J . CHEM. SOC. DALTON TRANS. 1985 605Carlo A. Ghilardi, Paolo Innocenti, Stefan0 Midollini," and Annabella Orlandinilstituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione,C.N.R., Via F. D. Guerrazzi27, 50132 Firenze, ItalyBy reaction of [Fe( H,O),] [ BF,], with 1,1,1 -tris(diphenylphosphinomethyl)ethane, tppme, andan excess of sodium tetrahydroborate, in boiling tetrahydrofuran, the red crystalline complex[ FeH (H,BH,) (tppme)] has been synthesized. Its crystal structure has been determined from counterdiffraction data [orthorhombic, space group Pn2,a, a = 20.803(7), b = 16.964(5), c = 10.242(3)A, and Z = 41 and refined by full-matrix least squares to R and R' of 0.042 and 0.041 respectivelyfor 1 191 reflections having I 3 3 ~ ( 1 ) .The iron atom is six-co-ordinated by the three phosphorusatoms of the ligand, two hydrogen atoms of the BH, group, and a hydridic hydrogen atom.Variable-temperature 'H and 31P n.m.r. studies have shown that at low temperature ( <253 K) themolecule is rigid also in solution. On raising the temperature, scrambling of the three metal-boundhydrogens occurs and the ligand phosphorus atoms, co-ordinated to the metal, becomemagnetically equivalent.The tetrahydroborate ion BH,- has been shown in variousstructural studies to bond to a wide variety of metal ions as auni-, bi-, or tri-dentate ligand.'., However, the use of n.m.r.spectroscopy to determine the type of M-BH, bonding insolution was generally unfruitful. Indeed the protons of thetetrahydroborate ligand have been found to be equivalent innearly all of these compounds, due to rapid equilibration onthe n.m.r.time-scale., Only recently some non-fluxionaltetrahydroborate complexes were r e p ~ r t e d . ~ In particular,[RuH(H,BH,)(bdpp)] [where bdpp is the linear-chain ligandPPh(CH,CH,CH,PPh,),] shows discrete H n.m.r. signals atroom temperature for the metal hydride, each of the twobridging protons, and the two terminal B-H protons. On raisingthe temperature, complete scrambling of the BH,- protonsoccurs.We now describe the synthesis, i.r. and n.m.r. spectroscopicand X-ray structural characterization of the new complex[FeH(H,BH,)(tppme)] [where tppme is the tripodal ligandCH,C(CH,PPh,),J. This complex, although isoelectronic with[RuH(H,BH,)(bdpp)], containing the same donor-atom set,shows quite different molecular dynamics in solution.A briefcommunication has been published previ~usly.~ExperimentalUnless otherwise stated, all reactions and manipulations wereperformed under a nitrogen atmosphere; solvents were purifiedby standard methods. Infrared spectra were measured on aPerkin-Elmer 283 instrument from 200 to 4000 cm-' usingNujol mulls. Phosphorus-31 n.m.r. spectra were recorded on aVarian CFT-20 spectrometer, equipped with a ' P probe whichoperated at 32.2 MHz. Chemical shifts are reported in p.p.m.relative to 85y0 H3P04, downfield values being taken aspositive. Proton n.m.r. spectra were obtained o n a Varian CFT-20 instrument equipped with a 'H probe which operated at 80t Hydrido(te1rahydroborato-HH')[ 1.1.1 -tris(diphenylphosphino-methy1)ethane-PP'P'Iiron.Supplenienlurj' duru uiuiluhle (No.SUP 561 34, 4 pp.): thermalparameters. See Instructions for Authors, J . Chem. Soc., Dalton Truns.,1985, Issue 1 , pp. xvii-xix. Structure factors are available from thee d i t o r i a l office.MHz, and a Varian XL-200 spectrometer of the University ofSiena. Selective phosphorus-decoupling experiments werecarried out on a Bruker WM 250 instrument of theEidgenossische Technische Hochschule (E.T.H.) (Zurich);SiMe, was used as internal standard.Prrpurution of [FeH( H,BH,)(tppme)].