J. CHEM. SOC. DALTON TRANS. 1991 1385A Bridge-bond Isomerism. X-Ray Crystal Structure, Spectraland Magnetic Properties of Two Dinuclear Isomeric Complexes[Cu( bipyo)Cl,] (bipyo = 2,2'-bipyridine N,"-dioxide) tPeter Baran/ Marian Koman,a DuSan Valigura *AI and Jerzy Mrozinskiba Department of Inorganic Chemistry, Slovak Technical University, 8 I237 Bratislava, CzechoslovakiaInstitute of Chemistry, University of Wroclaw, 50383 Wroclaw, PolandCopper(i1) chloride reacts with 2,2'-bipyridine N,N'-dioxide (bipyo), in methanolic solution, in amanner depending on the reaction conditions (temperature, ligand : metal ion ratio, concentration, etc.) toform different products. Two of them, a green and a yellow-orange isomer of [Cu,(bipyo),CI,], have beencharacterized by elemental microanalysis, electronic, I R and ESR spectra and magnetic susceptibilitymeasurements. The crystal and molecular structures of both isomers have been determined fromX-ray diffractometer data by the heavy-atom method and refined by full-matrix least-squares.Thestructures of the green and yellow-orange isomers, respectively, have been refined to R = 0.054 (for1366 reflections) and 0.063 (for 1283 reflections): green isomer, triclinic, space group P j , a =8.664(2), b = 8.732(2), c = 9.099(3) A, u = 95.62(2), p = 107.04(2), y = 114.05(2)", and Z = 1;yellow-orange isomer, monoclinic, space group P2,/c, a = 7.912(2), b = 9.878(2), c = 15.086(3) A,p = 99.86(2)" and 2 = 2. Both crystal structures comprise centrosymmetric dimeric [Cu,(bipyo),CI,]molecules. The copper(ii) atoms of the green isomer are bridged by two chlorine atoms, those of theorange-yellow isomer by the oxygen atoms of two bipyo ligands.Considerable attention has been given to the donor propertiesof aromatic N-oxides.' Two basic classes of copper(I1) com-plexes have been prepared and characterized: (a) those havingthe N-oxide ligands as bridges, which usually result in lowmagnetic moments; and (b) those containing non-bridging N-oxide ligands, which exhibit normal magnetic moments.2,2'-Bipyridine N,N-dioxide (bipyo) has a further possibility as achelating ligand. However, despite this, significantly less workhas been done in the preparation and structural characterizationof the copper complexes with bipy~.~-'As a part of our interest in chelating ligands which stabilizethe copper(I1) oxidation state, we have studied the bipyocomplexes of copper(I1) salts.Two different isomeric complexesof composition Cu(bipyo)Cl, have been obtained. Since thegreen and the yellow-orange isomers differ significantly in theirmagnetic and other properties we decided to solve theirstructures. The results of the X-ray structure determinationtogether with their syntheses, spectroscopic characterizationand magnetic properties are presented in this paper. A briefpreliminary report of the preparation and some properties hasappeared.6Results and DiscussionStructures of [Cu,(bipyo),Cl,].-The crystal structures ofboth isomers consist of dimeric molecules [Cu,(bipyo),Cl,] 1held together by van der Waals interactions and weak hydrogenbonds.Only half of the dimeric molecules are crystallo-graphically unique in both isomers. The final fractionalpositional coordinates of the non-hydrogen atoms of bothisomers are listed in Table 1.The structure of the green isomer of 1 and the atomt Supplementary data available: see Instructions for Authors, J. Chem.SOC., Dalton Trans., 1991, Issue 1, pp. xviii-xxii.Non-SI units employed: Oe = lo3 A m-', G = 10-4 T.Fig. 1of CCu, ( ~ ~ P Y o ~ C L 1Molecular structure and atomic numbering of the green isomernumbering are shown in Fig. 1 while the relevant interatomicdistances and bond angles are presented in Table 2. The dimericmolecule is centrosymmetric with the symmetry centre betweenatoms Cu and Cu'.Two chlorine atoms [Cl(l) and C1(2)] andtwo oxygen atoms [O(l) and 0(2)] of the bipyo ligand arebonded to the copper atom in the independent part of 1 thusforming the distorted square base of a nearly square-pyramidalco-ordination. The co-ordination is completed by the chlorineatom Cl(2') of the symmetrically dependent part of 1 thusforming two square-pyramidal polyhedrons connected via thecommon body edge. The Cu-Cl and Cu-0 bond distances[2.265( l), 2.278( 1) and 1.964(3), 2.014(3) A, respectively] arewithin the range typical for the basal plane of a square pyramid,while the apical Cl(2') atom is at a distance typical for thisposition [2.626(1) A]. The copper atom is displaced from thebasal plane of the donor atoms by 0.270(1) 8, and the apicalCl(2') atom is slightly moved (0.31 A) from the regular axia1386 J.CHEM. SOC. DALTON TRANS. 1991Table 1 Final positional parameters ( x lo4) with estimated standard deviations (e.s.d.s) in parentheses(a) Green isomer (b) Yellow-orange isomerAtom Xla Ylb Zlc Xla Ylb2206( 1)2246( 1)487(1)3882(4)3254(4)5618(4)2377(6)2764(7)4359(8)5567(6)5186(5)6 3 69 (6)8 19 1 (6)9261(6)8470(6)6606(6)3593(4)1466( 1)3088(1)3567(3)293(3)3922(4)- 1126(1)7W4)- 202(6)167(7)1488(7)2 52 7 (6)2 158(5)3253(5)3745(6)4898(6)5473(6)4954(5)1055(1)3 178( 1)1444(1)665(3)- 88(3)1313(4)- 1335(4)- 2767(5)- 4 1 29( 5)- 3944(6)- 2450(5)- 1142(5)470(5)1088(6)2568(6)3440(5)2783(5)658(1)3076(2)- 1014(2)- 1303(4)- 23 16(5)- 3835(6)- 4920(7)- 4429(8)- 2862(6)- 1797(7)- 186(6)-21 l(7)1349(4)1326(5)1275(8)2796(8)279 1 (6)1307( 1)2469( 1)2783( 1)- 6(3)- 102(3)- 353(4)- 1409(4)- 97(6)- 1076(6)- 1710(6)- 1356(5)- 2083(5)- 3485(5)- 41 54($)- 3425(6)- 2057(6)265(5)Z l c4450( 1)4824(1)3603(1)4480(2)3467(2)3 703 (2)3672(3)3479(4)2710(4)2167(4)2412(4)3188(3)3534(3)3671(4)4002(4)4178(4)4009(4)Table 2parenthesesInteratomic distances (A) and angles (”) with e.s.d.s in(a) Green isomer2.265( 1)2.278( 1)1.964(3)2.014( 3)2.626( 1)3.409( 1)1.3 3 1 (4)1.3 15(5)1.332(5)1.367(6)1.367(5)1.341(5)1.340(9)1.385(7)1.3 59( 7)1.470(7)1.368(7)1.389(6)1.369(7)1.399(6)1.409(7)(b) Yellow-orange isomer2.219( 1)2.028(3)2.173(4)2.061(3)3.333( 1)1.349(6)1.328( 6)1.339(8)1.366(8)1.3 54(7)1.346(8)1.369(9)1.367(10)1.382(9)1.368(9)1.479(8)1.400( 8)1.367(9)1.388( 10)1.367(9)2.221( 1)C1( l)-Cu-C1(2)C1( l)-cu-O( 1)C1( 1 )-Cu-0(2)C1( l)-cu-C1(2’)C1(2)-CU-O( 1)C1(2)-CU-O(2)C1(2)-CU-C1(2’)O( l)-Cu-0(2)O( 1 )-Cu-C1(2’)0(2)-Cu-C1(2’)cu-O( 1 )-CU’0(2)-N(2)-C(1)0(2)-N(2)-C(5)O(1 )-N( 1 )-CWO( 1 FN(1 FC(10)Cu-O(l)-N( 1)CU-0 (2)-N( 2)C1( l)-Cu-C1(2)Cl(1)-cu-O( 1)Cl( l)-Cu-0(2)Cl( 1)-cu-O( 1 ’)C1(2)-CU-O( 1)C1(2)-CU-O(2)C1(2)-Cu-O( 1’)O( l)-Cu-0(2)O( 1)-cu-O( 1’)0(2)-cu-O( 1’)CU-C1(2)-CU’0(1)-N(1)-C(1)0(1)-N(ltC(5)0(2)-N(2)-C(6)0(2)-N(2)-C(10)Cu-O( 1)-N( 1)C~-0(2)-N(2)95.1( 1)90.2( 1)154.1( 1)107.9( 1)172.6( 1)89.1 (1)92.2( 1)83.9(1)90.9( 1)97.4(1)1 13.6(2)119.9(2)109.2( 1)11 8.0(3)120.1( 3)119.8(3)119.0(3)102.2(1)1623 1)101.5( 1)92.1(1)92.9(1)102.7( 1)154.7(1)83.6(1)70.8( 1)89.1( 1)87.8(1)119.1(1)116.6(3)118.6(5)119.2(5)119.3(5)120.3(5)position [see also Table 2 for the angles including the Cu-Cl(2’)bond].nWFig. 