J. CHEM. SOC. DALTON TRANS. 1994 65 1Highly Stereospecific Carbene Insertions into Platinum-Halide Bonds. Crystal Structure of [ PtCI(S-CHCISiMe,)-(S,S-Ph,PCHMeCH,CH MePPh,)] tPaola Bergamini,*.8 Emiliana Costa? A. Guy Orpen! Christian Ganterb and Paul G. Pringle *JJa Dipartimento di Chimica dell' Universita di Ferrara e Centro di Studio su Fotoreattivita e Catalisi del CNR,Via 1. Borsari 46, 44100 Ferrara, ItalySchool of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 ITS, UKTrimethylsilyldiazomethane reacted with [PtX,LJ [X = CI, Br or I; L, = cycloocta-l,5-diene (cod) orPh,P(CH,),PPh, where n = 1-41 to give the corresponding [PtX(CHXSiMe,)LJ as a racemic mixture inhigh yields. The cod ligands in [PtX(CHXSiMe,) (cod)] were readily displaced by diphosphines.Whenoptically active complexes [PtX,LJ [X = CI, Br or I; L, = (2S,3S)-2,3-bis(diphenylphosphino) butane,(2S,4S) -2,4-bis(diphenylphosphino)pentane (SS-skewphos) or (4R,5R)-4,5-bis(diphenylphosphino-methyl ) -2,2-dimethyl-l.3-dioxolane] were treated with trimethylsilyldiazomethane the products [ PtX-(CHXSiMe,)L,] were mixtures of diastereoisomers in different proportions ranging from 1 : 1 to 15: 1.The major diastereoisomer of [ PtCI(CHCISiMe,) (SS-skewphos)] was readily separated by fractionalcrystallisation and its crystal structure revealed that the configuration at the a-carbon was S. Thiscompound is configurationally stable for at least 1 4 d in CDCI, solution as shown by jlP NMRspectroscopy. The high diastereoselectivities obtained in some of the reactions may originate from stericeffects.Chloromethylplatinum(xr) complexes have been used for thesynthesis of co-ordinated phosphorus ylides,, platinaoxacyclo-butane^,^ platinaazacyclobutanes and for some novel C-Cbond-forming reactions.We have previously shown thatdiazocarbonyl compounds react with complexes of the type[PtX,L,] (X = C1 or I, L, = cyclooctadiene or a diphosphine)to give functionalised chloromethylplatinum(xr) complexescontaining an asymmetric carbon atom a to the platinum andthat this reaction is diastereoselective when L, is a chiraldiphosphine (diastereomeric ratios of up to 7 : I). We now reportthat, in the absence of water, the readily available diazocompound Me3SiCHN, reacts similarly with complexes of thetype [PtX,L,] to give [PtCl(CHClSiMe,)L,] and when L, =chiral diphosphine diastereomeric ratios of up to 15 : 1 havebeen achieved.A preliminary account of part of this work hasbeen given previously and recently McCrindle and McAlees *have also reported observations on some of these reactions.Results and DiscussionTreatment of [PtX,(cod)] (X = C1, Br or I; cod = cycloocta-1,5-diene) with 1.1 equivalent or an excess of Me,SiCHN, gavethe monoinsertion products [PtX(CHXSiMe,)(cod)] (X = C1,la; Br, lb; or I, lc) in high yields which have been fullycharacterised (see Tables 1 and 2 and Experimental section fordetails). The 'H NMR spectrum of la shows four distinctresonances for the alkenyl protons of the co-ordinated cod(multiplets of 6 4.84, 4.97, 5.51 and 5.62), consistent with thepresence of the asymmetric CHClSiMe, group