--A solution ofNaBH, (8 mmol) in ethanol (25 cm3) was added to a solution of[Fe(H,O),][BF,J, ( 1 mmol) and tppme (1 mmol) in tetra-hydrofuran ( t h o (40 cm3).The resultant solution was heated toreflux for 5 rnin to give a bright red solution and a white solid.The solution was filtered and the solvent was distilled off untilbright red crystals precipitated. The mother-liquor wasdecanted off and the crystals were washed twice by decantationwith ethanol; then they were filtered off, washed with lightpetroleum, and dried in a stream of nitrogen, yield 65%) (Found:C, 70.6; H, 6.4; Fe, 7.9. Calc. for C,,H,,BFeP,: C, 70.7; H, 6.35;Fe, 8.004)).The deuteriated complex [FeD(D,BD,)(tppme)] was pre-pared analogously, by using [Fe( D20)J[BF4],, NaBD,, andC,H,OD.Cr?~JtuIIO~~Tuf~?~.-The crystal used for X-ray data collectionwas an irregular prism of dimensions cu.0.10 x 0.30 x 0.40 mm.The unit-cell paranleters were obtained from a least-squares fitof 20 reflections, carefully centered on a Philips PW 1100automatic diffractometer.Crj-stul dutu. C4, H,,BFeP,, M = 696.4, orthorhombic,s ace group Pn2 u, u = 20.803( 7), h = 16.9b4( 5 j, c = 10.242( 3 j 1 L' = 3 614 A3, Z = 4, D, = 1.279 g cm ', F(OO0) = 1464.Data collection was carried out using the o 20 scantechnique and graphite monochromated Mo-K, radiationwithin 26 < 40 . After correction for background, the standarddeviation, o(f), of the intensity 1 was calculated as describedelsewhere5 using the value of 0.03 for the instability factor k .The intensity data were corrected for Lorentz-polarizationeffects and for absorption [p(Mo-K,) = 5.74 cm '3, thetransmission factors ranging from 0.95 to 0.87.Both the A f ' andA f " components of anomalous dispersion were included for allnon-hydrogen atoms.6 Of the total 1 954 reflections, 1 191 wereconsidered observed [f 3 30(1)].Solution und rrfrnement of'thr structure. The calculations were~ ( M o - K , ) = 0.7107 A606 J. CHEM. SOC. DALTON TRANS. 1985Table 1. Positional parameters ( x lo4) of complex (2)X-659(1)3 4 4 0 )- 544(2)-960(2)56(5)- 369( 5 )- 349( 5 )601(4)1 161(4)1 345(4)968(4)407(4)224(4)975(4)1 593(4)2 068(4)1 924(4)1 307(4)832(4)103(4)479(4)943(4)1033(4)658(4)367(6)598(6)Y-1 320*-1 278(3)- 146( 2)- 872(2)957(7)255(7)-271(7)579(7)- 203( 7)-1 899(4)- 1 751(4)- 2 260(4)-2 916(4)- 3 063(4)-2 555(4)- 1 566(5)- 1 249(5)- 1 503(5)- 2 074( 5 )-2 391(5)-2 137(5)3W4)71 l(4)8 15(4)237(4)- 444(4)Z626(2)1 224(3)- 160(3)2 579(4)2 947(11)2 252( 1 1)1 726(12)1 133(11)3 255(11)2 62 l(9)3 328(9)4 331(9)4 627(9)3 920(9)2 9 17(9)56(7)1 26( 7)- 740(7)- 1 676(7)- 1 746(7)- 880(7)- 1 363(7)- 1 377(7)- 2 343(7)- 3 296(7)-3 282(7)Chosen to be in accord with the isormorphous complex [Cu(BH,)(tppme)].r193(4)-1 219(4)- 1 291(4)- 1 799(4)- 2 234(4)-2 162(4)- 1 654(4)- 1 1 16(4)- 857(4)- 1 008(4)- 1 417(4)- 1 676(4)- 1 526(4)- 1 702(4)- 1 825(4)-2 368(4)-2 789(4)-2 666(4)- 2 123(4)-1 317(12)- 1 349(51)- 1 197(42)- 1 704(63)- 46343)- 775(53)L’- 548(4)317(5)1 134(5)1490(5)1 029(5)212(5)-1 545(4)- 144(5)-1 431(4)- 1 952(4)-2 588(4)- 2 702(4)-2 181(4)- 2 17(5)147( 5 )626( 5 )687(5)269( 5)- 210(5)-2 261(14)- 1 484(75)-2 281(62)-2 661(54)-2 341(76)- 1 582(52)-2 316(7)- 1 030(8)- 997( 8)- 1 664(8)-2 365(8)-2 398(8)- 1 731(8)3 953(7)5 195(7)6 203(7)5 969(7)4 727(7)3 719(7)2 734(7)3 880(7)3 9JO(J)2 914(7)1 767(7)1677(7)162(25)54( 100)510( 102)846( 126)- 654(92)-624(95)carried out using the SHELX 76 system of programs’ on a SEL32/77 computer.Atomic scattering factors for non-hydrogenatoms were taken from ref. 8, those for hydrogen atoms fromref. 