2orange isomer of [Cu2(bipyo),Cl4]Molecular structure and atomic numbering of the yellow-The structure of the yellow-orange isomer of compound 1 andthe numbering scheme are presented in Fig.2. Selected bonddistances and angles are shown in Table 2.The centrosymmetricmolecule has the symmetry centre between the Cu and Cu’atoms. Besides two chlorine atoms [Cl(l) and C1(2)] and twooxygen atoms of the bipyo molecule [O(l) and 0(2)] of theindependent part of 1, the co-ordination polyhedron of the Cuatom is completed by the bridging atom O(1’) of a centro-symmetrically related bipyo molecule. The C1( l), C1(2), O( 1)and O(1’) atoms form a rather distorted square base of a square-pyramidal polyhedron; thus two polyhedrons share a basaledge. The basal Cu-C1( l), Cu-C1(2), Cu-O( 1) and Cu-O( 1’)distances are 2.219(1), 2.221(1), 2.028(3) and 2.061(3) A,respectively. The apical Cu-0(2) distance [2.173(4) A] isgreater than the other two Cu-0 distances, but the differencesare unexpectedly small.The Cu atom is dis laced above thebasal plane of the donor atoms by 0.244(1) x, and the apicaJ. CHEM. SOC. DALTON TRANS. 1991 1387O(2) atom is shifted from its regular position by 0.527 A[see also Table 2 for the angles involving the Cu-0(2)bond].According to the data the square pyramid of the Cu atom inthe yellow-orange isomer seems to be more distorted than in thegreen isomer. In spite of this the parameter proposed fordistinguishing between tetragonal-pyramidal and trigonal-bipyramidal co-ordination geometries has a smaller value (z =13.0%) for the yellow-orange than for the green isomer (z =30.8%). However, in both cases the values are closer to thevalue for a pure tetragonal-pyramidal geometry (z = 0%).Thus the greater distortion of the yellow-orange isomer cannotbe explained by the deformation leading to the trigonal-bipyramidal polyhedron, rather as a consequence of thebridging mode of the bipyo ligand.The other difference between the two isomers is in theirchromophores, CuCl30, for the green isomer and CuCl,03 forthe yellow-orange isomer, arising from the different bridgingligands.This difference raises the question of the classification ofthe isomerism. Generally speaking these isomers differ one fromanother in the bonding mode of the ligands and from this pointof view could be classified as bonding isomers. However, there isa significant difference between this pair of isomers and others;bond isomerism is known for ligands able to bond via differentatoms.In this case the same atoms of the ligands (two oxygenatoms of bipyo and two C1 atoms per copper atom) are used forbond formation, but the modes of bonding are different. Themain difference between these two isomers is in the atomswhich form the bridges between two copper atoms (Cl in thegreen isomer and 0 in the yellow-orange one), while othercharacteristics (co-ordination polyhedron, chelation, mode ofbipyo, etc.) are the same. Thus the term bridge-bond isomerismis suggested.Moreover, the present structures and properties can becompared with those of copper complexes with other aromaticN-oxides. A few pairs of isomers 8-11 show the stoichiometry[Cu(N-oxide),Cl,] (N-oxide = pyridine N-oxide, 4-methyl-, 3-methyl-, 2,6-dimethyl- or 2,4,6-trimethyl-pyridine N-oxide) butthe structures have been ~ o l v e d ~ ~ , ' ~ only for the 4-methyl-pyridine N-oxide pair.The green isomer is monomeric with anearly planar co-ordination polyhedron and the yellow isomerhas a dimeric structure [(CuL,Cl,},] with bridging N-oxide (L)ligands and this complex is partly suitable for comparison withthe yellow-orange isomer of 1. The structures of some dimericcomplexes of