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1978 673The Chemistry of Polynuclear Compounds. Part 30.l Some Reactionsof Dodecacarbonyltetrahydridotetraosmium : The Molecular and CrystalStructure of Trihydrido-~,-iodo-cyc/o-tetrakis(tricarbonylosmium)-(40s-0s)By Brian F. G. Johnson, Jack Lewis," Paul R . Raithby, George M. Sheldrick, and Kenneth Wong, Uni-Mary McPartlin, Department of Chemistry, The Polytechnic of North London, Holloway, London N7 8DBversity Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EWDodecacarbonyltriosmium reacts with H, under pressure to produce [Os4(CO),,H4] almost quantitatively.Methanolic K[OH] converts this into [Os,(CO),,H,] - which gives the title compound [OS,(CO)~,H,I] with ICrystals of [Os,(CO),,H,I] are monoclinic, with a = 17.307(8), b = 8.040(3), c = 17.760(9) 8, p = 11 9.80(2)';space group C2/c, and Z = 4.The final residuals for 2 724 unique observed reflections are R 0.036 and R' 0.039.The four 0 s atoms are in a ' butterfly ' configuration : one edge has been extended and a bridging iodine inserted[Os-I 2.749(4) 83. A crystallographic two-fold axis passes through the iodine atom and bisects the oppositeOs-0s bond. The Os-0s distances are 2.876(1) (two), 2.927(3), and 3.052(1) 8 (two) : the crystallographicsymmetry, l H n.m.r. spectrum, and geometrical arrangements of the carbonyls indicate that the three longest Os-0sbonds are hydride-bridged. Methanolic Na(0Me) converts [Os,(CO),,H,] into [Os4(CO),1(C0,Me) Ha] -,isolated as the [N (PPh,),] + salt.THE direct reaction of H, with [Os,(CO),,] at elevatedtemperatures and atmospheric pressure gives samplesof [Os,(CO),,H,] contaminated with [Os,(CO),,H,],[Os,(CO),,H,], and [Os,(CO),,], which are difficult topurify by standard techniques.We have developed animproved synthesis of [Os,(CO),,H,] from [Os,(CO),,]which has enabled us to investigate some of the chemistryof this elusive hydride. All attempts to grow crystalssuitable for X-ray diffraction have failed. The prepar-ation and single-crystal X-ray study of [Os,(CO),,H,I]provide the first structure determination of an Os,cluster and indicate a possible structure for [Os,(CO),,H,].KESULTS AND DISCUSSIONTreatment of [Os,(CO),,] with hydrogen gas at 100 "Cunder pressure (120 atm) t leads almost quantitativelyto [Os,(CO),,H,]. This compound dissolves slightly inmethanol to give a pale yellow solution.On stirringwith K[OH] more [Os,(CO),,H,] dissolves to give amore intensely coloured solution, from which the anion[Os,(CO),,H,]- can be precipitated as the "Me,]+ or[N(PPh,),]+ salts; these were characterised by C and Hanalysis, conductivity measurements, and i.r. and n.m.r.spectroscopy. The lH n.m.r. spectrum of the aniont Throughout this paper: 1 atin = 101 325 Pa; 1 eV z1.60 x 10-19 J.1 Part 29, C. R. Eady, B. F. G. Johnson, and J . Lewis, J . C . S .Daltow, 1977, 838.over the range 0 to -70 "C showed three signals at7 28.1, 28.7, and 30.8. At >0 "C coalescence occurs anda t ca. 30 "C a single broad resonance is observed. Thisbehaviour parallels that found for the rutheniumanalogue,, and suggests the presence of two isomers(Figure 1) at low temperature, with fluxional behaviourand rapid interconversion at elevated temperatures.Unlike the ruthenium analogue, no lH-lH coupling isobserved for [OS,(CO)~,H,]-~ Low-temperature i.r.spectra of CH,Cl, solutions were also consistent with thepresence of two isomers, but because of limited solubilitythe spectra were of poor quality.Treatment of [Os,(CO),,H,]- with I, (1 mol) in2 S.A. R. Knox, J. W. Koepke, M. A. Andrews, and H. 11.3 J. W. Koepke, J. R. Johnson, S. A. R. Knox, and H. D.Kaesz, J.'Amev. Chem. SCC., 1975, 97, 3942.Kaesz, J . Amer. Chem. SOC., 1975, 97, 3947674 J.C.S. Daltonmethanol gives [Os4(CO),,H,I]. The composition afterrecrystallisation was confirmed by C and H analysis andmass, n.m.r., and i.r.[v(CO)] spectroscopy. The l HFourier-transform n.m.r. spectrum at 0 "C showed twosharp singlets ( T 27.75 and 27.35) in the intensity ratio0 s (l)-Os (2)OS( 1)-Os( 2')Os(2)-Os(2')Os(1)-I( 1)Os( 1)-C( 11)Os( 1)-C( 12)0 s (2)-C( 2 1)OS( 1)-C( 13)TABLE 1Bond lengths (A) *2.876( 1)3.052(

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