J. CHEM. SOC. DALTON TRANS. 1985 2277The Palladium-catalysed Reaction Between [Re,(CO),,] and Phosphines andthe Crystal and Molecular Structure of Diaxial [ Re,(CO),( PMe,Ph),] tGillian W. Harris, Jan C. A. Boeyens, and Neil J. Coville"Department of Chemistry, University of the Witwatersrand, Johannesburg, Republic of South AfricaThe reaction between [Re,(CO),,] and PR, [PR, = PPh,, PMePh,, PMe,Ph, PMe,, P(CH,Ph),, orP(OMe),] in xylene is catalysed by Pd/C, Pd/CaCO,, and PdO and yields the complexes[Re,(CO),,,(PR,),] (n = 1 or 2) as the major products (1 5 4 0 % ) . Attempts to synthesise[Re,(CO),,,(PR,),] from [Re,(CO),,] and PR, in the presence of NMe,O under milder conditions(e.g. refluxing CH,CI,) also gave the above products with n = 1 or 2 as well as high yields of fac-[ReCI(CO),( PR,),].An X-ray crystal structure determination of diaxial [ Re,(CO),( PMe,Ph),] wascarried out: space group C2/c, Z = 4, a = 22.779(9), b = 7.1 66(3), c = 17.249(7)A, p = 95.1 3(3)",R = 0.0602. The structure has a staggered geometry with torsion angles deviating by f 6" from theideal (45"). The Re-Re bond length was found to be 3.044(1 )A, cf., 3.041 (1) A for [Re,(CO),,],indicating that this parameter is almost invariant on substitution of CO by PMe,Ph.In recent publications we have reported on the catalysedreaction between [M,(CO),,] (M = Mn or Re) and isonitrileswhich yields [M,(CO),,(CNR),] (n = 1 - 4 ) . In thesereactions the stereochemistry of all products isolated could beexplained in terms of the dominance of electronic over stericfactors, which resulted in equatorial isonitrile substitution. In acontinuation of this work we now report on the reaction(catalysed and otherwise) between [Re,(CO),,] and Group 5donor ligands (PR,) to give the products [Re,(CO),,,((PR,),](n = 1 or 2) in which steric factors now influence the course ofthe reaction.The reaction (both thermal and photochemical) between[Re,(CO),,] and PR, has been investigated by many groups4and in most cases the major product(s) obtained are either axial[Re,(CO),(PR,)] and/or diaxial [Re,(CO),(PR,),] togetherwith minor amounts of products such as [ReH(CO)3(PR3),].SThere have also been reports of the synthesis of equatorial[Re,(CO)lo-,,(PR,),] (n = 1 or 2)6-8 as well as[Re,(CO),,,(PR,),] (n = 3 or 4).9 The use of alternativeroutes,",' ' e.g.using substituted rhenium dimers, also yieldsa range of [Re,(CO),(PR,),] complexes of mixed stereo-chemistry '' (see below).Although numerous techniques have been used to establishthe stereochemistry of the resulting complexes ( e g . H and 31 Pn.m.r. spectroscopy and mass spectrometry), i.r. spectroscopy isthe most ubiquitous means of product identification. However,i.r. spectroscopy is not highly reliable in this regard as thenumber of v(C0) bands predicted by group theory does notnecessarily correspond with those observed in practice. In thispublication we have thus attempted to correlate the observed i.r.spectra (number of bands, intensity) of the complexes[Re,(CO),L,] (L = substituting ligand) with product stereo-chemistry using all available reported chemical and physicaldata on these complexes, with particular emphasis on dataobtained from X-ray crystallographic studies.The methodcan be extended to other derivatives of [Re,(CO)l,].'2As no crystallographic data are available on any [Re,-(CO)lo,(PR3),,] (n > 1) complexes in the literature, wehave also carried out the molecular and crystal structure~~ ~ ~t