general formula [{Cu(pyo)LCl,},] [pyo =pyridine N-oxide; L = pyo,l4 H,0,15-17 or dimethyl sulphox-ide (dmso) 18] are more suitable for structural comparisonbecause they contain pyo bridging ligands and the apicalpositions are occupied by oxygen-donor ligands. It is interestingthat the Cu-O(apica1) distance [2.187(1) A] for L = pyo l4 isvery close to that found in the yellow-orange isomer of 1 [seeTable 2 for the Cu-0(2) distance].The unexpectedly shortCu-O(apica1) distances in both these complexes seem to becaused by the nature of the ligands because for the otheroxygen-donor apical ligands this distance is slightly longer[2.336(3), 2.342(4), 2.266(4) and 2.279(6) A for L = H2O,''H20,16 H2017 and dmso,18 respectively]. All other structuralparameters (e.g. copper-basal plane distance, mutual orienta-tion of pyridine N-oxide ring and basal plane, etc.) are very closeto each other for all complexes compared.No structure of an aromatic N-oxide-copper(I1) halidecomplex is known which could be compared with the dimericstructure of the green isomer of 1.There are known 19,20 onlysome polymeric complexes of stoichiometry [(Cu(pyo)Cl,},] or[(Cu3L,C16(H,0),},] (L = 2-methylpyridine N-oxide) whichcontain alternate chloride and N-oxide bridges. The copper(I1)atoms in those complexes show a wider variety of co-ordinationpolyhedrons.Magnetic Susceptibility and ESR Spectra of [Cu,(bipyo),-Cl,].-Variable-temperature magnetic susceptibility measure-Table 3 ESR spectroscopic splitting factors' of the green isomer ofCCU,(biPY o)Zc1414.2 2.04, 2.07, 2.321 2.14," No monomeric form of the complex was observed. gaV2 = (gx2 +gyz + gz2)/3.Table 4 Magnetic data" for the green isomer of [Cu2(bipyo),C14]T/K4.2102550100150200250290106xCllb/cm3 mol-I59 40029 30015 0008 2004 1702 8002 11017101 480CLefl'1.411.531.731.811.831.831.841.851.85a Data selected from 66 experimental points.xcu = Mxc0"/2, whereM = molecular weight; x(S1) = (47r x lo6) x(cgs). peff = 2.83(xcu7)+.Table 5 Magnetic parameters of the green isomer of [Cu2(bipyo),C14]Curie constant, Weiss constant,T/K Clem3 K mol-' 0/K4.2-25 0.413 - 3.850-290 0.433 - 3.84.2-290 0.433 -4.1Table 6 Magnetic data for the yellow-orange isomer of [Cu,(bipyo),-C14ITIK17418319520521422223224225 126 127 128 1298106xcu/cm3 mol-141124303646687899108133147181Peff0.070.130.190.220.250.290.350.390.450.470.540.570.66ments and ESR spectra of complex 1 revealed great differencesin the magnetic properties of the two isomers.The ESR spectrum of the green isomer was measured at threetemperatures, e:g.room, 77 K and liquid-helium temperature.The spectrum shows a rhombic signal with two features in theperpendicular region (gy = 2.07 and g , = 2.04) and one in theparallel region (g, = 2.32). It does not change with temperature(Table 3). The spectrum is consistent with the square-pyramidalco-ordination sphere of the copper(r1) magnetic centre, and thesingle unpaired electron is located in an essentially d,~-~2orbital. In the region between the signal components cor-responding to g , and gy an incomplete copper hyperfine patternhas been observed (five lines only). The average interlinespacing was ca.44 Oe. This pattern is a clear indication of thepresence of a very weak magnetic interaction between thecopper centres in the dimeric unit.The magnetic properties of the green isomer over th1388 J. CHEM. SOC. DALTON TRANS. 1991I- I100 200T /KFig. 3 Variation of the reciprocal molar susceptibility and xcuT withtemperature and variation of XgH with H for the green isomer ofCCU,(biPY 0)2C1,1temperature range 4.2-290 K are given in Table 4 and plotted inFig. 