Edward Maslowsky, Jr. Loras College Dubuque, Iowa 52001
The Structures of Metal-Cyclopentadienyl Derivatives
Metal derivatives o f cyclopentadiene can be classified as either ionic cyclopen tadienides or covalent cyclopen tadienyl. Although these structural extremes are common, other structures also have been found; some possess dynamic structural aspects. The isolation of his(cyclopentadienyl)iron or ferrocene by tsvo independent moups (1,2) over a quarter of a century ago contributed significantly to a rapid development of organometallic chemistry. The countless metal derivatives of cyclnnentadiene that have since been characterized have been - - ~ placed in either the ionic cyclopentadienide or covalent cyclnoentadienvl suberouos. - . The covalent structures are commonly described using the hapto (designated as 7 ) nomenclature system proposed by Cotton (3). In most texts, the monohapto (7') or 0 type structure (I) and the pentahapto (q5) or r type structure (11) ~
~
~.~ ~
are descrihed as common for covalent cyclopentadienyl derivatives. Although these structural extremes are common, other structures have also been found for several covalent cyclopentadienyl compounds. In addition to characterizing cyclopentadienyl compounds in terms of static structural extremes, there is a dynamic structural aspect associated with several of these compounds. The following is intended as a discussion of the static and dvnamic structural features determined for metal derivative; of cyclopentadiene. Although the examples discussed are mainly neutral ryclopentadienyl Edward Maslowsky, Jr., chairman of the chemistry department at Loras College, received his B.S. from Canisius College in Buffalo, New York, in 1965 and his PhD from Illinois Institute of Technology in 1970. He pursued postdoctoral studies at the University of Guelph and Princeton University and then spent two years on the faculty of Rochester Institute of Technology before going to Loras College in 1974. His research has centered around the use of infrared and Raman spectroscopy to study the structures and bonding of organometallic and inorganic coordination compounds. He is also Edward Maslowsky, Jr. interested in the development of Loras College new teaching techniques and media for college level courses. He has recently published a book on "Vibrational Spectra of Organometallic Compounds." Professor Maslowskv's . naner . . is the third in a series of Resource Pnperi, intended primarily for college and university kachcrs. The publication of rhrs reries is supported m pan try a grant fmm the Reiedrch Corporation. ~
~
276 / Journal of Chemical Education
comnounds. i t has proved necessarv in some instances to cite as examplei compounds in which iigands other than the cyclopentadienyl ring are also bonded to the metal atom. lonlc Compounds Since there is no rigid dividing line between ionic cyclopentadienide and nonionic cyclopentadienyl compounds, i t is difficult to place some compounds with certainty into either suherounine. " . The derivatives of the heavier group Ia elements, that is CsHsM (M = K, Rh, Cs), have been characterized as having the ionic cyclopentadienide structure (46). While a structure consistine of ion pairs has also been found for 1M tetrahvdrofuransolutions of CsHsLi and C5H5Na (7), in the solid state, there is a small covalent contribution to the interaction between the metal cation and the delocalized cyclopentadienide anion (6). Early solid state studies of the heavier group IIa element derivatives, namely (C5H5)~Sr and (C5H5)~Ba, have indicated the presence of an ionic structure (8).While the bonding in solid (CsH&Mg is highly ionic, there is a degree of nonionic character in the magnesium-ring interaction (9).Spectral data for a tetrahydrofuran solution of (CsH&Mg, however, indicate the oresence of ion triolets (7). . . The solid state structures of ( c ~ H & B ~and ( c ~ H & c ~are nonionic and unrelated to those of each other or to that of (C5Hd9Me.and are discussed
- --- ...
Of the croup " - IIIa element derivatives. hiehlv ionic bonding
has been proposed to be present for C ~ H ; Tboth ~ vapor (10j and solid state (11, . 12) . studies as well as for CsHsIn . . in the solid state (12). have The phvsical and chemical roper ties of (CsHAMn .been reported to he consistent-with an essentially ionic structure (13). This is the only derivative of a first row transition element for which such a structure has been proposed. Highly ionic structures have been proposed for the dicyclopentadienide (14-16) and tricyclopentadienide (15-17) derivatives of the lanthanide elements. A single crystal X-ray study of (C5H5)3Sm(18) has shown this compound to he an excention since hrideine cvclo~entadienvlrines are oresent in the solid state. Although a higher beg& of covalent bonding is found in actinide than in lanthanide derivatives, it has been concluded that actinide tricyclopentadienide compounds also have basically ionic structures (16). ~
Monohapto Cyclopentadienyl Rings Although monhapto cyclopentadientyl rings are found in several compounds that have other ligands bonded to the central metal atom, few compounds containing only monohapto cyclopentadienyl rings have been fully characterized. The most thoroughly studied monohapto derivative has been
In dicyclopentadienyl derivatives with parallel rings, the rings may be staggered or eclipsed relative to each other. (C5H5)zHg. The studies have included mainly solution and solid state vibrational data (19,20). Although the vibrational data are consistent with the presence of monohapto rings, the relative orientation of these rings has not been determined. Two extreme orientations are conceivable, namely transoid (111) and cisoid (IV).