Bis[tetracarbonyl(dimethylphenylphosphine)rhenium](~e-Re).Supplemenrary data available (No. SUP 56287, 4 pp.): thermalparameters, least-squares planes. See Instructions for Authors, J . Chem.Suc., Dalton Trans., 1985, Issue 1 , pp. xvii-xix. Structure factors areavailable from the editorial office.determination of [Re,(CO),(PMe,Ph),]. As this complex waspredicted to have a diaxial arrangement of PMe,Ph ligands(relative to the Re-Re bond) we were also interested inobtaining bond-length and -angle data on the effect of PR,substitution on the molecular parameters of the substituteddimer.ExperimentalGeneral.-The complex [Re,(CO), ,] was purchased fromStrem Chemicals.The phosphines were obtained from StremChemicals [PMe,Ph, PMePh,, PMe,, P(CH,Ph),, andAsMe,Ph], Merck Chemicals (PPh,), or British Drug HousesLtd. [P(OMe),]. The catalysts Pd/C (10% Pd) and Pd/CaCO, (10% Pd) were obtained from Merck Chemicals, andPdO from Johnson Matthey Chemicals Ltd. The compoundNMe30-2H,0 was purchased from Fluka AG. All chemicalswere used as obtained from the manufacturers, without furtherpurification.Xylene was distilled from sodium under nitrogen prior to use.All reactions were performed under argon.Thin-layerchromatography was carried out on Merck silica gel 60F-254plates, and preparative layer chromatography was performedusing Merck silica gel plates (20 x 20 cm, layer thickness 2mm). Column chromatography was carried out using Merck 60(60-200 pm) silica gel.Infrared spectra were recorded on a JASCO IRA Ispectrophotometer. Melting points were determined on aKofler micro hot-stage apparatus and are uncorrected.Microanalyses were performed by the MicroanalyticalLaboratories, Council for Scientific and Industrial Research,Pretoria.Synthesis of [Re,(CO), o,(PR3),] [PR, = PPh,, PMePh,,PMe,Ph, PMe,, P(CH,Ph),, or P(OMe),; n = 1 or 2]from theReaction between [Re,(CO),,] and PR, in the Presence ofP alladium Catalysts.-The complex [Re,(CO) ,] ( 163 mg, 0.25mmol) and catalyst (Pd/C, Pd/CaCO,, or PdO; 15 mg) werestirred in xylene (10 cm3) and to this solution was added PR,(0.75 mmol).The solution was refluxed at CQ. 140 "C for 1-9 h,depending on PR, (see Table l), and the progress of the reactionmonitored by t.1.c. [silica gel; eluant, ethyl acetate-lightpetroleum (b.p. 8&-100 "C) (1 : 3)]. The products were isolatedby preparative layer chromatography (silica, eluant as for t.l.c.),extracted with benzene, and recrystallised from benzene-hexan2278 J. CHEM. SOC. DALTON TRANS. 1985Table 1. Details of catalytic syntheses of [Re2(CO),,-,(PR3),] (n = 1 or 2) complexesReactionCatalyst time (h) Product aa Traces of other products were also observed but were not isolated.Isolated yields.Yield (%)b20202040603282384141514185525solutions to yield colourless to yellow solids characterised by i.r.spectroscopy and elemental analyses.Synthesis of [Re2(C0),,,(PR,),] [PR, = PMePh,, PMe,,or P(CH,Ph),; n = 1 or 21 by the NMe,O-assisted Reactionbetween [Re,(CO),,] and PR,.-The complex [Re,(CO),,](163 mg, 0.25 mmol) was dissolved in CH2Cl, (10 cm3) to whichPR, (0.75 mmol) had been added, and NMe30-2H,0 (60 mg,0.54 mmol) dissolved in degassed MeOH (5 cm3) was added.The solution was then warmed at 40 "C for 8 h and the progressof the reaction monitored by t.1.c.[silica gel; eluant, ethylacetate-light petroleum (b.p. 80-100 "C) (1 : 3)]. The productswere isolated by preparative layer chromatography (silica gel,eluant as for t,l.c.), extracted with CH,Cl,, and recrystallisedfrom CH,Cl,-pentane solutions to yield colourless to yellowsolids characterised by i.r. spectroscopy and elemental analyses.Preparation of [Re , (CO) 8 ( PMe , Ph) ,] , isomer (b).