The total structure determination of thiol-protected Au clusters has long been a major issue in cluster research. Herein, we report an unusual single crystal structure of a 25-gold-atom cluster (1.27 nm diameter, surface-to-surface distance) protected by eighteen phenylethanethiol ligands. The Au25 cluster features a centered icosahedral Au13 core capped by twelve gold atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly symmetric structure is distinctly different from recent predictions of density functional theory, and it also violates the empirical golden rule "cluster of clusters", which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand-gold core interactions. The Au25(SR)18 clusters exhibit multiple molecular-like absorption bands, and we find the results are in good correspondence with time-dependent density functional theory calculations for the observed structure.

Many metal-oxo (M=0) and -imido (M=NR) complexes are known, but in most the M=X linkages are notoriously inert. The absence from the literature of monomeric group IV metallocenes of this class ((t75-C5R5)2M=0 and (7js-C5R5)2M=NR', M = Ti, Zr, Hf) suggests that if these species could be generated, they might exhibit more extensive chemistry than do oxo and imido complexes that are presently known. We now wish to report the

This paper outlines the generation of the transient monomeric imido complexes CpzZr==NR (la-e), their successful trapping with unsaturated organic molecules and dative ligands, and their use in activating C-H bonds. The generation of Cp,Zr=NR has been accomplished by thermolysis of the alkyl amides CpzZr(NHR)(R') (2a-d) and bis(amides) CpzZr(NHR)2 (3a-c) via a-abstraction of alkane and amine, respectively. The imido intermediate la, generated from alkyl amide 2a (R = 4-tert-butylphenyl, R' = Me) in THF, resulted in the formation of the bridging imido dimer 4a in 70% yield. In contrast, the generation of intermediates lb-d ( l b R = tert-Bu; IC R = 2,6-dimethylphenyl; Id R = Si(CMe3)Mez) under similar conditions furnished the monomeric imido T H F complexes 5b-d in 71, 69, and 72% yields. Imido dimer 4a and monomer 5b were characterized crystallographically. The T H F ligands of 5b-d are weakly bound, as demonstrated by the exchange of bound and free T H F in 5c on the NMR time scale and by facile substitution of Ph3PO for T H F in 5b, giving the triphenylphosphine oxide adduct Cp2Zr(NCMe3)(OPPh3) (6b). Thermolysis of the alkyl amides 2b and 2d a t 85-95 "C in benzene resulted in the addition of the C-H bond of benzene across the Zr-N linkage to provide the phenyl amides 7b and 7d. Similarly, the T H F adduct 5b reacted with benzene under the same conditions to give the C-H activation product 7b. The imido intermediates Cp,Zr=NR undergo cycloaddition reactions with a variety of unsaturated organic compounds. In addition to formation of azametallacyclobutenes (e.g., 8b, 8c, 9c, lob) from alkynes, certain olefins react with Cp2-Zr=NR to give azametallacyclobutanes. Thus, upon reaction of norbornene or ethylene with imido complex 5b an equilibrium mixture of 5b, free olefin, free THF, and the metallacycles 12b and 13b, respectively, were observed. Addition of a large excess of norbornene shifted the equilibrium almost entirely to the azametallacyclobutane 12b and facilitated its isolation. The structure of 12b was determined after crystallization from hexamethyldisiloxane. Complex 5b also reacted with benzophenone and tert-butyl isocyanate to provide the tert-butylimine of benzophenone and di-tert-butylcarbodiimide in 89 and 65 % yields, respectively. Monomer 5b reacted with tert-butyl isocyanide to provide the couplingltrapping product CpzZr-(N(CMe3)C=NCMes)(CNCMe3) (16), while reaction of 5b with isobutene oxide gave the epoxideopened product Cp2Zr(NHCMe3)(0CHzC(Me)=CH2) (17). Finally, heating the alkyl amide CpzZr(NHCHPh2) (CH2CH2CMe3) (2e) (which contains hydrogens both a and / 3 to nitrogen) with diphenylacetylene yielded the azametallacyclobutene product formed via a-abstraction in 64% yield.

An x-ray crystal structure that confirms the soccer ball-shaped carbon framework of C(60) (buckminsterfullerene) is reported. An osmyl unit was added to C(60) in order to break its pseudospherical symmetry and give an ordered crystal. The crystal structure of this derivative, C(60)(OsO(4))(4-tert-butylpyridine)(2), reveals atomic positions within the carbon cluster.

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