9.On account of the isomorphism with [Cu(BH,)(~ppme)],’~the final parameters of the copper complex were used asstarting parameters for the present iron one.The functionminimized was Zw(IF,,I - IFJ)’, where wj = 1/o2(F0). Severalcycles of full-matrix least-squares refinements using anisotropicthermal parameters for the iron and phosphorus atoms andisotropic ones for the remaining non-hydrogen atoms loweredthe conventional R factor to 0.060. Throughout the refinementthe phenyl rings were treated as rigid bodies of D,,, symmetry.The hydrogen atoms of the methyl group of the tppme ligandwere located from a AF Fourier map, while the remaininghydrogen atoms of this ligand were introduced in calculatedpositions (C-H 0.95 A). All the parameters of these hydrogenatoms were kept constant throughout the refinement.Owing tothe polar nature of the space group, the absolute configurationwas determined by refining the two possible configurations,which gave R values of 0.046 (x, y, 2 ) and 0.047 (X, j , 5 )respectively. At this point a Fourier difference map, calculatedusing the reflections having sinO/A < 0.35 A-’, revealed thepositions of the tetrahydroborate hydrogen atoms and thehydridic hydrogen among the first six peaks, whose electrondensity ranged from 0.7 to 0.5 e A-3. Several full-matrix least-squares cycles, in which the tetrahydroborate hydrogen and thehydridic hydrogen atoms were refined isotropically togetherwith all the other parameters, using the entire set of data, led toconvergenceat R ( = C IIFoI-IFcII /CIFoI) and R ‘ [ = C~i(lF,ol-IFCI)’/CwFo2] of 0.042 and 0.041 respectively.The largestshiftlerror for the hydrogen atom parameters in the final cyclewas 0.2. Final positional parameters with their estimatedstandard deviations are given in Table 1.Results and DiscussionAs previously reported,I ’ iron([]) salts such as FeCI,, FeBr,,and [Fe(H,O),][CIO,], react, in the presence of tppme, withsodium tetrahydroborate to yield the dimeric blue hydride[(tppme)Fe(p-H),Fe(tppme)] + (1) [equation (l)]. We havenow found that an analogous reaction, using the tetrafluoro-borate salt [Fe(H20)6][BF4]2, produces only the red com-plex [FeH(BH,)(tppme)] (2) [equation (2)]. Reaction (2) isfavoured by an excess of NaBH, and high temperature, but theuse of the tetrafluoroborate salt seems to be the decisive factorJ.CHEM. SOC. DALTON TRANS. 1985 607Table 2. Selected bond distances (A) and angles (")Fe-P( 1) 2.175(3) Fe-BFe-P(2) 2.162(4) B-H( 1 )Fe-P(3) 2.230(4) B-W)Fe-H(l) 1.58(11) B-H(3)Fe-H(2) 1.65( 10) B-H(4)Fe-H(5) 1.41(9) P(1 K ( 3 )P( 1 )-Fe-P( 2)P( 1 )-Fe-P( 3)P(2)-Fe-P(3)P( I )-Fe-H( 1 )P( 1 )-Fe-H(2)P( 1 )-Fe-H(5)P(2)-Fe-H( 1)P( 2)-Fe-H( 2)P(2)-Fe-H(5)P(3)-Fe-H( 1 )P(3 )-Fe-H(2)P( 3 )-Fe-H( 5)H( 1 )-Fe-H(2)H( 1 )-Fe-H(5)88.2(2)90.3( 1)92.9( 1 )170(4)lOl(4)90(4)97(4)154(4)85(4)98(4)11 l(4)178(4)71(5)82(5)2.16(2)1.32( 12)1.1 8( 10)1.10(9)1.08( 13)1.860( 12)71(5)134.0(7)132.1(6)105.7( 7)96(7)98(7)96(7)9 W )90(7)121(9)126(9)1 12.4(4)124( 7)1.8 56( 9)1.842(9)1.843( 12)1.849(9)1.840( 9)1.840(11)1 18.4( 3)119.5(3)103. I( 5 )102.7(4)98.3(4)1 1 1.6(4)118.5(3)119.3(3)103.1 ( 5 )102.4(4)99.q 4)110.7(4)1 2 1.6( 3)1.841(8)1.857(9)1.53(2)!.54(1)1.55( 1)1.54( 1)!19.8(3)102.5( 4)101.8( 5 )97.4(4)107.8(9)1 08.0( 9)108.3( 10)1 1 1.4( 10)1 1 O.O( 9)1 1 1.2(9)1 I5.0(8)114.0(8)! 