-Thecomplex [Re,(CO),,] (653 mg, 1 mmol) was dissolved inacetonitrile (10 cm3) and the solution heated at 80 "C. To thissolution was added NMe3O-2H,O (234 mg, 2.1 mmol)dissolved in degassed methanol (10 cm'). Reaction to[Re,(CO),(NCMe),] was instantaneous as evidenced by thecolour change from colourless to pale yellow.The compoundPMe,Ph (2.1 mmol) was added to the pale yellow solution andthe mixture heated at 80 "C for 2 h. The product was isolated bycolumn chromatography (2 x 40 cm silica column, gradientelution with hexane-benzene solution starting with hexane andending with benzene). Recrystallisation from CH,Cl,-pentanesolution gave a cream solid, m.p. 142-144 "C (Found: C, 33.5;H, 2.7. C,,H,,O,P,Re, requires C, 33.0; H, 2.6%).Preparation of [Re,(CO),(PMePh,),], isomer (b).-Thecomplex [Re,(CO),,] (667 mg, 1.02 mmol) and PMePh, (4.0mmol) were refluxed in xylene (17 cm3) at ca. 140 "C for 15 h.During this time the solution changed from colourless to intenseyellow. The xylene and excess of phosphine were removed undervacuum to yield a yellow solid which after chromatographicprocedures ' yielded the required product with spectroscopicand analytical properties similar to those reported previously.'Crystal and Molecular Structure of[Rez(CO),( PMe,Ph),].-Data collection.The compound diaxial [Re,(CO),(PMe,Ph),]was synthesised via the PdO-catalysed reaction between[Rez(CO)lO] and PMe,Ph. Recrystallisation from diethylether-pentane solution under nitrogen at 20 "C yielded finecolourless needle-like crystals. A crystal measuring0.19 x 0.09 x 0.09 mm was selected for crystallographicanalysis.Preliminary investigation by standard Weissenberg photo-graphy established the space group as either C2/c or Cc, fromthe conditions hkl, h + k = 2n, h01,l = 2n (h = 2n), and OkO,k = 2n. Refined cell constants were obtained during datacollection on a Philips PW 1 100 four-circle diffractometer,using graphite-monochromated Mo-K, radiation (h = 0.7107A) at room temperature (20 "C).Crystal data.C24H2208P Re,, M = 872.76, a = 22.779(9),b = 7.166(3), c = 17.249(7) 1, p = 95.13(3)", U = 2 804.33 A3,F(0oO) = 1640, 2 = 4, D, = 2.07 g ~ m - ~ , p(Mo-K,) = 84.01cm-'.Lorentz and polarisation corrections were applied, and thedata were empirically corrected for absorption effects using theprogram DIFABS,'," as adapted by Kruger.',' Data werecollected in 03-20 scan mode in the region 3 < 0 < 23". A totalof 1 768 reflections as measured at a speed of 0.03" s-l with ascan width of 0 . 9 " ~ . After omission of all unobserved reflectionswith F < a(F) a unique data set of 1473 observations wasretained.Structure solution and refinement.Structure analysis andrefinement were carried out using the program SHELX 82.Initial co-ordinates for the rhenium atoms were derived from aPatterson synthesis, and the positions of the other non-hydrogen atoms were found by Fourier difference syntheses.The structure was refined in the centrosymmetric space groupC2/c (no. 15), the two halves of the molecule being related by acrystallographic two-fold axis. Positional parameters for allatoms, and anistropic thermal parameters for the rhenium andphosphorus atoms, were refined by full-matrix least-squaresanalyses. In the initial stages the phenyl rings were treated asrigid hexagons, but all restraints were removed in the laterstages of refinement.Least-squares refinement was consideredcomplete when all parameter shifts were less than 0.50. At thisstage the conventional R was 0.0602. Unit weights were used.Scattering factors for Reo were taken from ref. 146 andanomalous dispersion correctionsFinal atomic co-ordinates for all non-hydrogen atoms aregiven in Table 2, an ORTEP" diagram indicating thewere made for rheniumJ. CHEM. SOC. DALTON TRANS. 