3, while the Curie and Weiss constants are listed in Table 5.The course of the xcuT us. T plot was in accord with the ESRspectra, namely the observed antiferromagnetic interactions arevery weak. At higher temperatures the xcUT product waspractically constant (about 0.425 cm3 K mol-') and decreasedonly at the lowest temperatures (below 30 K), to 0.249 cm3 Kmol-' at 4.2 K.In such situations the parameter J can bedetermined only by means of the magnetization equation.,' Theapplication of the Bleaney-Bowers susceptibility expressionwould be inappropriate becau_se J z gpH. The Hamiltonian isgiven by equation (1) where S is the total spin operator (9 =,!?A + &) and J expresses the intramolecular exchange inter-action between spins. The magnetic susceptibility xCu isexpressed by equation (2) and is corrected, in the molecular fieldapproximation, for the presence of magnetic interaction withneighbouring dimers 2 2 [equation (3)] where z is the number ofnearest-neighbour dimers and J' is the lattice interactionparameter.All other symbols have their usual meanings. Thebest fit to the data yielded J = - 0.8 cm-', zJ' = - 2.9 cm-' andthe agreement factor R = 3.39 x lW3.The ESR spectrum of the yellow-orange isomer of compound1 at room temperature gives a broad single line of 6Hpp x 500Oe for v = 9.385 GHz. The spectroscopic splitting factor for thecentre of the line was 2.1 1. The sample was free from monomericimpurities.The magnetic susceptibility data for this isomer are presentedin Table 6. The low values of the magnetic susceptibility andof the magnetic moment at temperatures close to roomtemperature (Table 6) suggest strong antiferromagneticcoupling of the magnetic centres in the complex. This wasconfirmed by the drastic decrease in both values at decreasingtemperature, to zero below 170 K.The magnetic susceptibilityof two interacting copper(I1) ions can be calculated using theisotropic Heisenberg-Dirac-Van Vleck model 23 resulting inequation (4) where 2Jis the singlet-triplet energy gap defined bythe phenomenological Hamiltonian fi = -2J3, * s',, wkere Jexpresses the intramolecular exchange interaction and S , and3, are quantum spin operators. The least-squares best fit of thedata yielded 2J = 730 cm-', with R = 3.34 x lC9.Infrared Spectra of[Cu2(bipyo),C1,].-The Nujol (mull) andKBr (pellet) IR spectra of compound 1 in the region 4W1700cm-l are rather complicated, due to the presence of thebipyridine moiety, with many absorptions most of which are notsensitive to the co-ordination mode.Two absorptions at 1220and 1204 cm-' for the yellow-orange isomer and at 1218 and1196 cm-' for the green isomer could be assigned to the N-0stretching vibrations and owing to co-ordination they aresignificantly shifted from their positions for the free ligand 24(1260 and 1253 cm-'). Smaller shifts were found for theabsorptions assignable to the N-0 bending vibrations (844 and833 cm-' for the yellow-orange isomer, 846 and 828 cm-' for thegreen isomer, 849 and 837 cm-' for the free ligand24).Absorptions at 409 cm-' for the yellow-orange isomer and 401cm-l for the green isomer may be assigned to the Cu-0vibrations and they are close to those ones found for the otherN-oxide complexes (e.g. 388 cm-' for [ { C ~ ( p y o ) ~ C l ~ } ~ ] , ~ ~ 425cm-1 for [{C~(pyo)Cl,(H,0))~],~~ and 41 1 cm-' for [{Cu(pyo)-Cl,(dmso)) '1' 5).Absorptions assignable to the Cu-Clvibrations were found at 321 and 310 cm-' for the yellow-orangeisomer and at 362,285,280 and 249 cm-' for the green isomer.The complex [{Cu(pyo),Cl,},] exhibits those bands at 316 and297 cm-', [{Cu(pyo)C1,(H20)},] at 314 cm-' and [{Cu(pyo)-Cl,(dmso)},] 25 at 313 and 274Electronic Spectra of [Cu,(bipy o) ,C14] .