The most direct method of determining the relative orientation of the rings and the nature of the mercurv-cvclonentn~, dienyl ring int&wtion in the solid state would be through the use of sinale crvstal X-rav diffrnctiun. LtnfortunateI\f. .. single crystals of (c;H~)zH~are light-sensitive. A recent single crystal X-ray study of the complex [(C6H5)3PC5H4HgI2jzr however, has confirmed that the mercury atom is honded to the cyclopentadienvl rina in a monohapto fashion (21) in this corn&x:lnt'rared data Cswd for solid (C5H5j3AI(6)and infrared and proton nmr data for rGH..hAs . .. (22) havealso Open interpreted as consistent with those expected for the monohapto structure.
.
~
rings, one of which is coordinated to the calcium atom in a trihapto manner (28). Pentahapto Cyclopentadienyl Rings Compounds with a covalent, pentahapto cyclopentadienyl ring have been further subdivided by one investigator (6) into centrally a bonded and "genuine" or centrally a bonded structures. The former group includes principally complexes of the main group and f-block elements and also some d-block elements in all of which the metal-ring interaction is relatively weak. The latter group includes complexes of the d-block elements with relatively strong metal-ring interactions. The basis for this division was that the centrally a honded derivatives, except for those of relatively light metals, showed no hands above 250 cm-I that could he attributed tometal-ring skeletal modes. I t has been noted, however, that this division is artificial and that a strict classification of all pentahapto compounds into one of these two subgroups is not always possible. In fact, i t has been reported (29) that the infrared spectrum of (C&)zGe in benzene-d6 and methylene chloride solutions exhibits hands whose frequencies fall within the ranges expected from centrally a and centrally s honded cnmnonnda. --- The two rings of pent ah apt(^ diryclopentadien\4 rornplexes may he nonparallel ~ V or I parallel (V1j with respect to each other.
. ~ ~ ~ ~ ~ - ~
Dlhapto Cyclopentadienyl Rings Spectral data for (C5H&Ti have been interpreted in terms of a pentahapto structure (6.23) or a structure with one monohnptr~andtwo pentahapto rings ( 2 1 ) .A single crystal X-ray studs of rCr,H-):;I7 has now shown. however. that althoueh in ~the solid state there are two pent&apto rings, the thirdUring is honded to the titanium atom through only two adjacent carbon atoms (25). An electron diffraction study of C5HsAI(CH& vapor has been interpreted as showing that this compound is monomeric and that the cyclopentadienyl ring is honded to the aluminum atom in a dihapto fashion (26,27). In the solid state, however, an associated structure in which each (CH&AI unit is bonded to two fi-($:11-C5H5) rings has been proposed for CsH5Al(CH& (26,27). ~
Trlhapto Cyclopentadienyl Rings An ionic structure had originally been proposed for solid (C5Hs)zCa (6). A single crystal X-ray study of (C5H5)~Ca, however, has shown the presence of a polymeric structure in which each calcium atom is bonded to four cyclopentadienyl
The nonparallel structure has been found for hoth (C5H5)zSn and (CsHdzPb in the vapor phase using electron diffraction data (30). Structural data reported (3140) for the dicyclopentadienyl derivatives of the first row transition elements listed in the table are consistent with the presence of parallel rings. Similar structures have been found in electron diffraction and single crystal X-ray studies of hoth (CSH5)zMg 132,41,42) and (CsHJzRu (34,43). The table also includes the electron configurations determined (44,451 for the highest molecular orbitals of the dicyclopentadienyl derivatives of the first row transition elements. For the compounds listed in the table, attempts have bern reported to relate the electronic strilcture, structural data, and strength of the metal.rina interaction. Molecular orbital calculations reported-for
Electron Conflguratlons (44, 45) and Structural Data for Known Dlcyclopentadienyl Derlvatlves of the First Row Transltlon Elements In the Vapor and Solld States
Compound
Phase
(CsHs)rV (CsHr12Cr
Vapor Vapor Vapor Vapor
(CsHskMn (CsH&Fe (CsH,kCo (CsHMi
MefaLCarbon Bond Length
C, Ring C H Band
(A)
Angleb
Refs.
