Magnesocene: Structure and bonding. Covalent or ionic?

weak covalent d2$p3 hybridization overlapping the ionic electrostatic binding between the metal and cyclopentadienyl ions. Based on magnetic, spectral...
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A. G. Pinkus Baylor University Waco, Texas 76703



Magnesocene: Structure and Bonding Covalent or ionic?

Although the most common state of coordination for magnesium in its compounds is four ( I ) , examples of three-(2), five-(3), six-(4), and eight-coordination (5) have also been reported. An unusual type of bonding for magnesium which does not fit into any of these categories occurs in magnesocene? Magnesocene is a member of the general class of or: ganometallic compounds known as metallocenes (6) or "sandwich" comnounds of which the first examole to he recognized is the well-known ferrocene. Although it is generally aereed that bondine in metallocenes of transition metals aDG a r s to he predomkantly covalent, the nature of bondingin magnesocene is currently in dispute. This paper will present a hrief review of the structures and bonding proposed for magnesocene leading up to the most recent work. Although magnesocene was first prepared (7) by addition of cyclopentadiene to ethylmagnesium bromide and removing ether solvent in vacuo, it can also he prepared (8)by the reaction of metallic magnesium with cyclopentadiene (equ. (1)). 2CsHs + Mg Hz + M g ( C 6 H d z (1) Mugnesocene is a useful intermedintr for the preparation (9) oiothrr memllmxmrs either in the molten state or in solution (eqn. (2)). The high free energy of formation of MgClz favors the metathesis. M~(CSH&+ M C l z - M ( C s H s ) 2 + ME12 (2) Magnesocene has heen reported (10) to have been prepared in a rose-colored modification in addition to the better-known form. Weiss and Fisher (11) determined that Mg(CsHd2 in the crystalline state had a sandwich-like structure and proposed weak covalent dZsp3hybridization overlapping the ionic electrostatic hinding between the metal and cyclopentadienyl ions. Based on magnetic, spectral, and chemical investigations, Wilkinson, Cotton, and Birmingham (12) concluded that ionic bonding was present; the two cyclopentadienide anions would he expected on electrostatic grounds to align themselves on opposite sides of the cation with parallel planes and to very closely resemble ferrocene. The concept of ionic honding in Mg(CsH& was further elaborated by Cotton and Reynolds (13). Lippincott, Xavier, and Steele (14) studied the infrared and Raman spectra of solutions and the solid state and in comparison with studies on ferrocene confirmed the sandwich-type structure. However, they concluded that their results were not in agreement with the ionic bonding concept hut were better explained by covalent ring-to-metal bonding which was, however, weaker than the metal-to-ring bonding in ferrocene, From a gas-phase electron diffraction study, Haaland, et al. (15,161, determined that the best agreement hetween calculated and experimental intensities was obtained by assuming eclipsed CsHs rings ( D 5 h symmetry) (see figure). However, a model with staggered symmetry (DSd)could not ~~~~~~~




he conclusively ruled out. They also concluded that there was no significant deviation from planarity for the C5H5rings and that magnesocene is best regarded as a covalent rather than an ionic compound. More recently, Aleksanyan and coworkers (17) obtained Raman and infrared snectra on the comnound. Raman results were ior solid and m h forms and iniluded depolarization measurements while infrared data for the solid covered the range 400-3200 cm-I. Their analysis of the data led to the conclusion that the Mg-Cp bond is mainly ionic. Literature Cited (1) Bailar, J. C., Jr in "TheChem~tryoftheCmrdinationCompoundr: (Editor: Bailar, J.C.,Jr.l,Reinhold,New York, 1956,p.243. 12) For areeent review:Pinkur.A. G.. Coord. Chem. Re", 25,173 (1978). (3) Studty,G.,and Rund1e.R. E., J . Amrr. Chem. Soe., 86,4821 (1964):Vallino,M.,O~ganomelol. Chem., 20.1119691. (4) Schroder, F., and Spandau.H.,Notunuissanscholten. 53,360 (1966);Schrider. F. A., Chem Ber. 102.2035 (1969): Atqwd. J. L., and Stuckey, G. D.. J. O ~ o n o m d a l . C h m . , 13, 52 119681;Tonay, J. D.,and Slucky, G. D.. J. Orgonometal. Chem., 28.