-The reflectancespectra of the solid complex in the region 200-2500 nm showsd-d bands (broad band at 870 nm with unresolved shoulder onthe low-energy side for the yellow-orange isomer, and broadband at 815 nm with significant shoulder at lo00 nm for thegreen isomer), charge-transfer bands (at 430 nm with shouldersat 410 and 392 nm for the yellow-orange complex, and twoshoulders at 460 and 378 nm for the green isomer) and bandsassignable to intraligand transitions or charge-transfer bandsin the UV region (at 332 and 280 nm for the yellow-orangeisomer, and at 348 and 280 nm for the green isomer).In thisregion the free ligand exhibits bands at 348 and 258 nm with ashoulder at 268 nm.ExperimentalChemicals.-2,2'-Bipyridine and copper(r1) chloride dihydratewere of analytical grade (Lachema). Other chemicals were ofreagent grade (Lachema) and all were used as received. Allsolvents were purified and/or dried by standard methods.Preparations.-2,2'-Bipyridine N,N'-dioxide was preparedby 2,2'-bipyridine oxidation with hydrogen peroxide in glacialacetic acid.' A mixture of 2,2'-bipyridine (10 g) and hydrogenperoxide (13 cm3) in glacial acetic acid (75 cm3) was heated for3 h at 80°C.Then another portion of hydrogen peroxide (9cm3) was added and the mixture heated for 19 h at 80°C.The mixture was then cooled and acetone (1 dm3) added tocrystallize the product. The crude product was recrystallizedfrom water by adding acetone. Yield ca. 75% (Found: C, 64.60;H, 4.25; N, 14.60. Calc. for CloH,N202: C, 63.85; H, 4.30; N,14.90%).[Cu,(bipyo),Cl,]. Green isomer. A solution of CuCl,~2H2O(10 mmol, 1.70 g) in dry methanol (5 cm3) was added to a hotsolution of bipyo (10 mmol, 1.88 g) in dry methanol (95 cm3).The clear green solution was allowed to cool and to crystallize.The green product was filtered off, washed with small portionJ.CHEM. SOC. DALTON TRANS. 1991 1389of hot dry methanol and dried in vacuo. Yield ca. 80% (FoundCu2N404: C, 37.25; H, 2.50; Cu, 19.70; N, 8.70%). Crystalssuitable for X-ray analysis were obtained by slow crystallizationfrom methanol4ioxane (1 : 2 v/v).Yellow-orange isomer. A solution of bipyo (20 mmol, 3.76 g)in methanol (350 cm3) was added to a solution of CuC12~2H20(20 mmol, 3.41 g) in methanol (30 cm3). The clear green solutionwas then refluxed. The green substance which started toprecipitate at the beginning became yellow-orange under reflux.After 2.5 h of reflux the mixture was allowed to cool and theyellow-orange product filtered off, washed with small portionsof hot dry methanol and dried in vacuo.Yield ca. 65% (Found:C, 37.20; H, 2.45; Cu, 19.65; N, 8.65). Crystals suitable for X-rayanalysis were obtained by slow crystallization from water-dioxane (1 : 5 v/v).C, 36.95; H, 2.45; CU, 19.65; N, 8.55. Calc. for C2oH16C14-Physical Measurements.