2.280
c
2.169
2.9 (1.lI0
0
2.383 2.064 2.045
3.7 (0.9)O
I
2.119
2.1 (0.8)'
2
2.096 2.196
c
31 31.32 33 32.34 35 .36.37 38 39.40
Elechon Configuration
Electron Imbalance*
alDi+g2 atD' e2113 a*.' esg2eqg2
3 2 5
algZe2g'
Solid vapor Solid
alo2e2,'
Vapor
ale2 egg4 ejg2
el,'
d
m e eienron imbalance b detinea In Ref. (30as the number of holes in me e,,and erg orbitals plus the number of electrons in me el,orbnsls. This angle is defined as being positive uhen
G H band is hem M of me Cs plane toward the metal awn.The numbers in parentheses equal me estimated standad deviations of
me angles. "Deviation olthe U1bond trom me csplane was judged to be inelgnificant.
40verlapping of peeks in the radial di3hibution curve gave a very unreliable value of 6.5 (1.4)'.
Volume 55, Number 5, May 1978 / 277
(C5H5)~Fe(4648) agree that the highest completely filled orbitals are weakly bonding alp and ezgorbitals and that the lowest empty orbital is the antibonding el, orbital. Assuming that removal of an electron from the bonding alp o r e % orbitals weakens the metal-ring interaction to the same extent as the introduction of an electron into the antibonding el# orbital permits the definition of an "electron imbalance" (31) as the number of holes in the a,. and e?, orbitals olus the number of electrons in the el, Gbitals.% follows that the electron imbalance is inversely proportional to the strength of the metal-ring interaction. From the data in the table, it is also observed that the electron imbalance is directly proportional to the metal-carbon bond length and inversely proportional to the angle formed when the C-H bond is bent out of the plane formed by the Cs ring toward the metal atom. Therefore, of the complexes in the table, (CsH5)~Feis expected to have the strongest metal-ring interaction since it has an electron imbalance of zero, the shortest metal-carbon bond length, and the largest angle between the C-H bond and the plane of the C5 ring. For dicyclopentadienyl derivatives with parallel rings, the rings may be staggered (VII) or eclipsed (VIII) relative to each other.
vapor state (54). The distorted structures found for ( C a 5 ) ~ B e have been explained (50,52,53) using a steric argument in which it is assumed that the small size of the beryllium atom causes the rings to be brought too closely together, thereby introducing strain that is relieved through the adaption of a structure with lower symmetry than that found for the other pentahapto dicyclopentadienyl compounds. More recently, i t has also been areued (55) that the structure of (C4H&Be . " might possibly arise since there are four too many electrons for the number of available metal-ring bonding orbitals in the undistorted structure, and that this excess is relieved by a structural distortion. Studies have been reported of a green form of titanocene, (CsH&Ti (56). and the zirconium (57) and hafnium (58) . analogs of titanocene in which it was concluded that these three com~oundshave a structure with oarallel.. . oentaha~to rings. he green compound with t h e empirical formha CloHlaTi, however, was observed (59) to be dimeric and a proposed structure (59) that contains a fi-($:q5-fuIvalene) group (XI) has now been confirmed (60, 61) for this compound.
~
VII
VIII
Single crystal X-ray studies have shown that the rings are staggered in solid (C5H5)2Fe (35), (C5Hs)zCo (38), and (C5H5)~Mg (42), and eclipsed in solid (CsH5)zRu (43). The eauilibrium rine conformation for the (CsHs)?Fe molecule in the \,apor state,'howewr, is t.clipsed (34).~ l e c i r o ndiffraction data for (C5Hi),,V . . .. (311,rCHshCr ( 3 1 ) . (C;H&hln (33). (C5H&Co (36,37), ( C ~ H ~ (39,40), Z N ~ (c~H&M~ (411, and (CSHARU(34) vapor have also been interpreted a s consistent with the presence-of the eclipsed struct&e, although in all cases the presence of staagered structure could not be ruled . . out. The characterization of (CsHdzBe has added another dimension to the structural chemistry of pentahapto dicyclopentadienyl compounds. With the exception of (C5H5)~Be, the metal atom has been found to be eauidistant from both rings in pentahapto dicyclopentadienyl herivatives. Electron diffraction data reported for (C5Hs)zBe vapor, however, have been interpreted as consistent with a distorted structure (IX) in which the narallel. staeeered. o e n t a h a ~ t orines are not equidistant from the beryiFum atom ( 4 G l ) .