which rendersthese protons diastereotopic.Similarly, treatment of [PtX,L,][X = Cl or I; L, = Ph,P(CH,),PPh,, n = 1 (dppm), 2 (dppe),3 (dppp) or 4 (dppb)] with Me3SiCHN, gave the mono-insertion products Id-li (see Tables 1-3 for characterisingdata); some of these compounds have also been made byt Supplementary data available: see Instructions for Authors, J, Chem.SOC., Dalton Trans., 1994, Issue I, pp. xxiii-xxviii.Table 1 Elemental analytical" and IR data (cm-')Analysis (%)Complex C H v(Pt-C1)la 31.10 (31.30) 4.75 (4.80) 300l b 26.45 (26.45) 4.10 (4.00)l c 23.05 (22.40) 3.55 (3.45)Id 47.25 (47.30) 4.30 (4.40) 290le 37.80 (37.90) 3.40 (3.50)If 48.00 (48.00) 4.50 (4.55) 280h! 38.85 (38.60) 3.60 (3.65)Ih 48.05 (48.75) 4.70 (4.75) 2802a, 3a" 47.30 (47.20) 4.95 (4.80) 2902b, 3b' 43.00 (42.90) 4.50 (4.30)2c, 3cd 37.35 (37.90) 3.90 (3.80)4a, 5a 45.95 (46.40) 4.70 (4.80) 2904c, 5' 39.90 (39.50) 4.25 (4.05)6a, 7a 49.00 (49.40) 5.55 (5.00) 290a, 7b 44.85 (44.75) 4.55 (4.50)6c, 7c' 42.00 (42.30) 4.25 (4.70)" Calculated values in parentheses." Contains 0.5 CH2C12.d Contains 1 CH,C12.Contains 1 Et20.Spectra measured in CsI pellets.substitution of the cod ligands in complexes la and lc by theappropriate diphosphine (see Scheme 1 and Experimentalsection). Complexes la-li are racemic mixtures.Treatment of complex la with (2S,3S)-2,3-bis(diphenyl-phosphino)butane (S,S-chiraphos) in CDCI, gave a 1 : 1 mixtureof diastereoisomers 2a and 3a (Scheme 2).The presence ofthe two isomers is particularly evident in the 31P NMRspectrum which shows two equally intense AX patterns, theparameters for which are closely similar (see Table 2). In thesame manner, 1 : 1 mixtures of diastereoisomers 2b with 3b, 2cwith 3c and the corresponding complexes of (2S,4S)-2,4-bis(dipheny1phosphino)pentane (S,S-skewphos) (4a-k with5a-5c) and (4R,5R)-4,5-bis(diphenylphosphinomethyl)-2,2652 J. CHEM. SOC. DALTON TRANS. 1994IdleIfl hli2a, 3a2b, 3b2c, 3c4a, 5a4b, 5b4c, 5c6a, 7a6b, 7b6c, 7c-42.8- 52.840.842.62.515.641.342.541.644.139.141.713.314.412.813.78.89.72.15.04.12.20.1- 1.340503614417739104109425740894255403041 303828392341 1641284066407438963902425242694128422640454075-42.8- 57.240.840.5- 0.813.441.743.240.543.237.141.314.110.712.18.77.55.94.03.9-0.5-0.9-8.1- 6.0136214571816185116751738176717341795178118181808171916601760170718201792167716951714172417721796495000241715151313121224242324222415151517171710551522211" Spectra (36.2 MHz) measured in CDCI, at 28 "C unless otherwise stated.Chemical shifts (6) in ppm (k 0.1) to high frequency of 85% H,PO,.Coupling constants ( J ) in Hz ( 2 3). Atom PA is trans to the halogen and PB trans to the carbon.In cases where two diastereoisomers are formednumbers in this column refer to ratios of the intensities of the 31P NMR signals (data for the major isomer are given first).Table 3 Selected proton NMR data"Complex 6(PtCH) 'J(PtCH) ,J(PPtCH) 6(SiCH,)lalbl cIdl eIflhl i2a, 3a2b, 3b2c, 3c4a, 5a4b, 5b4c, 5c6a, 7a6b, 7b6c, 7c4.484.382.4'4.123.483.503.102.983.033.303.3 13.032.922.892.803.32d3.65d3.84d91 .O84.248.952.031.033.534.837.229.332.735.724.5n.0.n.0.n.0.n.0.n.0.7.311.5, 