1985 2279Table 2. Final atomic co-ordinates for the non-hydrogen atoms of diaxial [Re,(CO),(PMe,Ph),j with estimated standard deviations in parenthesesXla-0.048 q1)-0.124 9(2)-0.125 3(6)-0.007 8(7)0.9 17 4(8)0.041 2(7)- 0.096 3(9)-0.021 8(9)-0.068 5(9)Ylb Z l C0.187 9(1) 0.182 7(1)0.196 8(8) 0.082 4(3)0.001 3(20) 0.301 l(8)0.581 l(25) 0.242 3(9)0.365 9(24) 0.078 9(9)0.067 9(29) 0.256 3( 11)0.432 5(28) 0.221 5(10)-0.206 7(24) 0.126 9(9)-0.055 l(30) 0.148 6(11)Xla0.008 5(9)-0.147 9(12)-0.193 7(9)- 0.206 7( 1 1)-0.258 O(12)-0.295 9(12)- 0.284 3( 1 1)- 0.232 O(10)-0.111 2(10)Ylb0.295 6(30)0.429 9(39)0.081 5(34)0.095 2(30)-0.088 9(35)-0.174 l(41)-0.078 2(41)0.113 l(36)0.200 l(33)Zlc0.1194(11)0.047 5(14)0.109 3(11)0.089 6(13)0.1166(15)0.1564(15)0.169 7(13)0.149 O(12)- 0.009 3( 13)numbering system used is given in Figure 2, and a packingdiagram in Figure 3.DiscussionThe Palladium-catalysed Reaction between [Re,(CO) ,] andPR,.-As part of an extension of the Pd-catalysed reactionof [Re2(CO)10] with isonitriles' to other ligands the reactionof [Re,(CO),,] with Group 5 donor ligands, PR,, wasinvestigated.I6 The thermal reaction between [Re,(CO),,] andPR, is extremely sluggish even at elevated temperat~res,~ e.g.we have observed that the reaction between [Re,(CO),,] andPMe,Ph (140 "C, 10 h) yielded only small amounts of[Re,(CO),(PMe,Ph)] and [Re,(CO),(PMe,Ph),] (as well astrace amounts of other uncharacterised products) and > 80%[Re,(CO),,] was recovered.Use of palladium catalysts [e.g. Pd/C (lo%), Pd/CaCO, (lo%), and PdO] yields mono- and di-substitutedderivatives [Re~(CO)lo~(PR3),J (n = 1 or 2) in poor to goodyield after moderate reaction times.Examples of this reactioncovering a range of PR, groups varying in both steric andelectronic properties are given in Table 1.In favourable casesthe disubstituted complexes can be obtained in good yields asthe sole reaction product after relatively short times, e.g. 84%[Re,(CO),(PMe,Ph),] after 1 h. Reactions with PR, =PMePh, and PMe,Ph also yielded trace amounts of productswhich were isolated, and by comparison with the i.r. spectrumoffac-[ReH(CO),(PPh,),] [i.r. (CHCI,), v(C0) 2 009s, 1 9&,and 1 900s an-'] and elemental analyses were characterised asfac-[ReH(CO),(PMePh,),] [i.r. (CHCl,), v(C0) 2 OOOs,sp,1 910s, and 1 870s cm-'1 and fac-[ReH(CO),(PMe,Ph),][i.r.(CHCl,), v(C0) 2030s,sp, 1946s, and 1883s CM-'].~ Suchproducts were almost certainly formed with other PR, ligandsbut product decomposition on silica plates precluded theirisolation.Comparison of the i.r.data for the [Re,(CO),,,(PR,!,](n = 1 or 2) products (Table 2) with that of axial[Mn,(CO)g(PMe,Ph)]17 [i.r. (benzene), v(C0) 2 094w, 2 016s,1993vs, 1969sh, and 1 938m cm-'3 and diaxial [Mn,(CO),-(PMePh,),] 1i.r. (benzene), v(C0) 1 983w and 1 954vs an-'],' 'for which axial substitution has been established from X-raycrystallographic structure determinations,'8w19 indicates thatthe products isolated are either axial [Re,(CO),(PR,)] ordiaxial [Re2(CO),(PR3),]. An X-ray crystal and molecularstructure determination of [Re,(CO),(PMe,Ph),] has alsoconfirmed the diaxial substitution geometry (see below).Alternative Routes to [Re,(CO),,,(PR,),] (n = 1 or 2)Complexes.