16.0( 8)Figure 1.Perspective view of the complex [FeH(H,BH,)(tppme)J.ORTEP drawing with 30% probability ellipsoidsIndeed, experiments under different conditions (temperature,solvent, molar ratio of the reagents) have shown that complex(2) can be obtained only when iron(ir) tetrafluoroborate is usedas starting material, whereas compound (1) can not be obtainedfrom this salt.Complex (2), which is diamagnetic and very air sensitive, ismoderately soluble in methylene chloride and slightly soluble intetrahydrofuran, benzene, and toluene.It does not react withcarbon monoxide at room temperature and atmosphericpressure.Description of the Structure.-The molecular structure ofcomplex (2) consists of discrete [FeH(H,BH,)(tppme)] mole-cules, shown in perspective in Figure 1. Selected bond distancesand angles are given in Table 2.The metal atom is co-ordinated by the three phosphorusatoms of the tpprne ligand, by two bridging hydrogen atoms ofthe BH,- group, and by an hydridic hydrogen atom in adistorted octahedral environment.The bridging hydrogenatoms and the hydride atom are each trans to the threephosphorus atoms of tppme, the corresponding axial anglesbeing significantly different; while the angle involving thehydridic hydrogen, 178(4)", is very close to 180°, the other twoangles involving the bridging tetrahydroborate hydrogens,owing to the BH,- bite angle, differ significantly from theideal value [ 170(4) and 154(4)"]. Moreover, the reduced stericrequirements of the hydride with respect to the other ligands isevidenced by the displacement of the metal atom (0.25 A) fromthe equatorial plane, the four basal ligands being bent towardsthe hydride atom.The BH4- ligand is definitely bidentate, as demonstratedby the Fe-B distance of 2.16(2) A. This value is fully comparablewith those reported for 3d metal complexes where the tetra-hydroborate anion acts as a bidentate ligand; M-B separationsranging from 2.14(1) to 2.21(3) A have been reported forc ~ b a l t , ' ~ " ~ nickel,', and copper com lexes.In this contextthe value of the Cu-B distance [2.44(2) R 3 in the isomorphouscopper derivative [Cu(BH,)(tppme)]," where the BH4- actsas a unidentate ligand, is of interest.The Fe-H, (b = bridging) bond distances are 1.58( 11) and1.65(10) A, while the Fe-H, (t = terminal) bond distance,[1.41(9) A3 is shorter than the usual values (1.49-1.57 A)reported for such distances. ' Although this difference may notbe significant considering the large values of the standarddeviations, the stronger attachment of the terminal hydrogenligand is supported by the lengthening of the trans Fe-P bonddistance with respect to the other ones which are cis to thehydridic hydrogen ligand [2.230(4) us. 2.175( 3) and 2.I62(4) A].As regards the BH4- group, the B-H, distances [1.32(12)and 1.18( 10) A3 are, as expected, longer than the terminal ones[1.10(9) and 1.08(13) A].Considering the isomorphous five-co-ordinated cobalt ' andfour-co-ordinated copper derivatives, containing a bidentateand a unidentate BH4- respectively, noteworthy are theaverage values of the P-M-P angles, which take account of thesteric hindrance of the ancillary ligand [P-M-P(av.) 90.5(present compound), 91.5 (five-co-ordinated cobalt complex),94.0" (four-co-ordinated copper complex)].f.r.Spectra.