7.210.0, 7.312.9, 5.913.2, 6.014.1, 6.310.6, 8.011.7, 7.313.1, 6.814.0, 7.0+ 0.22 + 0.26 + 0.30-0.12-0.10-0.12- 0.05- 0.05-0.12-0.12-0.12- 0.08-0.100.000.00-0.10, -0 .I 1-0.00, -0 .020.00, -0 .02Spectra (300 MHz) measured in CDCI, at 25 "C; chemical shifts (6) inppm ( k 0.01) to high frequency of SiMe, and coupling constants ( J ) inHz ( f 0.1). n.0. = Not observed because signal is obscured. Signalobscured but assigned on the basis of observed nuclear Overhauserenhancement between this signal and that of SiMe,. Assignmentconfirmed by correlation spectroscopy.Xl a CIl b Brl c I( p\ Pt 2' 'xScheme 1 (i) N,CHSiMe,; (ii) diphosphine/CHXSiMe3 pp\xnX P PI d CI dppml e I dppmI f CI dppel h CI dpppl i CI dppb19 I dPpedimethyl-l,3-dioxolane (R,R-diop) (6a-6~ with 7a-7c) havebeen made and characterised (Tables 1-3).When the optically active complexes [PtX,L,] (X = C1, Bror I; L, = S,S-chiraphos, S,S-skewphos or R,R-diop) weretreated with Me,SiCHN, the same diastereomers 2-7 wereformed (see Scheme 2) but in the proportions shown in Table2, as determined by integration of the 'H NMR signals forthe Me& groups or integration of 31P NMR signalscorrected for Nuclear Overhauser and relaxation effects bycomparison with spectra measured under the same conditionsof 1 : 1 mixtures generated from la-lc and the chiral diphos-phine.A single recrystallisation of the mixture of S,S-skewphoscomplexes 4a and 5a from CH,Cl, and EtzO gave pure 4a.Noepimerisation of the a-carbon configuration was observed whenpure 4a or a 1 : 1 mixture of 4a and 4b was redissolved in CDCl,,over a period of 14 d even in the presence of [AsPhJCI. Thisdemonstrates that under these conditions this compound isoptically stable for kinetic reasons.Crystals of complex 4a suitable for X-ray crystallographywere grown from CH,Cl,-Et,O.The molecular structure isshown Fig. 1 and selected molecular dimensions are listed inTable 4. The crystal structure consists of isolated molecules of4a separated by normal contacts. The molecule consists of aplatinum atom which is chelated by a skewphos ligand andfurther ligated by a chloride ligand and CF bonded to the chiraJ. CHEM. SOC. DALTON TRANS. 1994 653Xl a CIl b Brlc I2a2b2c4a4b4c6a6b6cScheme 2 (i) N,CHSiMe,; (ii) diphosphineFig. 1 Molecular geometry of complex 4a, showing the atom labellingscheme.The platinum atom is represented as an ellipsoid enclosing 50%probability density. All hydrogens other than H(1a) have been omittedfor clarityalkyl ligand through C( 1) [Pt-C(1) 2.104( 12) A]. The variationin trans influence is indicated by the different Pt-P distances forthe two skewphos phosphorus atoms [Pt-P(2) 2.232(4) trans toCl( 1) and Pt-P( 1) 2.318(4) trans to C(1); Pt-Cl(1) 2.360(4)A].As is to be expected for a platinum(I1) complex the co-ordination at Pt is close to planar (mean deviation 0.057 A)with a slight twist (6.0") in the co-ordination plane between thePtP, and the PtCCl units. The cis and trans angles deviateslightly from 90 and 180", the largest deviation for a cis anglebeing for the skewphos ligand [P(2)-Pt-P( 1) 94.0( l)"].Theconformation adopted