-The magnetic susceptibility of apolycrystalline sample of the green isomer of compound 1 wasmeasured by the Faraday method in the temperature range 4.2-290 K using a sensitive RG-HV electrobalance. The fieldapplied was 6.25 kOe. The magnetic susceptibility of apolycrystalline sample of the yellow-orange isomer wasmeasured by the Gouy method in the temperature range 77-290K using a sensitive RM-2 Cahn electrobalance, at a magneticfield strength of 9.9 kOe. The calibrant employed in both caseswas Hg[Co(SCN),], the magnetic susceptibility of which wastaken26 as 16.44 x 1 t 6 cm3 g-'.All of the measuredsusceptibilities were corrected for diamagnetism of the con-stituent atoms calculated by the use of Pascal's constants27and found to be - 354 x 1t6 cm3 mol-' for both isomers. Thevalue 60 x 1W6 cm3 rno1-l was used for the temperature-independent paramagnetism of copper(I1). The magnetism ofthe samples was found to be field independent. The exchangeparameters were determined by the least-squares procedure andminimization of the function (5) was the criterion used todetermine the best fit. The effective magnetic moment wascalculated from the equation peff = 2.83(xCuT)*.The ESR spectra of polycrystalline samples at roomtemperature and 77 K were recorded on a JEOL-Me X-bandspectrometer using a nuclear magnetometer (MJ 110R) andmicrowave frequency meter (JES-SH-30X) and ESR standards.The spectrum at liquid-helium temperature was measured witha X-band Radiopan SE/X 2543 spectrometer. Solid diphenyl-picrylhydrazyl (dpph) was used as the reference and themagnetic field was calibrated with proton and lithium probes.The IR spectra in the region 40@-4000 cm-' were measured inNujol mulls with a Specord 75 IR spectrophotometer and asKBr pellets with a Perkin-Elmer 180 spectrophotometer.Spectra obtained from the two instruments did not showsignificant differences. The FIR spectra were measured in Nujolmulls on a Perkin-Elmer 180 spectrophotometer in the range30-500 cm-'.The electronic spectra of the solid complexes diluted inLi,C03 were measured in the 200-700 nm region on a Hitachimodel 356 spectrophotometer and those of undiluted sampleswere recorded at 400-2000 nm on a Beckman UV 5240spectrophotometer.Spectra obtained from the two instrumentsdid not show significant differences.Crystal and Molecular Structures of [Cu2(bipyo),C1,] Iso-mers.-Data collection. Irregularly shaped crystals of averagedimension 0.25 mm (0.20 x 0.25 x 0.35 mm for the greenisomer of 1, and 0.25 x 0.20 x 0.30 mm for the yellow-orangeisomer) were used for data collection. Preliminary crystallo-graphic data were obtained from oscillation and Weisenbergphotographs and were refined by using the positions of 15reflections centred on a Syntex P2, instrument.Intensitymeasurements were carried out on the Syntex P2, four-circle computer-controlled diffractometer using graphite-mono-chromated Cu-K, radiation (h = 1.541 78 8,) and a scintillationcounter. 1614 Independent reflections were collected for thegreen isomer in the range 0 < 28 < 55" and 1366 withI > 3o(I) were used in the analysis. For the yellow-orangeisomer, 1754 independent reflections were collected in the range0 < 28 < 55" and 1283 with I2 30(I) were used in theanalysis. The 8-20 scanning technique with a variable scanspeed (4.88-29.3" mi&) was used in both cases. Two checkreflections at intervals of 100 were measured and no significantintensity changes were observed. Lorentz and polarizationcorrections were applied in the usual way.Since the crystalswere nearly cylindrical no absorption corrections were made(p = 68.9 cm-l, pR = 0.9, where R is the cylinder radius of thecrystal).Crystal data. Green isomer, C20H,6C14CU2N404, M =645.28, triclinic, space group Pi, a = 8.664(2), b = 8.732(2),c = 9.099(3) A, a = 95.62(2), p = 107.04(2), y = 114.05(2)",U = 582.1(3) A3, D, = 1.83 g ~ m - ~ , 2 = 1, Dc = 1.84 g ~ m - ~ ,F(OO0) = 322.Yellow-orange isomer, monoclinic, s ace group P2,/c, a =1161.6(4) A3, D, = 1.84 g cmP3, 2 = 2, Dc = 1.84 g ~ m - ~ ,F(OO0) = 644.Structure solution and refinement. The structures of bothisomers were solved using the heavy-atom method and refinedby full-matrix anisotropic least-squares methods for all non-hydrogen atoms (hydrogen atoms were calculated). Calculationswere performed with the SHELX 76 program system.28 Thefinal positional parameters of the non-hydrogen atoms arelisted in Table 1.The structure of the green isomer was refined toR = 0.054, R' = 0.064 ( W = k[o2(F0) + gFo2], k = 1.0000,g = 0.009 362) for 154 parameters (data/parameter ratio =8.87: 1). The structure of the yellow-orange isomer was refinedg = 0.001 699) for 154 parameters (data/parameter ratio =8.33 : 1). Final Fourier difference maps showed peaks withApmax,min = 0.55/- 1.16 e A-3 for the green isomer andAP,,,~~,,,,~,, = 1.7 (ca. 1 8, from the Cu atom)/-0.84 e A-3 for theyellow-orange isomer.Additional material available from the Cambridge Crystallo-graphic Data Centre comprises H-atom coordinates, thermalparameters and remaining bond lengths and angles.7.912(2), b = 9.878(2), c = 15.086(3) K , p = 99.86(2)", U =to R = 0.063, R' = 0.064 {W = k[o2(F0) + gFo2], k = 2.8811,References1 W.H. Watson, Znorg. Chem., 1969,8, 1879 and refs. therein.2 P. G. Simpson, A. Vinciguerra and J. V. Quagliano, Znorg. Chem.,3 A. Vinciguerra, P. G. Simpson, Y. Kakiuti and J. V. Quagliano,4 S. K. Madan and W. E. Bull, J. Inorg. Nucl. Chem., 1964,26,2211.5 E. J. Halbert, C. M. Hams, E. Sinn and G. J. Sutton, Aust. J. Chern.,6 P. Baran, D. Valigura, M. Koman and J. Mrozinski, Proc. 12th Con$7 A. W. Addison, J. Chem. Soc., Dalton Trans., 1984,1349.8 J. V. Quagliano, J. Fujita, G. Franz, D. J. Phillips, J. A.Walmsley and9 W. E. Hatfield and J. C. Morrison, Znorg. Chem., 1966,5,1390.1963, 2,282.Inorg. Chem., 1963,2,286.1973,26,951.Coord. Chem., Bratislava-Smolenice, 1989, p. 437.S. Y. Tyree, J. Am. Chem. SOC., 1961,83,3770.10 R. Whyman and W. E. Hatfield, Znorg. Chem., 1967,6,1859.11 R. Whyman, D. B. Copley and W. E. Hatfield, J. Am. Chem. SOC.,12 D. R. Johnson and W. H. Watson, Znorg. Chem., 1971,10,1070.13 D. R. Johnson and W. H. Watson, Znorg. Chem., 1971,10,1281.14 J. C. Morrow, J. Cryst. Mol. Struct., 1974,4,243.15 E. D. Estes and D. J. Hodgson, Inorg. Chem., 1976,15,348.16 J. A. Paulson, D. A. Krost, G. L. McPherson, R. D. Rogers and J. L.17 M. Gawron, R. C. Palenik and G. J. Palenik, Acta Crystallogr., Sect.1967,89,3135.Atwood, Inorg. Chem., 1980,19,2519.C, 1988,44,168J. CHEM. SOC. DALTON TRANS. 199118 R. J. Williams, W. H. Watson and A. C. Larson, Acta Crystallogr.,19 R. S. Sager and W. H. Watson, Znorg. Chem., 1968,7,2035.20 R. S. Sager, R. J. Williams and W. H. Watson, Znorg. Chem., 1969,8,21 B. E. Mayers, L. Bergerand and S. A. Friedberg, J. Appi. Phys., 1969,22 J. S. Smart, Effective Field Theories of Magnetism, Saunders,23 B. Bleaney and K. Bowers, Proc. R. SOC. London, Ser. A, 1952,214,Sect. B, 1915,31,2362.694.40, 1 149.Philadelphia, 1966.451.24 E. J. Halbert, C. M. Harris, E. Sinn and G. J. Sutton, Aust. J. Chem.,25 T. P. E. Auf Der Heyde, C. S. Green, D. E. Needham, D. A. Thornton26 B. N. Figgis and R. S. Nyholm, J. Chem. SOC., 1958,4190.21 E. Konig, Magnetic Properties of Coordination and OrganometallicTransition Metal Compounds, Springer, Berlin, 1966.28 G. M. Sheldrick, SHELX 76 Program for Crystal StructureDetermination, University of Cambridge, 1976.1973,26,951.and G. M. Watkins, J. Mol. Struct., 1981,70, 121.Received 12th December 1990; Paper 0/05596