 19.32.00 38.01 .oo 0.50 7.51 .oo 16.81.40 24.61.70 30.02.00 36.00.50 0.50 7.01.25 20.62.00 35.0Table 3 The dependence of first-order rate constants, kobs (25 "C),for the [Fe(H,0)613- oxidation of ~,CUS,(H,O),,]~+, (0.50-1.00) x lo4 M, on oxidant concentration and [H+], I = 2.00 M(LiClO,)1 1 /'ov I I0 1 .o 2.01 03[Co(dipic)2J/MFig.5 The dependence of first-order rate constants, kobs (25 "C), forthe [Co(dipic),]- oxidation of [W,CUS~(H~O)~~]~'; [H'] = 1.80 (*),1.00 (0) and 0.50 M (V) HCIO,; I = 2.00 M (LiClO,)[H + ]/M1.90 2.04.06 .O8.010.01.40 2.55.07.510.01 .oo 2.04.06.08.010.00.70 5.07.510.00.50 2.55.07.59.01 O3 [ Fe"']/M kobs/s-l0.991.862.903.94.91 S O3.14.96.31.583.45.16.37.85.07.29.83.36.59.511.10 1 .o 2.01 03[Cu'(aq)YMFig.4 The variation of first-order rate constants, k,, (25 "C), for thereaction of Cu'(aq) with LW3S,(H20),]4+ (5 x M), [H'] = 1.90M, I = 2.00 M (LiClO,)J0 4 8 121 03[ Fe"')/MFig. 6 The dependence of first-order rate constants, kobs (25 "C), for the[Fe(H,0),I3+ oxidation of ~,CUS,(H,O),,]~+; [H'] = 1.90 (a),1.40 (B), 1.00 (A), 0.70 (V) and 0.50 M (*) HCIO,; I = 2.00 M(LiClO,)On varying [H'] within the range 0.50-1.90 M, Fig. 6, k,,shows a dependence on [H+]-', Table 4, equation (11). Thek,, = a + b[H+]-' (1 1)dependence illustrated in Fig. 7 implicates reaction pathsinvolving [Fe(H20)J3 + (a), and [Fe(H20)5(OH)]2 + (b),where a = (2.68 k 0.04) x lo2 M-' s-l and b = (5.0 k0.3) x lo2 s-'. At 25 "C the acid dissociation process (12) is[Fe(H20),I3+ & [Fe(H20),(OH)]2+ + H' (12)known to have K, = 1.0 x lop3 M, I = 2.0 M (NaC10,),28which allows the second-order rate constant b' for the reactioJ.CHEM. SOC. DALTON TRANS. 1994 2813Table 4 Second-order rate constants, kFc (25 "C), for the oxidation of~ 3 C ~ S 4 ( H 2 0 ) 1 0 ] 5 + with [Fe(H20)J3+, Z = 2.00 M (LiC104)[H+]/M k,,/M-' ssl1.90 490 2 201.40 650 2 301 .oo 800 +_ 700.70 980 k 300.50 1260 f 60" ~~ I 0 1 .o 2.0[H + J'/M-Fig. 7 The linear dependence of second-order rate constants, kFe(25 "C), on [H']-' for the [Fe(H,0),13+ oxidation of w3CuS4-(H20)lo]5+ at I = 2.00 M (LiC104)of [Fe(H20)5(OH)]2+ ( =b/K,) to be determined as 5.0 x lo5M-1 s-lDiscussionRecent work including quantitative redox studies, together withUV/VIS and EPR spectroscopic investigations, has confirmedthe existence of two oxidation states for the Mo,CuS, cluster,namely [Mo,CuS,(H,O) 0]4+ and [Mo,CuS,(H20), 0] +.'Both are obtained from [Mo,S,(H~O),]~+, the 4+ cube byreaction with Cu metal, and the 5 + cube by reaction with Cu'.The structural characterisation of w3CuS4(p-O2CMe)( S2-P(OEt),} ,I(py)] has indicated the existence of the cuboidalW,CUS,~ + core with a tetrahedrally co-ordinated coppersubsite. The same 5 + core has been obtained as an aqua ion inthis work by reacting w3S4(H20),I4+ with Cu+(aq) or solidCuC1. Surprisingly, the reaction of w,S,(H20),I4+ with Cumetal also gives ~ , C u S , ( H 2 0 ) l o ] 5 ~ with no evidence for theexistence of ~,CuS,(H20)lo]4+ as a stable entity.Formation of the 5 + product on reacting w,S,(H20)9]4fwith Cu metal suggests an instability of incipient W,CuS,-(H20)10]4+ in the presence of H+ and/or H20, or the presenceof some Cu' which reacts directly with w3S,(H20)9]4f.However, the formation of Cu+ from the interaction of Cumetal with dilute acid in the absence of air is not normallyobserved, and prior activation of the Cu with concentratedacid is expected to remove any surface oxide.The highreactivity/instability of the 4+ product can be understood interms of a quantitative bonding model proposed by Harris.29Thus the metal-based electrons on [W,CuS,(H20)lo]5 + total16, all of which occupy bonding or non-bonding orbitals. Theadditional electron which has to be