by the chiral alkyl has the a-hydrogennear the co-ordination plane of the platinum and pointingtowards P(2) [torsion angle P(2)-Pt-C( 1)-H( la) 7"]. Thisorientation allows the bulkier C1 and SiMe, substituents atC( 1) to avoid the crowded co-ordination plane with the largerSiMe, group between the pseudo-equatorial phenyl groups[cfi Cl(2) which is closer to the pseudo-axial phenyl rings, seeFig. I]. Rather similar gross geometries have been observedin [PtCI(R-CHClCO,Et)(R,R-diop)] and in [PtCl(R-CHMeCO,Et)(S,S-diop)] .' The skewphos six-membered ringadopts a flattened chair conformation (see Fig. 1) with themethyl substituents in axial [C(9)] and equatorial [C(5)] sitesrespectively.3a3b3c5a5b5c7a7b7cXCIBrICIBrICIBrInP PS, SchiraphosS, SchiraphosS, SchiraphosS, S s kewp hosS, Sskewp hosS, Sskewp hosR, R-diopR, R-diopR, R-diopTable 4 Bond lengths (A) and selected bond angles ("1 for complex 4aC1( 1 )_Pt-P( 1)P( 1 )-Pt-P(2)P( 1 )-Pt-C( 1 )Pt-P( 1)-C(8)C(8 j P ( 1)-c(3C(8)-P( 1)-C(4Pt-P(2)-C(6)C(6tP(2)-C( 1C(6)-P(2)-C(2C( 1 kSi-C(2)C(2)-Si-C(3)C(2)-Si-C(4)Pt-C( I)-C1(2)C1( 2kC( 1 )-SiP(2)-C(6)-C(7)C(6tC(7)-C(8)P( l)-C(8)-C(9)2.360(4)2.232(4)1.843( 13)1.824(11)1.838(14)I .829(8)1.847( 18)1.886( 17)1.52 1 (20)1.534(21)89.1(1)94.0( 1 )177.5(4)116.6(5)105.4(6)1 15.8(5)100.6(6)107.6(6)114.7(7)110.3(8)105.8(8)1 1 1.2(6)108.3(6)112.9(10)116.8(13)108.9(10)99.9(5)Pt-P( 1)Pt-C( 1 )P( 1 )-C(8)P( 1)-C(41)P(2)-C( 1 1)Si-C( 1 )Si-C( 3)C(5tC(6)C(7)_C(8)Cl( 1 )_Pt-P( 2)C1( 1 jPt-C( 1 )P(2)-Pt-C( 1 )Pt-PpC(31)Pt-P-C(41)Pt -P(2)-C( 11)Pt-P(2)-C( 2 1 )C(lltP(2) C(21)C( 1 kSi-C(3)C( 1 jSi-C(4)C( 3)-Si-C( 4)Pt-C( 1 )-SlC(31)-P(1) C(41)P(2)--C(6) C(5)C( 5)-C(6)-C( 7)P( 1 )-C( 8)-C( 7)C(7jC(8) C(9)2.3 18(4)2.104( 12)1.846(14)1.811(11)1.848( 12)1 .890( 14)1.854(18)1.548(22)1.508(20)173.3(1)88.4(4)88.5(4)1 18.7(3)11 1.0(4)103.5(5)1 15.0(4)109.7(3)107.4(4)110.8(7)106.9(7)l08.0(8)115.1(7)115.1(10)1 09.1 ( 1 2)1 13.0( 10)11 1.9(12)It had been reasoned that if steric effects influence the ratio ofdiastereoisomers, higher diastereoselectivity should be obtainedusing N,CHSiMe, as a consequence of the bulkiness of theSiMe, substituent. Indeed the diastereoselectivities of some ofthese insertion reactions are high (see Table 2) and, for thechlorides, higher than for the corresponding reactions with ethyldiazoacetate or diazoacetophenone consistent with stericeffects being one factor in determining the diastereoselection.For the bromides and iodides, where the difference in sizebetween the halogen and the trimethylsilyl substituents is less,the diastereoselectivity is less than for the correspondin654 J.CHEM. SOC. DALTON TRANS. 1994chlorides as would be expected on steric grounds. Howeverthere appear to be no consistent trends in diastereoselectivityamong the different diphosphines and further work is inprogress to probe the other factors that are important indetermining the diastereoselectivities of these insertions.It is known" that CH,N, and Me3SiCHN, have similarchemical reactivity but the corresponding reaction of [PtX,-(diphosphine)] with diazomethane gives the bis insertionproducts [Pt(CH,X),(diphosphine)].' ' This contrasting beha-viour is most likely a consequence of kinetics (hinderedapproach of Me,SiCHN, to the bulky [PtX(CHXSiMe,)-(diphosphine)]} and thermodynamics ( unfavourable stericcongestion that would be present in the product, [Pt(CHX-SiMe ,) ,(diphosphine)] ) .It has been shown that the bulky diazoalkane N,CHSiMe,reacts smoothly with complexes of the type [PtX,(diphos-phine)] to give the products of monoinsertion into the Pt-Xbonds.When X = CI and the diphosphine is chiral, un-precedentedly high diastereoselectivity is obtained. Singleisomers which are configurationally stable are readily separatedwhich should make these species valuable for mechanisticstudies.ExperimentalAll reactions were carried out under an atmosphere of nitrogen,though the products could be handled in air. All thediphosphines (Aldrich or Strem), and a 2 mol dmp3 hexanesolution of N,CHSiMe, (Aldrich) were used as purchased. TheFT-IR spectra (CsI discs) were obtained using a Nicolet 510Pspectrometer, 31P-(1H} NMR spectra at 81 MHz and 28 "Cusing a Bruker AM200 spectrometer and 'H NMR spectra at300 MHz at 25 "C using a Varian Gemini 300 spectrometer.Preparations.-[PtCl(CHClSiMe,)(cod)] la.A solution ofN,CHSiMe, (0.29 cm3 of a 2 mol dm-, solution in hexane, 0.58mmol) was added over 5 min under nitrogen to a solution of[PtCl,(cod)] (200 mg, 0.53 mmol) in CH,Cl, (15 cm3), themixture was stirred for 2 h and then the volume was reduced to1 cm3. Addition of diethyl ether (25 cm3) precipitated the off-white product (220 mg, 90%). The bromo (Ib) and iodo (Ic)analogues were made similarly in 84 and 82% yield respectively.13C NMR (CDCI,): la, 6 53.12, ['J(PtC) 750, CPt] and 0.01(CH3Si); lb, 6 41.07 ['J(PtC) 750 Hz, CPt] and 0.90 (CH,Si);lc, 6 30.22 ['J(PtC) not observed, obscured by allylic codresonances, CPt] and 1.97 (CH,Si).[PtCl(CHClSiMe,)(dppm)] Id from [PtCl,(dppm)].A solu-tion of N,CHSiMe, (0.69 cm3 of a 2 mol dm-, solution inhexane, 1.38 mmol) was added over 5 min under nitrogen to asolution of [PtCl,(cod)] (300 mg, 0.46 mmol) in dry CH,CI, (20cm3), the mixture was stirred for 18 h and then the volume wasreduced to 1 cm3. Addition of diethyl ether (30 cm3) precipitatedthe ofT-white product (307 mg, 91%). Complexes le-li weremade similarly from the appropriate [PtX,(diphosphine)] inthe following yields: le, 74; If, 73; Ig, 71; Ih, 97; li, 94%.[Ptl(CHISiMe,)(dppe)] Ig from [PtI(CHISiMe,)(cod)] lc.A solution of dppe (1 24 mg, 0.3 1 mmol) in CH,Cl, (30 cm3) wasadded dropwise to a vigorously stirred solution of [PtI(CHISi-Me,)(cod)] (200 mg, 0.31 mmol) in CH,Cl, (20 cm3).Theresulting yellow solution was stirred for I h and then the solventwas evaporated to dryness under reduced pressure. Addition ofpentane (50 cm3) to the residue gave the yellow product (270mg, 93%). Complexes Id and If have also been prepared in thisway in > 90% yields.