'I'he structure of solid (C-,H!ABe is even more intriguing. A single crysml X-ray study at -120°C showed the presence of a .'slio" sandwich structure IX) in which the ~arallel,svmmetric rings are staggered and perpendicular distances between the bervllium atom and each rine are again - unequal (52). This structure has been described as containing one monohapto ring and one pentahapto ring (52) or alternatively two pentahapto rings one of which has slipped approximately 1.20 A sidewavs (51). A sinale X-ray studv a t room - crystal temperature aiso showed the presence of a slip structure, but one in which the equilibrium orientation of the rings is somewhere h~tweenthe staggered and eclipsed extremes (53) Inn var~ahletemperature, nolution nmr study between -SO0 and -135"C, it-was not possible to determine whether (C5H5)zBeretains the structure found in the solid state or the 278 1 Journal of Chemical Education
Another relatively reactive compound with the emprical formula CloHloTi, and which is dimeric in benzene, has also been reported (62).A more detailed structure, however, could not be determined. Heating of a toluene solution of this compound gave an almost quantitative yield of the green complex (62). Other "titanocene" complexes have also been reported (63, 64) although their structures have not been fully characterized. T o date, therefore, a monomeric compound with the molecular formula (CsHJ?Ti has not been characterized. The ." fact that the green titanocene complex is isomorphous with the comoounds orieinallv formulated as (CsH&Zr - -. - and ( C ~ H ~ ) ~indicatecthat ITI~ 'the structures of these latter two compounds are far from certain. p-(ql:~l-Cyclopentadlenyl) Rings
A structure with monohapto rines had orieinallv been proposed for solid (C5H&In (65)on t i e basis of &frare"d data. More recentlv. a sinele crvstal X-rav studv . .(66) . has shown that in the soid sta& polberic stru&re is present in which each indium atom is bonded to two monohapto rings and two p-(ql:ql-CgH5) rings. Similarly, a single crystal X-ray study of (C5H5)& (67) has shown that each scandium atom is bonded to two pentahapto rings and two p-(ql:ql-C5H5) rings to give rise to a polymeric structure. p-(q':q5-Cyclopentadienyl) Rings
Solid state and spectroscopic evidence were reported to indicate (68) that niobocene is best formulated as the dimer [ ( C ~ H ~ ) ( C ~ H ~ ) with N ~ Hpentahapto ]Z and p-(+:q5-C5Ha) rings (XII).
- n
This structure has been confirmed in a single crystal X-ray study (69,70) of the niobocene dimer. A similar structure has
-
The eclipsed rings o f (C5H5)zFehave an internal rotational barrier o f 4 kJ/mole. Therefore, in the vapor, the rings appear to be "jumping"about the five-fold axis from one equivalent site to another.
been nronosed for the corres~ondingtantalum compound [ ( c ~ H & ~ ~ H ~ ) Tsince ~ H ]it~ is , i~omo;~houswith and has the same snectral nroperties as the niobocene dimer (68-70). More recently,>t has been reported that monomeric niohoceue, (CsH&Nh, can be synthesized in solution and that a pentahapto structure is likely in this phase (71, 72). The monomeric compound could not, however, be isolated as a solid. Colorless crystals of a thorium compound that analyzed as [(C5H5)z(CsH4)Th]z have been studied using single crystal X-ray diffraction (73). This study shows that each thorium atom is bonded in a nentaha~tomanner to two CSHS .rines . and that the two thorium atmns in earh dimeric unit nre held ~ I I eether hv two u-ln':n5-(:iHa, rincs to give a structuresimilar to that fbund fo; the niobocene ;lime; (XI11 ~
~
Rings .u-(n5:n5-Cyclopentadlenyl) ~ . . . ~
A polymeric structure with highly ionic rr-(@f-C5H5) rings (XIII) has been found in single crystal X-ray studies of CsH& and CsH5Tl(12).