-The i.r. spectrum of complex (2) (Nujol mull)shows a strong doublet at 2 380-2320 cm-I due to B-H,stretching, a broad strong band at 1440 cm-' due to bridgedeformation, and a strong absorption at 1 182 cm-' due toB-H, deformation.' Moreover, there is a broad medium band at1 910 cm-' which can be assigned either to B-Hb stretching orto the iron-hydride stretching. We prefer the first assignmentbecause of the shape of the band; those for metal-hydridestretching vibrations are generally sharp. The spectrum of thedeuteriated derivative of (2) shows two medium absorptions a608 J. CHEM. SOC. DALTON TRANS. 1985I T I K3133012831 253I 1 1 I # I 1 I * , I , * ' , l , , , , l150 100 50 0 - 5 061PPmFigure 2. The 32.2-MHz " P - ( I H ) n.m.r.spectra of complex (2) in[2H,]tetrahydrofuran over the temperature range 253-328 K1 792 and 1 710 cm-' due to v(B-D,) and a medium absorptionat 1 060 cm-' which can be assigned to bridge deformation. Theother absorptions due to the BH,- and BD,- anions and tothe Fe-H and Fe-D groups are either very weak or notdistinguishable from those of the ligand.N . M . R . Specrrcr.-The n.m.r. spectra of complex (2) appearcomplicated by resonances due to impurities, either present inthe starting complex or arising from its decomposition particu-larly at higher temperatures. The poor solubility of the complexobviously enhances this problem.The low-temperature ( < 253 K ) 3 1 P-{ ' H 1 and 'H n.m.r.spectra demonstrate a rigid stereochemistry as usually expectedfor six-co-ordinated molecules.The 3 ' P-( 'H). spectrum in[ZH,Jtetrahydrofuran, at 253 K (Figure 2), essentially consistsof an A2B pattern (the spectrum is practically first order) with adoublet at 79 p.p.m. and a triplet at 35.5 p.p.m. [intensity ratio2: 1, 'J(P-P) = 28 Hz]. As far as we know, this is the first timethat the three phosphorus atoms of the tppme ligand, all co-ordinated to a metal centre, have appeared non-equivalent onthe n.m.r. time-scale, even if at low temperature. For instance,the six phosphorus atoms in the dimeric hydride [(tppme)-Fe(p-H),Fe(tppme)] + are magnetically equivalent also at 183K.I6The low-temperature 'H n.m.r. spectrum in CD,Cl,, in thehydridic hydrogen range (upfield of SiMe,), shows a pseudo-quartet at -5.1 p.p.m.( 1 H) and a singlet at - 14.7 p.p.m.(2 H) (Figure 3). Also present is a weak broad absorption at-12 p.p.m. Since the relative intensity of this signal changeswith the sample, this absorption must be attributed to animpurity present in the starting complex. The multiplet at 6-5.1 p.p.m. is an example of an A,BX pattern and can besimulated using the parameters 'J(A-X) = 54.34, 'J(B-X) =51.26, and 'J(A-B) = 28.0 Hz. Selective decoupling of themagnetically different phosphorus nuclei at 223 K (Figure 4)shows that the hydridic hydrogen atom is coupled to thephosphorus atoms of the ligand. All the other absorptions areunaffected by phosphorus decoupling. Downfield of SiMe, the- T I K306J------e296A255 -I I 1 1 I I0 - 5 -10 -15 - 20 - 256 1 P P mFigure 3.The 200-MHz ' H n.m.r. spectra (hydridic hydrogen region) ofcomplex (2) in CD,CI, over the temperature range 255-306 Kspectrum shows multiplet patterns typical of the tppme ligand,centred at 6 7.30 and 6.90, 2.20 (6 H), and 1.30 (3 H) p.p.m.,assigned to the C,H5 (first two), CH,, and -CH, protonsrespectively . No other absorptions unambiguously attributableto complex (2) were detected.