[PtC1(CHC1SiMe3)(S,S-skewphos)] as a mixture of dia-stereoisomers 4a and 5a and the separation ofpure 4a. A solutionof N,CHSiMe, (0.32 cm3 of a 2 mol dm-3 solution in hexane,0.64 mmol) was added over 5 min under nitrogen to a solutionof [PtCI,(S,S-skewphos)] (1 50 mg, 0.21 mmol) in dry CH,CI, (5cm3), the mixture was stirred for 16 h and then the volume wasreduced to 1 cm3. Addition of diethyl ether (20 cm3) precipitatedthe off-white product (1 20 mg, 72%) which is a 15 : I mixture ofdiastereoisomers 4a and 5a; one recrystallisation from CH,Cl,-Et ,O gave pure 4a.The complexes 2-7 were all made similarly(though reaction times varied from 30 min to 16 h) in thefollowing yields: 2a, 3a, 76; 2b, 3b, 74; 2c, 3c, 79; 4a, 5a, 72; 4b, 5b,79; 4c, 5c, 61; 6a, 7a, 75; 6b, 7b, 63; 6c, 7c, 74%. The isomer ratiosobtained by this method are given in Table 2. In each case a 1 : 1mixture of diastereoisomers has been generated by adding 1equivalent of the appropriate diphosphine to la-lc in CDCI,.Crystal Structure Determination of [PtCl(S-CHClSiMe,)-(S,S-skewphos)] 4a.-Crystal data. C3,H,oCl,P,PtSi, M, =792.7, orthorhombic, space group P212,21 (no. 19), a =11.152(5), b = 16.102(4), c = 19.021(5) A, U = 3415(2) A,,2 = 4, D, = 1.54 g cm-,, x = 0.710 73 A, p(Mo-Ka) = 44.1crn-', F(OO0) = 1576, T = 295 K.A single crystal of complex 4a (approximate dimensions0.25 x 0.36 x 0.50 mm) was mounted in a thin-walled glasscapillary under N, and held in place with epoxy glue.Alldiffraction measurements were made at room temperature (295K) on a Siemens R3m/V diffractometer, using graphite-monochromated Mo-Ka X-radiation. Unit cell dimensionswere determined from 34 centred reflections in the rangeTable 5 Atomic coordinates ( x lo4) for complex 4aX- 59( 1)- 888(4)- 2 769(4)1823(3)598(3)- 2 580(4)- 1 799( 11)-3 564(14)- 1 480(16)- 3 594( 15)2 555( 14)2 132(12)3 055( 14)3 119(12)3 231(16)- 242- 937( 1 1)-1 526- 1 420- 725Y9 889(1)9 086(3)10 608(3)9 695(2)10 772(2)9 033(3)10 033(9)8 600( 1 1)8 241(10)9 277( 1 1)11 191(12)10 592(10)10 585(10)9 81 l(9)9 025( 1 1)10 75011 415(5)11 36510 6519 986z1 009(1)1926(2)1214(3)1 500(2)191(2)353(3)599(6)1 044(10)61(10)-413(8)-714(8)- 128(7)461(7)905(7)454(8)- 646- 882(6)-1 527- 1 938- 1 702X- 136528230(8)2575828808532 176I 545(8)19102 9053 5363 1712 0843 084(8)3 2002 31513151199Y10 036 -11 83411 951(4)12 74613 42413 30812 5138 6917 971(7)7 2067 1607 8798 64510 4221 0 940( 7)11 49611 53311 01510 460Z10575291234(4)152511114061151 8921718(5)1 9872 4302 6042 3342 2102 251(5)2 8103 3283 2872 72J .CHEM. SOC. DALTON TRANS. 1994 65516.0 -= 20 c 30.0'. A total of 3397 diffracted intensities,including check reflections, were measured in a unique octant ofreciprocal space for 4.0 < 20 < 50.0" by Wyckoff o scans. Twocheck reflections ( 1 -6 3, 2 -2 10) remeasured after every 98ordinary data showed a decay of ca. 6% over the period of datacollection; an appropriate correction was therefore applied. Ofthe intensity data collected, 3373 unique observations remainedafter averaging of duplicate and equivalent measurements anddeletion of systematic absences, of these 2528 with I > 241)were retained for use in structure solution and refinement. Anabsorption correction was applied on the basis of 420 azimuthalscan data, maximum and minimum transmission coefficientsbeing 0.438 and 0.30 1 