-These complexes can also be synthesised by theNMe,O-assisted reaction in CH,Cl, between [Re,(CO),,]and PR,.' ' Typically, axial [Re,(CO),(PR,)] (ca.40% yield),diaxial [Re,(CO),(PR,),] (ca. 30%), and [ReCl(CO),(PR,),](ca. 10%) are the major products obtained after productseparation and purification. The last product is almost certainlyproduced from reaction of initially formed [ReH(CO),(PR,),]with the solvent CH,C1,.20 The i.r. spectra confirm the facstereochemistry,6 which gives a spectrum quite different fromthat of mer-trans-[ReX(CO),L,] complexes [e.g. X = H,L = PPh,; v(C0) 1935s ~m-'].~ The synthesis of the facisomer, the kinetic reaction product, rather than the mer-transproduct which has been obtained by other workers from thethermal reactions between [Re,(CO),,] and PR,, reflects themild reaction conditions (40 "C) employed in this work as facisomers are known to isomerise readily to the mer-transisomers.' 'Since the complex [Mn,(CO),(AsMe,Ph),] has been shownby X-ray crystallography ' to have a diequatorial stereochem-istry [Figure l(c)], an attempt to prepare arsine derivatives of[Re,(CO),,] was made.The NMe,O-assisted reaction between[Re,(CO),,] and AsMe,Ph yielded only axial [Re,(CO),-(AsMe,Ph)] (74%) and fac-[ReCl(CO),(AsMe,Ph),] (15%)characterised by i.r. spectroscopy (Table 3) and elementalanalysis. No [Re,(CO),(AsMe,Ph),] was detected.The reaction of [Re,(CO),,] and PMe,Ph in the presenceof NMe,O in CH,CN rather than CH,Cl, yielded a creamproduct which analysed for [Re,(CO),(PMe,Ph),] [referred tohere as isomer (b)] but had an i.r.spectrum different from thethermally synthesised isomer diaxial [Re,(CO),(PMe,Ph),](Table 3). The similarity to the spectrum of axial-equatorial[Re~(CO)8(PPh,),]'o suggests that isomer (b) is axial-equatorial [Re,(CO),(PMe,Ph),]. Attempts to obtain crystalssuitable for X-ray crystallographic analysis, to confirm thisproposal, have so far been unsuccessful. The above reactionalmost certainly occurs via intermediate formation of di-equatorial [Re2(CO),(NCCH,),].Z2 Subsequent reaction withPMe,Ph results in displacement of the CH,CN ligands, stericconsiderations determining the axiakquatorial substitutionpattern (PMe,Ph, Tolman cone angle 122°).23Product Characterization.-It is significant to note thatsidebands are often observed in the i.r.spectra of diaxial[Re,(CO),(PR,),] complexes. For the related diaxial[Mn2(C0),(PR3),], Lewis et have ascribed some of theobserved minor v(C0) vibrations to the "CO isotopeabsorptions. However, this is unlikely to be the explanation forthe [Re,(CO),(PMe,Ph),] complex as the minor v(C0) bandsdisappear on recrystallisation. The i.r. spectra (CHCl,) recordedfor the crystals of [Re,(CO),(PMe,Ph),] used in the X-raystudy show no sidebands. An i.r. spectrum of a crude sample ofdiaxial [Re,(CO),(PMe,Ph),] indicated weak sidebands, andfurther, when the above complex was heated in toluene (80 "C, 4h) the sidebands [i.r. (toluene): 2 070w, 2 02Om, and 1 930(sh)cm-'1 grew in intensity and corresponded to those of anotherisomer of [Re,(CO),(PMe,Ph),] [isomer (b), see above].Attempts to separate the two isomers by t.1.c.were unsuccessful.A reinvestigation of the reaction between [Re,(CO),,] an2280 J. CHEM. SOC. DALTON TRANS. 1985Table 3. 