Having 1 more electron than ferrocene i t is

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Unformatted text preview: the reactions of sodium cyclopentadiene and anhydrous CoC12 in thf. Having 1 more electron than ferrocene, i t is paramagnetic with a magnetic moment of 1 .76BM and, while it is thermally stable up to 250"C, its most obvious characteristic is its ready loss of this electron to form the yellowgreen cobalticenium ion, [Co"'(q5-C5H5)2]+. This resists further oxidation, being stable even in conc HN03 but, like the isoelectronic ferrocene, is susceptible to nucleophilic attack on its rings. Rhodocene, [Rh(q5-C5H5)2],is also known but is unstable to oxidation and has a tendency to form dimeric species. Claims for the existence of iridocene probabIy refer to Ir'" complexes. However, the yellow rhodicenium and iridicenium cations are certainly known and are entirely analogous to the cobalticenium cation in their resistance to oxidation and susceptibility to nucleophilic attack. Numerous "half-sandwich'' compounds of the type [M(q5-C5R5)L2], M = Rh, Ir; R = H, Me; L = CO, phosphine etc.) are known and are useful reagents. [Ir(q5-C5Me5)(C0)2]for instance is an excellent nucleophile and is also used in the photochemical activation of C-H in alkanes. It is particularly effective in the latter role when supercritical C02 is the solvent.(42) 4 ' S . MARTINENGO, CIANI nd A . SIRONI, . Chem. Soc., G. a J Chem. Commun., 1405-6 (1992). KOFF, More recently N has been encapsulated(41) in [Rh14(N>2(CO)25l2-and [Rh23(N)4(co)38l3-. The latter is the largest Rh cluster so far characterized. It consists of an irregular polyhedron of 21 Rh atoms encapsulating a pair of particularly close (257.1 pm) Rh atoms as well as 4 N atoms each of which is located in a semi octahedral site. Other derivatives of the carbonyls are of course numerous; Ir forms many carbonyl halides of the types [II(CO)3X], [Il(CO)2X2]-, [I$I1(C0)2&]- and [Irr1'(CO)X5]2-, but the stability of carbonyl halides falls off in the sequence Ir > Rh > Co and those of Co are only of the type [Co(CO)4X] and are very unstable. The bulk of derivatives are obtained by the displacement of C O by other ligands. These include phosphines and other group 15 donors, NO, mercaptans and unsaturated organic molecules such as alkenes, alkynes and cyclopentadienyls. Cyclopentadienyls 42 M . JOBLING,. M. HOWDLE, . A . HEALY nd M. POLIAS M a J . Chem. Soc., Chem. Commun., 1287-90 (1990)....
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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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