* The absorption at 6 - 14.7 p.p.m.is assigned to the two bridging protons, whereas the multiplet at6 - 5.1 p.p.m. is assigned to the unique hydridic hydrogen. Thevalues of these chemical shifts do not agree well with the morefrequently reported values for M-H-B (6 = -3.9 to -9.1p.p.m.) and for M-H (6 = - 10 to - 120 p.p.m.).' ' Howeverthe intensities of the two signals preclude a reversal of theirassignments.The resonance due to the two terminal BH,protons, expected in the range 6 = 3.9--7.8,3.18 is probablybroad and obscured by the ligand signals. The magnitudes ofthe 'J(P-H) values (54.3 and 51.3 Hz) are intermediate betweenthe ranges of ' J ( P-H) values reported for a cis ( 6 3 2 Hz) and atruns geometry (73 H Z ) . ~ ~On raising the temperature the , ' P and '14 n.m.r. spectra (forthe latter, the region upfield of SiMe,) change. The 3 1 P - [ 1 H )n.m.r. spectra are shown in Figure 2. As the temperature is* It is possible that some broad absorption due to complex ( 2 ) is presentin this range. With our spectrometer, which operates at 80 MHz, owingto the presence of the ligand resonances and signals of impurities, i t isvery difficult to identify such absorption. On the other hand, a fewspectra collected on higher field spectrometers in other laboratorieswere not totally consistent.Indeed, because of the high reactivity of (2),a spectrum can be considered well determined in this range only afterexperiments on different, freshly prepared, samples. Proton n.m.r.spectra with ' 'B decoupling might be useful in clarifying this problem.t These results are not in accord with the molecular structure ofcomplex ( 2 ) . the hydridic hydrogen being [runs with respect to onephosphorus atom and ci.s with respect to the other twoJ . CHEM. SOC. DALTON TRANS.1985 609AFe-H hydrogens. Because of rapid scrambling of the threemetal-bound hydrogens, the three phosphorus atoms becomemagnetically equivalent. A dynamic interchange of the fourBH, hydrogens with hydridic hydrogen was previouslyobserved in the complex [ZrH(BH,)(C,H,),].'9 At hightemperature the two terminal BH, hydrogens of complex (2)may also be involved in the dynamic process.ConclusionsThe complex [FeH(H,BH,)(tppme)] displays a distorted octa-hedral geometry, the distortion being mainly due to the shortbite of the bidentate BH,- ligand. The 'H and 31P n.m.r.spectra show a rigid stereochemistry in solution, at tempera-tures 6253 K. At higher temperatures, in solution, a fastinterchange between the Fe-H and bridging hydrogens occurs,so that the phosphorus atoms become magnetically equivalent.It is noteworthy that the isoelectronic complex [RuH-(H2BH2)(bdpp)J3 having the same donor-atom set, shows adifferent dynamic behaviour, where the single metal-boundhydrogen is not involved.The different anchoring mode of thetwo triphosphines, i.e. the tripod-like tppme and the linearbdpp, probably strongly affects the molecular dynamics.I 1 I Acknowledgements- L - 5 - 6 Thanks are expressed to Dr. N. Niccolai (University of Siena)6 / p . p . mfor recording o f some 'H n.m.r. spectra, Professor P. Pregosin(E.T.H., Zurich) for selective phosphorus-decoupling:experiments, Mr. F. Cecconi for technicai assistance i n thediscussions*Figure 4* The Fe-H region Of the 250-MHz proton n.m.r.Of synthesis of the complexes, and professor A. Vacca for helpful complex (2) in CD,CI,, at 223 K, without 3 ' P decoupling (c) and withselective decoupling on the two equivalent phosphorus atoms (a) andthe unique phosphorus atom ( h )increased above 253 K the resonances