1.r. spectral data for [Re,(CO),,-,(PR3),] (n = 1 or 2) and [ReCI(CO),(PR,),] derivativesComplex C(C0) "/cm-'[Re2(CO)dPPh,)]CRe,(CO),(PMePh,)lCRe,(CO),(PMe,Ph)I[Re,(CO),{P(CH,Ph),}][Re2(CO),(PMe,)l[Re2(CO)9 { P(OMe) 3 } 1tRe,(CO),(AsMe,Ph)I[Re2(C0)J3(PPh3)21 2 000(sh), 1 958vsCRe2(CO)dPMePh2)21 2015w, 1954vs[Re,(CO),{ P(CH,Ph),),] 2 OlO(sh), 1 954vs[Re2(C0)8(PMe3)21 2 000(sh), 1 943vs[Re,(CO),{P(OMe),),2 2 000(sh), 1 970vsCRe2(CO)8(PMe2Ph)21CRe2(CO),(PMePhd2I '[ReCl(CO),(PMePh,),][ReCI(CO),(PMe,Ph),][ReCI(CO),{P(CH2Ph),),1[ReCI(C0)3(PMe,)zl[ReCI(CO),{P(OMe),),][ReCl(CO),(AsMe,Ph),]2 113w,sp, 2 04Ow,sp, 1 998vs, 1 964(sh), 1 94Qm2 1 1 1 w,sp, 2 04Ow,sp, 1 995vs, 1 962(sh), 1 938m2 108w,sp, 2 038m,sp, 1 990vs, 1 960(sh), 1 932m2 1 lOw,sp, 2 034w,sp, 1 998vs, 1 962(sh), 1 938m2 108w,sp, 2 040m,sp, 1 990vs, 1 956(sh), 1 927m2 108w,sp, 2 04Ow,sp, 1 994vs, 1 952(sh), 1 930m2 11 2w,sp, 2 052m,sp, 1 998vs, 1 964w, 1 930mCRe2(CO),(PMe2Ph),I 2010w, 1949vs2 078mw,sp, 2 000m,sp, 1 974(sh), 1 964w,br, 1 932ms, 1 916s2 095w, 2 060m, 2 OOOs, 1 950s, 1 922m2 03Os,sp, 1 954s, 1 895s2 044s,sp, 1 957s, 1 896s2 024s,sp, 1 940s, 1 900s2 036s,sp, 1 950s, 1 894s2 04Os,sp, 1 970s, 1 910s2 OSOs,sp, 1 960s, 1 914s" Recorded in CHCI, unless otherwise stated. Recorded in hexane.Isomer (b) (see text; corresponds to the axial-equatorial isomer). ' Recorded inxylene.Table 4. Correlation between i.r. data and molecular geometry for [M,(CO),L,] (M = Mn or Re) complexesNumber ofv(C0) bandsIsomer a Ca1c.b Obs.'(a) Diaxial 2 2(b) Axiakquatorial 8 5(c) Equatorial-equatorial 8 5(6)(eclipsed) 8 4(5)(d) Axial-equatorial 8 5(staggered)(e) cis-Diequatorial 8 6(f) trans-Diequatorial 8Intensity pat tern Examplesw, vsw, m, s, s, sw, m, s,(sh), w, msw, m, s,(w), mm, vs, vs, w, m, m[I Mn,(CO),(PMePh ,),],d, ' - l9 [Re,(CO) ,( PMe,Ph) ,] d*e[Re2(CO),(PR3)2] (e.g. PR, = Bu' or Ph)9[Re,(CO),(CNC,H3Me,-2,6)2]d~2*3*J[Mn,(CO),(AsMe,Ph),] d*17*1[IM~,(CO),(CNBU'),]~*~~No examples knownm, s, vs, s, m CMn,(CO),(C4H6)I" See Figure 1.All complexes have the staggered conformation unless otherwise stated. Calculations were carried out assuming the ligand symmetryis ignored, i.e. local symmetry approximation holds. ' The number of bands may be affected by the solvent medium. Crystal structure determination.This work. J K. W. Lee, W. J. Pennington, A. W. Cordes, and T. L. Brown, Organometallics, 1984,3,404. M. L. Ziegler, H. Haas, and R. R. Skeline,Chem. Ber., 1965, 98, 2454.PMePh, using published procedures (xylene, 140 "C, 5 h)which was reported to yield the diequatorial isomer yielded acompound analysing as [Re,(CO),(PMe,Ph),] but with i.r.data also characteristic of the (b) isomer of [Re,(CO),-(PMePh,),], i.e.presumably the axiakquatorial isomer. Theabove data, together with recent studies by Nubel and Brown,"allow for a rationalisation of the data that have been publishedon the synthesis of [Re2(CO),(PR,), J complexes from[Re,(CO) o] and its derivatives. Electronically, equatorial COsubstitution by PR, is predicted and provided low-temperatureligand-substitution routes are used this is observed (e.g. bydisplacement of bridging ligands from [Re,(p-H)(p-olefin)-(co),] by PR,)." If, in the above reactions, the PR, ligandsare bulky the diequatorial isomer isomerises to an axial-equatorial isomer and this is observed for PR, groups of coneangle ca. 110-140".However, use of bulky ligands (cone angle ca.140") orreaction of [Re,(CO),,] with PR, under more forcingconditions yields diaxial [Re,(CO),(PR,),]. (This