XI11
As noted ~reviouslv.electron diffraction data for (.c -~ H s ) ? P h in the vapor state have been interpreted (30) as consistent with the Dresence of monomeric molecules in which the nentahapto rings are nonparallel with respect to each other. Solid (CsH5)zPh is found in either a monoclinic or orthorhombic crystalline modification (74). A single crystal X-ray study of the orthorhomhic modification shows the presence of a polymeric structure (XIV) with each lead atom bonded to two p-(q5:v5-C5H5)and one pentahapto rings (74).
0 XIV
Polyhapto or Tilted Cyclopentadlenyl Rings The inadeauacv . of attemntine . to classifv. cvclonentadienvl . . rings in terms of structurul extremes is illustrated hy the molecular structure determined for solid IC-HS),MONO. A . single crystal X-ray study has shown that while one of the (CsHdnMoNO rines is monohanto, the other two rines have a slructure that lies hetween thosiof the trihapto a"d pent a h a ~ t oextremes (7Fj). These rings . have been described as being polyhapto or tilted. Similarly, the molecular structure of (CsH&Zr was originally characterized as containing one monohapto and three pentahapto cyclopentadienyl rings on the basis of single crystal X-ray data (76). This contrasts with the structures determined in single crystal X-ray studies of (C&H&Ti (77) and (C5H5)4Hf (78) in which there are two monohapto and two pentahapto cyclopentadienyl rings about each metal atom. I t has been sueeested (77). however. that althoueh the structureof ((:;~;)~fFts(lifferent from thmseof ( ~ ; ~ ; ) ; i ' i a n d (C5H5)1Hf.rhe structure proposed for [C'5H&Zr is nrither
-
-
justified by the reported data nor by generally held bonding considerations. Rather, it has been proposed (77) that although there appears to he one monohapto and one pentahapto ring, the other two rings are probably polyhapto or tilted analogous to two of the rings found for solid (C5H5)3MONO. Dynamlc Behavior Up to this point, the structures of cyclopentadienyl derivatives have heen discussed in static terms. Any complete discussion, however, must include a treatment of dvnamic structural aspects, which are a factor in all phases. Although the rings of (C5H5)*Fevapor have been shown to he eclipsed in an analysis of electron diffraction data, this structure represents only a configuration of minimum potential energy with the harrier to internal rotation found to be 4 i 1 kJ/mole (27). Therefore, the rings are "jumping" about the five-fold axis from one equivalent site to another. Using "wide line" nmr spectroscopy, a similar type of motion has been found (79) in crvstalline (CsHxLFe. I t will he recalled ~ ~." that the staggered stricture represents the configuration of minimum uotential enerev for crvstalline (CsH6)oFe (36). . " , . A value of 9.6 kJ/mole has L e n estrmated for the harrier to internal rotation of the cvclonentadienvl rines in crvstalline (C5H&Fe (79). ~ o t a t i o naliout the five-foid axis 02 the cyclopentadienvl rinm is also e x ~ e c t e dfor the other dicvclopentndienyl ;lerivati\es of the iirst row transition e~ernknts. It has heen pn)pnsed (31)that the harrier to internal rotation should decrease for these derivatives as the metal to ring distance increases. Therefore, it would he expected that of the first row derivatives, (CsH&Fe would have the largest harrier to internal rotation, since the data in the table show that this compound has the shortest metal-carhon bond distance. The electron diffructinn studies of the first row derivatives referred to in the table seem to support this proposd with (CiH5)zFe having the highest harrier 10 internal rotation. Several n)mpounds with monohapto cyclopentadienyl rings havr been shown to cxhihir fluxional heha\,ior in which the point of metal-ring interaction changes and the ring structure changes to accommodate this change. This behavior has mainlv been studied usine variable temnerature. solution nmr spect©. The proton nmr spectra of ( ~ $ , H ~ ) zand H~ CsHsHgX (X=CI.Br.I) in oreanic solvents all exhibit a sinele nmr signal at room temperatke although all four compougds have a monohapto structure (80). On lowering the temperature of a tetrahydrofuran-ds solution of C5HbHgCI to a minimum of -113'C. the spectrum was resolved into that expected for a monohapto cyclopentadienyl ring (80). Broadening of the signal resonance of (C5H&Hg was also ohserved below -100°C. The pathway of the rearrangement could not he established in the solution nmr study of the monoha~to cyclopentadienyl derivatives of mercury. Variable temperature, proton nmr solution studies (81) of other main . moup . and transition element derivatives containing,. monohanto cvcln~"~~ pentadienyl ring5 have been interpreted, however, in terms uf rearrangements in\wlving 12; shifrs (XV-XVII) with activntinn cnergied of approximately h)k.1 mole. ~