included for the 4+analogue occupies an anti-bonding orbital, hence there is agreater potential reactivity.Whereas in the molybdenum caseboth the 4+ and 5+ cubes are obtained, with tungsten therelative stability of the 5d level appears to favour immediateconversion of the 4 + ion to the 5 + state.Unlike the aqua ion [Mo,S,(H~O),]~+, the cyano andHnta' - complexes [MO~S,(CN),]~ - and [Mo3S4(Hnta),12 -(H,nta = nitrilotriacetic acid) can be reduced to products inwhich the average molybdenum oxidation state is less thanIV.30-32 In previous studies on the heterometallic clustersMo,MS,,+ it has been suggested that the oxidation states ofthe molybdenum atoms may be less than IV. A number ofcontradictions result when more detailed assignments areattempted. Thus in the case of [MO,CUS~(H~O)~~]~+, X-rayphoto electron spectroscopy (XPS) experiments seem to favoura Cu' assignment,, whereas EPR studies provide evidence foran unpaired electron interacting with the Cu nucleus." Theconflicting evidence here does not help in the assignment ofoxidation states to the molybdenum atoms.The difficulty inreducing trinuclear WIV3 32 indicates a possible preference for aCu' assignment in ~ 3 C ~ S 4 ( H 2 0 ) , o ] 5 +. Similar features inthe UV/VIS spectra of ~ 3 C ~ S 4 ( H 2 0 ) , o ] 5 + and [W,S,-(H20)9]4+, Fig. 2, suggest that both may have a Wlv,component, and a M01V3 assignment may likewise apply in thecase of [ M O ~ C U S ~ ( H ~ O ) ~ ~ ] ~ + , Fig. 3. The implication that the5 + ions may have a Cu' component, while in the [Mo3CuS,-(H20)lo]4+ cube the Cu" component is present with themolybdenum atoms in the (3.33) state, would be aninteresting if somewhat unexpected outcome.There has beenconsiderable discussion of the question of oxidation assign-ments in the case of the Fe4S4,+ core structures.,,Addition of Cu+(aq) to w3S4(H20),I4+ is fast and kineticstudies require the stopped-flow method. A uniphasic reactionexhibiting a [Cu'] dependence is observed. The same step hasbeen identified in the reaction of [Mo,S,(H~O)~]~+. lo SinceCu+ has a high affinity for S2-,,, this is assigned as Cu+attachment to one of the three p-S2 - core ligands. The second-order rate constant in the tungsten case (18.2 x lo3 M-' s-' ) iscomparable to that for molybdenum (980 M-' s-').Once theinitial Cu-S bond is formed subsequent re-orientation andattachment to the other p-S core atoms is presumably rapid. Inthe molybdenum case a second [Cu +]-independent kinetic stepis observed."The conversion of ~,CuS,(H20)lo]5+ to ~,S,(H20),]4+and Cu2 + requires the release of one electron, as has been con-firmed in the stoichiometry determinations with [Co(dipic),] -and [Fe(H20)6I3 + as oxidants, equation (1 3). This providesW,CuS,'+ - e- ---+ W3S4,+ + Cu2+ (13)additional evidence that the W,CuS, cube is indeed 5 + . Asingle rate-determining step, first-order in each reactant, isobserved with both oxidants. The [Co(dipic),] - reaction isassigned as outer-sphere, with inner-sphere bridging difficult toenvisage. There is moreover no [H'] dependence, in keepingwith previous studies on this ~ x i d a n t .~ > ~ ' , ~ ~ On the otherhand the oxidation with Fe"' exhibits an [H+]-' dependence,equation (1 4). Values of a and b are 268 M-' s-' and 500 s-l, andthe ratio a : b is consistent with outer-sphere and inner-spheremechanisms respectively. Thus if the acid dissociation constantfor [Fe(H20)6I3', K, = 1.0 x lop3 M-' at 25 OC, I = 2.0 M(NaC10,),28 is taken into consideration, the second-order rateconstant b' = 5.0 x lo5 M-' s-' exhibits a lo3 enhancementfactor over a, which is as expected for an OH- inner-spherebridged activated complex.37 Since the copper is likely to belabile, it is not clear how the inner-sphere activated complexassembles, and whether this is by substitution of Fe(0H) intothe Cu or of a Cu ligand into the Fe co-ordination sphere.Inthe latter case the Fe(0H) conjugate base will further labilisethe Fe"'-co-ordinated H20 ligands2814 J. CHEM. SOC. DALTON TRANS. 