s IPs8t.r.n P I . . " Cy 1 h . t ~ r t*domyl \ I . t a l'nmpn~nd:' n ' l m m m e t a l c '. M.r r 7-4.. I1 I llrlt#hdd Kru York. 196U.Chm ^ . p p i l h 3.9. .h, h... H A , v ~ . ~~ ~~I ,he~ rrrnvtlon I ~ ~\ I ~ ~~ ~ I.I". -O C J c ha". *... ID 3 ~ 2 &~ i r n i ~ ~ h C'M.,"SynthesisofCyclopentadienylMetal am. Compounds." in "Advances in Organometallic Chemistry," Vol. 2, (Editors: Stone, F. G. A,, and Wast,R.) Academic Press, New York, 1964,pp. 365-413;1dlChurchill,M. R..end Mason. R. "The Structural Chemistry of Organo-Transition Metal Complens: Some Recent Developments." ibid., vol. 5. 1967, pp. 93-135: (el Caia, M., and Lupin, M. S.," M a s Spectra of Metallmenes & Related Compounds." ibid.. val. 8, 1970, pp. 211.333; (1) Slocum. D. W., and Ernst. C. R., "Electronic EffeMin Metallocene~ and Certain Related Systems," ibid., vol. lo, 1972, pp. 79-111: i d Bruce, M. I., '"Onano-Transition Metal Chemistry-A Guide to the Literature 1950-1970,"ibid.. vol. 10, p p 275-346: (hl Rubezhou,A. Z and Gubin, S. P.."Ligend Substitution in Transition Metal r-Complexes," ibid, "01. 10, pp. 347417: 10 Bmm, M. [.;'The . Literature of Organo-Tranlition Metal Chemiatxy 1971," ibid., vol. 1 1 . 1 9 7 3 , ~ ~447 (I1 Bruce, M. I.. "The Literature of Organo Transition Metal Chemistry 1972," ibid.. vol. 12. 1974, pp. 379407: (k) Silverthorn, W. E., '"Arone Tmnaition Metal Chemintry," ihid.. uol. 13, 1975, pp. 47-137: (1) Raruvaw, G. A,, and Latyaeva, V. N., "Covslent Oqanometallir Compound. oftheTransitian Metals," Rvrr. Chem. Re", 36, 251.267 (1965); Usprkhi Khim., pp. 585 ff; (m) Rybin~kays,M. I., and Korneva, L. M.. "Eleetrophilie Substitution in Cyelopentadienid~Anions," ibid., 40,247-255 (1971); Uspekhi Khim. p p 444 ff: (n) Leonovs, E. V.. and Kmhetkova. N. S.,ibid., 42,27&292 (1973); UspehhiKhim., pp. 615% (01 Braterman, P. %and Croes. R. J.. "Organo-Tvansition-metalcomplexes: Stability. Reactivity and Orbital Conelations,"Chrm.Soc.R~~., 2,271 (1973); lp) Hunt,C.B.,Educ. Chem., 14,110 Chrm,.tn




\.",,,. ,,O,">

Wilkinson. G.. and Cotton. F. A,. Chom. Ind.. ILondonl.. 307 (1954): . . see a h Fischer. E. 0.. &d ~ s f n e rW.. , ~ot&orsch., 9b. 503 11954). Berber. W.A.,lnora Synlh., 6.11 (19M)). Hull, H. S., Reid, A. F., and Turnhull, A. G., Inorg. Chem.. 6,605 (19671 and refs.



prw. '(11) Weis, E.. and Fkhher. E. O.,Z.Anorg. AIlg. Chem., 278,219 11955). (12) Wilkinaon. G.. Catton.F.A.,and Birmingham, J. M.. J. 1norg.Nucbar Chrm., 2.95 (19561. (13) Cotton, F.A..andReynolds.L. J.,J. Amer Chem Sm.. 80,269 ((1958). (14) L$pinmtt,E.R., Xsvier, J.,andStede.D.,ilAmer. Chrm Soc.. 83,226211961). (IS] ~ ~ a lA.. ~LUSZ~Y~,J. ~ d , N O V ~ L .D. P., B ~ D V O I I ,J.. and starowieyski, K. B., J chpm. SOC,cham. comm.. M (19741. (16) Hasland, A,, Luwtyk, J.,Brnnvoll. J.,sndStamwieyski, K. B., J . O q o ~ m e l o lChem., . 85.279 (1975). (17) ~ k k s a n y a n v. , T., ~arbuzova,I. A,, ~svrilenko,v. v.,and Zakharkin. L. I . , J Organomrfol. Chem., 129.139 (1977).

'Chemical Abstracts nomenclature, from 1974 to the present date for this compound is bis(~5-2,4-cyclopentadien-1-yl)magnesium. In

earlier references the comoound is variouslv named: bis(cvclooen-



(rings perpendicular)

Structure of magnesacene ( 15. 16).

704 / Journal of Chemical Education


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