at 79 and 35.5 p.p.m.broaden and collapse. Above 3 13 K these two signals disappearand a broad resonance at 6 62.5 p.p.m appears. This trendindicates that at high temperature the three phosphorus atomsare magnetically equivalent. Moreover at 268 K a new species ofunknown composition is formed with an absorption at cu. 6 63p.p.m. On raising the temperature this signal becomes andremains sharp. At 301 K, a singlet at 6 -25 p.p.m., due to freetppme, appears, and upon addition of ligand to the solution thisabsorption is enhanced.* Also on raising the temperature thisresonance remains sharp.At temperatures lower than roomtemperature the equilibrium appears completely reversible,whereas at higher temperatures the spectral behaviour is notcompletely reversed. Indeed the absorptions due to the unco-ordinated ligand and to the unknown species remain uponlowering the temperature. These findings indicate that above300 K also a new, probably intermolecular, process occurswhich yields a new species. This process, when highertemperatures are reached, became irreversible.Concerning the 'H n.m.r. spectra (Figure 3), o n raising thetemperature above 255 K, the resonances at 6 -5.1 and - 14.7p.p.m. broaden and the multiplet collapses.At 306 K only a verybroad signal at 6 - 14.7 p.p.m. is still detectable. O n raising thetemperature the broad resonance at 6 - 12 p.p.m. remainspractically unchanged both in intensity and in shape. Itsintensity seems to increase only at higher temperatures. Thisspectral behaviour is also reversed on lowering the temperature.All these results can be explained by taking into account arapid, predominantly intramolecular exchange of bridging andReferences1 R. G. Teller and R. Bau, Strucf. Bonding (Berlin), 1981. 44, 3.2 T. J. Marks and J. R. Kolb, Chem. Rev., 1977,77,263 and refs. therein.3 J. B. Letts, T. J. Mazanec, and D. W. Meek, J. Am. Chem. Soc., 1982,104. 3898 and refs. therein.4 C. A. Ghilardi, P. Innocenti, S. Midollini, and A. Orlandini, J.Organornet. Chern.. 1982, 231, C78.5 P. W. R. Corfield, R. J. Doedens, and J. A. Ibers. fnorg. Chem., 1967,6, 197.6 'International Tables for X-Ray Crystallography,' Kynoch Press,Birmingham, 1974, vol. 4, p. 149.7 SHELX 76, System of Computing Programs, G. M. Sheldrick,University of Cambridge, 1976. adapted by C. Mealli.8 Ref. 6, p. 99.9 R. F. Stewart, E. R. Davidson, and W. T. Simpson, J. Chern. fhys.,1965.42, 3 175.10 C. A. Ghilardi, S. Midollini, and A. Orlandini, fnorg. Chem., 1982.21,4096.1 1 P. Dapporto, G. Fallani, S. Midollini, and L. Sacconi, J. Am. Chem.Soc., 1973,95,2021; P. Dapporto, S. Midollini, and L. Sacconi, tnorg.Chern., 1975, 14, 1643.12 P. Dapporto, S. Midollini, A. Orlandini, and L. Sacconi, fnorg.Cheni., 1976, 15, 2768.13 M. Nakajima, T. Saito, A. Kabayashi, and Y. Sasaki, J. Chtm. Soc.,Dalton Tran.y., 1977, 385.14 T. Saito, M. Nakajima, A. Kobayashi, and Y. Sasaki, J . Chem. Soc.,Dalton Trans., 1978, 482.15 S. J. Lippard and K. M. Melmed, fnorg. Chcm., 1967, 12, 2223.16 P. Innocenti and S. Midollini, unpublished work.17 J. P. Jesson, 'Transition Metal Hydrides.' Marcel Dekker, New York,18 S. W. Kirtley, M. A. Andrews, R. Bau, G. W. Grynkewich, T. J.Marks, D. L. Tipton, and B. R. Whittlesey, J. Am. Chem. Soc., 1977,99, 7 154.1971, p. 75.19 T. J. Marks and J. R. Kolb, J. Am. Chem. Soc.. 1975, 97, 3397.* The addition of an excess of ligand seems to stabilize complex (2) insolution. Receiued 9th Junuury 1984; Puper 4103