1994Rate constants a and b for the reaction with [Fe(H20)6]3'are not too different from the values 720 M-' s-' and 2380 s-'respectively for the corresponding reaction of [Mo,CuS,-(H20)10]5+. Again the numerical similarity of these rateconstants for a and b respectively suggests that the oxidation-state assignments Cu' and MIv, (M = W or Mo) may beappropriate.AcknowledgementsWe are grateful to SERC and Laborte Industries for a CASEStudentship (to C. A. R.).References1 T.Shibahara, Adv. Inorg. Chem., 1991,37, 143.2 P. W. Dimmock, G. J. Lamprecht and A. G. Sykes, J. Chem. SOC.,3 R. H. Holm, Adv. Inorg. Chem., 1992,38, 1.4 C. A. Routledge, M. Humanes, Y.-J. Li and A. G. Sykes, J. Chem.5 T. Shibahara, H. Akashi and H. Kuroya, J. Am. Chem. Soc., 1986,6 T. Shibahara, H. Akashi, M. Yamasaki and K. Hashimoto, Chem.7 T. Shibahara, M. Yamasaki, H. Akashi and T. Katayama, Inorg.8 T. Murata, H. Gao, Y. Mizobe, F. Nakano, S. Motomura, T. Tanse,9 T. Shibahara, H. Akashi and H. Kuroya, J, Am. Chem. SOC., 1988,10 M. Nasreldin, Y.-J. Li, F. E. Mabbs and A. G. Sykes, Inorg. Chem.,11 T. Shibahara, H. Akashi, M. Yamasaki and K. Hashimoto, Chem.12 G. Sakane, Y.-G. Yao and T. Shibahara, Inorg. Chim. Acta, 1994,13 T. Shibahara and G.Sakane, Inorg. Chem., 1993,32,777.14 H. Akashi and T. Shibahara, Inorg. Chem., 1989,28,2906.Dalton Trans., 1991,955.SOC., Dalton Trans., 1994, 1275.108, 1342.Lett., 1991, 689.Chem., 1991,30,2693.S. Yano and M. Hidai, J. Am. Chem. Soc., 1992,114,8287.110,3314.in the press.Lett., 1991, 689.216, 13.15 T. Shibahara, K. Hashimoto and G. Sakane, J. Inorg. Biochem., 1991,43, 280.16 M. Nasreldin, G. Henke, G. Kampman, B. Krebs, G. J. Lamprecht,C. A. Routledge and A. G. Sykes, J. Chem. SOC., Dalton Trans., 1993,737.17 T. Shibahara, A. Takeuchi, A. Ohtsuji, K. Kohda and H. Kuroya,Inorg. Chim. Acta, 1987,127, 145.18 C. A. Routledge and A. G. Sykes, J. Chem. Soc., Dalton Trans., 1992,325.19 M. Nasreldin, A. Olatunji, P. W. Dimmock and A. G. Sykes, J. Chem.SOC., Dalton Trans., 1990, 1765.20 B.-L. Ooi, A. L. Petrouand A. G. Sykes, Inorg. Chem., 1988,27,3626.21 T. Shibahara, H. Kohda, A. Ohtsuji, K. Yasuda and H. Kuroya,22 K. Shaw and J. H. Espenson, Inorg. Chem., 1968,8,1619.23 B.-L. Ooi, C. Sharpand A. G. Sykes, J. Am. Chem. SOC., 1989,111,24 E. Deutsch and H. Taube, Inorg. Chem., 1968,7, 1532.25 A. G. Mauk, C. L. Coyle, E. Bordignon and H. B. Gray,J. Am. Chem.26 H. Zhan, Y. Zheng, X. Wuand J. Lu, Inorg. Chim. Acta, 1989,156,277.27 L. J. Clark, Anal. Chem., 1962,34,348.28 C. F. Baes and R. E. Mesmer, in Hydrolysis of Cations, Wiley,29 S. Harris, Polyhedron, 1989,8,2843.30 T. Shibahara and H. Kuroya, Polyhedron, 1986,5, 357.31 A. Muller, R. Jostes, W. Eltner, C.-S. Nie, E. Diemann, H. Bogge,M. Zimmerman, M. Dartmann, U. Reinsch-Vogell, S. Che,S. J. Cyvin and B. N. Cyvin, Inorg. Chem., 1985,24,2872.32 T. Shibahara, M. Yamasaki, T. Watase and A. Ichimura, Inorg.Chem., 1994,33,292.33 See, for example, L. Noodleman and D. A. Case, Adv. Inorg. Chem.,1992,38,460.34 S. Ahrland, J. Chatt and N. R. Davies, Quart. Rev., 1958, 12, 265.35 P. W. Dimmock, D. P. E. Dickson and A. G. Sykes, Inorg. Chem.,36 P. W. Dimmock, D. M. Saysell and A. G. Sykes, Inorg. Chim. Acta,37 N. Sutin, Acc. Chem. Res., 1968, 1,225.J. Am. Chem. Soc., 1986,108,2757.125.SOC., 1979, 101, 5054.New York, 1976, p. 230.1990,29,5 120.1994, in the press.Received 26th April 1994; Paper 4/02459