~
~
~
~
~.~~
Volume 55, Number 5,May 1978 1 279
~
~
-
-
Solid state nmr studies of (C5H5)2Hg and C5H5HgX (X = Cl,Br,I) (82,83), and (7'-CsH5)(q5-C5H5)FeC0 (82) indicate that the reorientation process occurs not only in solution hut also in the solid state. Activation energies of 40 kJ/mole or less have been estimated (83) for the reorientation in the solid mercury compounds. Another type of fluxional behavior is exhibited by (CsH5)4Ti.A single crystal X-ray study (77) has shown that in the solid state there are two monohapto and two pentahapto cycloptnradienyl rings honded tu the titanium &m. If the monoha~torings are exhibitinc fluxima1 behavior, the proton nmr sulurion spectrum w o ~ i h heexpected l to exhihit twosignals of equal intensity; one due to thc t w o monohapro rings and the other due to the two pentahapto rings. The solution nmr spectrum of (CjH.);l'i, h w e v e r , exhibits only one signal at ternneratures as low as - R O T (01 -. ..---=-. .). This has been attritluted to the presence of an intramolecular rearrangement in which the monohapto and pentahapto rings interchange roles. At -30°C, this exchange is slow enough that the spectrum is resolved into two signals of equal intensity: one due to the monohapto rings, which are undergoing fluxional motion at such a rate that the nrotons on the monohapto rings . appear .~ equivalent, and the'orher due to the pentihapto rings. An activation enerrv of 67.4 f 1.2 kJ/mole has heen estimated for the interchange of the monohapto and pentahapto rings in ~~~~~~
~
~
~
~
--
(C5Hd4Ti
(81).
The last type of dynamic behavior to he discussed is exhibited by ( C S H ~ ) ~As B ~noted . previously, in the vapor state, both rings are pentahapto (49-51), although the beryllium atom is not equidistant from both rings. It has also been reported in these vapor state studies that the beryllium atom does not remain stationary hut alternates between the two equivalent sites of minimum potential energy. Similarly, there is a dynamic aspect of the solid state "slip" structures found at different temperatures (53), in that the rings are undergoing rotation and exchanging roles, while the beryllium atom is movine between alternate sites. In a recent variable temperature nmr study of (CsH5)2Be between -50' and -135"C, althoueh no conclusion could he reached as to whether rC:Hs)?Be is found with the pure pentahnpto structure or a slir~structure.it wasconcluded that the tmrier height between the two equi&ent potential wells of the beryllium atom is less than 0.59 kJ,mole ( ; i 4 J .
-
(22) Deuhler, B., Elisn, M., Fixher, E. O., and Fritz, H. P., Chem. B e t , 103,79911970). (23) Fischer, E. 0..and Loehner, A., 2.Nofurlorsch., 15b,266 11960). (24) Siegart, F. W.. and de Leifde Meijer. H. J.,J . O ~ g o n o m ~ aChem, l. 20.141 (19691. (25) ~ ~ ~ ~ , c . ~ . , ~ r e e n , ~ . , ~A.,snd o r d eProut,K., r , R . Chem. Commun., 97 (19731. 126) Drew. D. A,. and Haaiand. A,. Acto Chem Scond.. 27.3735 (1973).
(1967). (31) G a d , E., Hsaland, A,. Nouak, D. P., and Seip, R., J. Orgowmetol. Chem., 88,181 (1975). (32) Haaland, A.. Luszfyk, 3.. Nouak, D. P.. Brunvoll, J., and Starowieyski, K. B.. Chem. Commun., 54 (197$. (33) Almenningen. A,. Healsnd, A., and Motefeldt, T., in '"Selected Topies in Structmal Chemistry,"Univeraitetsforlaget, 0810, 1967,p. 105. (34) Hsaland, A,, and Nilsson. J. E., Acto Chem Seond., 22,2653 (1968). (35) Dunitz, J. D.,o,gel,L.E.,a"dRich,A.,AefoCryaf"llogr., 9,373(1956). 136) Almenningen,A..Gard, E., Ha&nd. A.,andBrunuoll, J . , J Orgnnometal. Chem. 107.
,.".",.
"71 ,,WE,
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