product isalso obtained from the NMe,O-assisted reactions in CH,Cl, orCHCl,' l ) . Formation of the axial-equatorial rather than thediaxial isomer when extended reaction times are used7 can beinterpreted in terms of ligand isomerisation. We tentativelypropose that this isomerisation occurs via metal-metal bondcleavage followed by ligand rearrangement on the radicalsfollowed by radical recombination. Consistent with this pro-posal is the formation of axial-equatorial [Re,(CO),(PR,),](PR, = PMePh,)' obtained by photolysis of [Re,(CO),,] andPR,. It thus appears that the size of the ligand as well as thesynthetic procedure employed determine the stereochemicaloutcome of the reaction between [Re,(CO),,] and PR,.Correlation of 1.r.Spectral Data with Stereochemistry.-Asurvey of the literature has suggested that a correlation existsbetween i.r. spectra (number and intensity of bands) and isomerstereochemistry for [M,(CO),,,L,] (M = Mn or Re, n = 1-4, L = ligand).' Herein we comment only on the disubstitutedproducts (n = 2). As shown in Figure 4, six disubstitutedisomers are possible (excluding different rotamers) and five ofthese isomers have been synthesised to date. Table 4 comparesthe expected number of bands calculated by Group Theory withthe number and intensity of the bands actually observed. ThJ . CHEM. SOC. DALTON TRANS. 1985 228 1C(2 1C(6)32.2 27.764.0 I 55.0C(4 1( C 1Figure 1.Newman projections of [Re,(CO),(PMe,Ph),]. (a) Viewdown the Re-Re bond with torsion angles indicated between bonds; (b)view down P( 1)-Re( ltRe-(2)-P(2) indicating CO groups, phenylrings, and methyl groups; (c) view down the P-Re bond indicatingtorsion anglesFigure 2. ORTEP l 5 diagram of [Re,(CO),(PMe,Ph),]Figure 3. Packing diagram of [Re,(CO),(PMe,Ph),] viewed down theb axisIL L Lpositions of the bands will be dependent on the metal, ligand,solvent, and degree of substitution, and care must thus beexcercised in predicting geometry from i.r. data alone. Ofinterest would be the determination of the structure of thepresently unknown [Re,(cO),(PR,),] to establish whether thePR, arrangement would be (a), (e), or ( f ) (Figure 1) andwhether the geometry could be predicted from the solution i.r.spectrum.Structure of Diuxiul [Re,(CO),(PMe,Ph),].-The molecu-lar structure of diaxial [Re,(CO),(PMe,Ph),] is shown inFigure 2, and an (010) projection of the unit cell is shown inFigure 3.Average bond-length and -angle data are summarisedin Table 5. The molecule adopts a staggered conformation, withdiaxial phosphine substitution as expected from i.r. data. Thisis in contrast to the structures of [Re,(CO),(CNC,H,Me,-2,6),] and [Mn,(CO),(CNBu'),] 2 5 where the two isonitrileligands occupy electronically favoured equatorial sites, and isan example of the dominance of steric factors over electronicsite preference in the case of the bulky phosphine ligand.Comparisons of the [Re,(CO),(CNC,H,Me,-2,6)2] and[Re,(CO),(PMe,Ph),] structures indicate that the groups onL * L( f )Figure 4.Possible isomers of [Re,(CO),L,]. In all cases astaggered ligand arrangement is indicatedthe isonitrile are much further removed from the Re atom thanthe groups on the P atom: adding average bond lengths for thelinear Re-CN (2.05 A), C=N (1.15 A), and N-C(R) (1.43 A)bonds gives an effective Re ..