respectively.Lorentz and polarisationcorrections were applied.The structure was solved by Patterson and Fourier methods.All non-hydrogen atoms were assigned anisotropic displacementparameters and all hydrogen atoms fixed isotropic displacementparameters. All non-hydrogen atoms were refined withoutpositional constraints except for the phenyl-group carbon atomswhich were constrained to D,, symmetry with C-C distances1.395 A. All hydrogen atoms were constrained to idealisedgeometries (C-H 0.96 A, H-C-H 109.5') with fixed isotropicdisplacement parameters. A parameter (q), defining the absolutestructure and hence the molecular chirality,12 refined to 1.07(4),thereby confirming the handedness of the molecules of complex4a in the crystal studied.An extinction parameter x (whereF,,,, = F [ ( 1 + 0.002~F')/sin 28]-0.25) was refined to a valueof 0.000 1 l(2). Full-matrix least-squares refinement of thismodel ( 1 86 parameters) converged to final residual indices R =0.044, R' = 0.046, S = 1.22, where R =C.IAI/XlF,[; R' =(CwA2/CwFo2)*; S = ~ w A 2 / ( N o - N v ) ] ~ ; A =Fo - Fc and No,N , = numbers of observations and of variables. Weights, w,were set equal to [ac2(Fo) + gFo2Ip1, where oc2(Fo) is thevariance in F, due to counting statistics and g = 0.0004 waschosen to minimise the variation in S as a function of IFo[. Finalelectron-density difference maps showed no features outside therange + 1 .1 to - 0.7 e k3, the largest of these being close to theplatinum atom. Table 5 lists the final atomic positionalparameters for the freely refined atoms, and Table 4 the selectedbond lengths and inter-bond angles. All calculations were carriedout on using programs of the SHELXTL-PLUS package.13Complex neutral-atom scattering factors were taken from ref. 14.Additional material available from the Cambridge Crystal-lographic Data Centre comprises H-atom coordinates, thermalparameters and remaining bond angles.AcknowledgementsWe thank Professors 0. Traverso and S. Sostero (Universitadi Ferrara) for useful discussions, Mr. P. Formaglio, Mr.M. Fratta and Mr. 0. Zerlotin for technical assistance, Ciba-Geigy for a Senior Research Fellowship (to P.G. P.),the Foundation Blanceflor Boncompagni-Ludovisi, nke Bildt,for a grant (to E. C.), the DAAD for a post-doctoral fellowship(to C. G.), NATO for the provision of a travel grant andJohnson-Matthey for a generous loan of platinum salts.References1 H. B. Friedrich and J. R. Moss, Adv. Organomet. Chem., 1991, 33,235; D. Steinborn, Angew. Chem., Int. Ed. Engl,, 1992,401.2 N. J. Kermode, M. F. Lappert, B. W. Skelton, A. H. White andJ. Holton, J. Chem. Soc., Chem. Commun., 1981, 698; C. Engelter,J. R. Moss, L. R. Nassimbeni, M. L. Niven, G. Reid and J. C. Spiers,J. Organomet. Chem., 1986,315,255; J. F. Hoover and J. M. Stryker,Organometallics, 1988, 7, 2082; R. McCrindle, G. J. Arsenault,A. Gupta, M. J. Hampden-Smith, R. E. Rice and A. J. McAlees,J. Chem. Soc., Dalton Trans., 1991,949.3 J. F. Hoover and J. M. Stryker, J. Am. Chem. Suc., 1989,111,6466.4 R. McCrindle, G. Ferguson and A. J. McAlees, J. 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