* C(R) distance of 4.63 A,compared to the average Re-P distance of 2.349(5) A. Thesmaller steric bulk of the isonitrile ligand is also reflected in the'fan-shaped' angles 26 for 2,6-Me2C,H,NC of 106" (wideness)and 53" (thickness). Axial substitution has also been observedin related [Mn,(CO),o,(PR,),] (n = 1; PR, = PMe,Ph;"n = 2; PR, = PMePh," or PEt,,') structures. In diaxial[Re,(CO),(PMe,Ph),] the Re-Re bond length is 3.044(1) A,similar to that in [Re,(CO),,].28 Comparison with theequatorially substituted [Re,(CO),(CNC,H3Me,-2,6),][Re-Re 3.047( 1) A] clearly indicates that the bond-length datado not reflect only electronic effects.The substitution of acarbonyl trans to the metal-metal bond by a poorer 7c-accepting 29 phosphine ligand, and the consequent increase inelectron density on the metal atoms, perhaps in antibondingorbital^,^' might have been expected to result in a significan2282 J. CHEM. soc. DALTON TRANS. 1985Table 5. Average bond length (A) and angle (") data for diaxialERe,(CO),(PMe*Ph),lRe-Re 3*044(1) P-Re-C 92.9(6)Re-C 1.94(2) R K - 0 1 7 7 3 18)P-C 1.83(2) Re-P-C 115.5(8)Re-Re-P 177.2(5) c-P-c 102.8( 11)Re-Re-C 87.2(5)Re-P 2.349(5) C-Re-C(cis) 89.9(8)increase in the metal-metal bond length.Such lengtheningeffects have been suggested for phosphine-substituted metalclusters, e.g. [Fe,(CO), 1(PPh3)],31 [Ru,(CO), 1(PPh3)],32but can be ascribed to ligand-packing effects, i.e. congestionbetween phosphine and carbonyl groups causes expansion ofthe metal core.33.34 Further, kinetic studies 35 have suggestedthat the Mn-Mn bond strength in [Mnz(CO)8(PPh3)z] is lessthan that in [Mn,(CO),,].The average Re-CO bond length is 1.94(2)& similar to that[1.976(6) A] for Re-CO trans to C o in [Re,(CO),(CNC,H,-Me,-2,6),]. The Re-P bond length is 2.349(5)A. Although thetwo sets of equatorial carbonyl groups are staggered withrespect to each other, the C-Re-Re-C torsion angles deviate byan average of 6.0" from the ideal 45" (see Figure 1).The conformation of the axial PMe,Ph groups with respectto the equatorial CO groups is interesting in that the phenyl ringof the axial phosphine group edipses a carbonyl ligand [Figurel(c)] with a torsion angle of 0.0" for C(l)-ReP-C(7).Theaverage Re-P-C and C-P-C bond angles of 115.5(8) and102.8(11)" and the average P-C bond length of 1.83(2) A aresimilar to those for the related [Mnz(CO)lo,(PR3),] (n = 1 or2) structure^.'^*'^*^^The a,P-Re-CO,q bond angles are obtuse [average92.9(6)"], ,,OC-Re-CO,, close to 90" [average 89.9(8)"], andthe Re-Re-CO,, are acute [average 87.2(5)"], i.e.theequatorial carbonyls bend in towards the Re-Re bond. Thiseffect of the bending in of the equatorial CO ligands towards themetal-metal bond is observed for related [Mn,(CO),,,(PR,),](n = 1 or 2) ~ o r n p l e x e s , ' ~ ~ ' ~ * ~ ~ as well as for the series[M,(CO),,,(CNR),] (M = Re, n = 1 4 ; M = Mn, n = 2 or4)12 and the parent carbonyls [Mz(CO),,] (M = Re or Mn).',Hence this phenomenon would appear to be an electronic effect,unaffected by steric considerations. Various explanations havebeen proposed to explain it, and Elian and Hoffmann36 havepredicted the effect on the basis of molecular-orbitalcalculations.[os3(co)l1 (P(oMe))31,33 and [0s6(c0)1 7{ p(0Me)3}41,34AcknowledgementsWe thank the University and the C.S.I.R. for financial assistanceand Mr.Jan Albain for crystallographic data collection.References1 M. 0. Albers and N. J. Coville, S. Afr. J. Chem., 1982, 35, 139.2 G. W. Hams and N. J. Coville, Organometaliics, 1985, 4, 908.3 G. W. Harris, J. C. A. Boeyens, and N. J. Coville, Organometallics,1985, 4, 914.4 G. Wilkinson, F. G. A. 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