Treatment of Cp2Zr(Cl)CH3 (6) with sodium cyclopentadienide gives Cp3ZrCH3 (7). Its reaction with dimethylanilinium tetraphenylborate yields the Cp3Zr(THF)+ cation. [Cp3Zr+CH3B(C6F5)3 -] (9) is generated by treatment of 7 with tris(pentafluorophenyl)borate. Nitriles add to 9 to form the ligand-stabilized tris(η5-cyclopentadienyl)zirconium cation systems Cp3Zr(N⋮CR)+. With tert-butyl isocyanide, 9 is transformed to yield the donor-ligand-stabilized [Cp3Zr(C⋮NCMe)3 +CH3B(C6F5)3 -] salt 12. Carbon monoxide adds to 9 to give the cationic metal carbonyl complex [Cp3Zr(CO)+CH3B(C6F5)3 -] (13). Both cation complexes 12 and 13 were characterized by X-ray diffraction.

~ ~~~~~~~The organometallic salt [Cp,Zr(N=C-CH,)

A density functional theory computational chemistry study has revealed a fundamental structural difference between [Ti(Cp)3]+ and its congeners [Zr(Cp)3]+ and [Hf(Cp)3]+/(Cp=cyclopentadienyl). Whereas the latter two are found to contain three uniformely η5‐coordinated Cp ligands (3η5‐structural type), [Ti(Cp)3]+ is shown to prefer a 2η5η2 structure. [Ti(Cp)3]+[B(C6F5)3(Me)]− (10⋅[B(C6F5)3(Me)]−) was experimentally generated by treatment of [Ti(Cp)3(Me)] (7a) with B(C6F5)3 (Scheme 3). Low‐temperature 1H‐NMR spectroscopy in CDFCl2 (143 K, 600 MHz; Fig. 8) showed a splitting of the Cp resonance into five lines in a 2 : 5 : 2 : 5 : 1 ratio which would be in accord with the theoretically predicted 2η5η2‐type structure of [Ti(Cp)3]+. The precursor [Ti(Cp)3(Me)] (7a) exhibits two 1H‐NMR Cp resonances in a 10 : 5 ratio in CD2Cl2 at 223 K. Treatment of [HfCl(Cp)2(Me)] (6c) with sodium cyclopentadienide gave [Hf(Cp)3(Me)] (7c) (Scheme 1). Its reaction with B(C6F5)3 furnished the salt [Hf(Cp)3]+[B(C6F5)3(Me)]− (8⋅[B(C6F5)3(Me)]−), which reacted with tert‐butyl isocyanide to give the cationic complex [Hf(Cp)3(C=N−CMe3)]+ (9a; with counterion [B(C6F5)3(Me)]− (Scheme 2). Complex cation 9a was characterized by X‐ray diffraction (Fig. 7). Its Hf(Cp3) moiety is of the 3η5‐type. The structure is distorted trigonal‐pyramidal with an average D−Hf−D angle of 118.8° and an average D−Hf−C(1) angle of 96.5° (D denotes the centroids of the Cp rings; Table 6). Cation 9a is a typical d0‐isocyanide complex exhibiting structural parameters of the C≡N−CMe3 group (d(C(1)−N(2))=1.146 (5) Å; IR: v˜(C≡N) 2211 cm−1) very similar to free uncomplexed isonitrile. Analogous treatment of 8 with carbon monoxide yielded the carbonyl (d0‐group‐4‐metal) complex [Hf(Cp)3(CO)]+ (9b; with counterion [B(C6F5)3(Me)]−) (Scheme 2) that was also characterized by X‐ray crystal‐structure analysis (Fig. 6). Complex 9b is also of the 3η5‐structural type, similar to the peviously described cationic complex [Zr(Cp)3(CO)]+, and exhibits properties of the CO ligand (d(C−O)=1.11 (2) Å; IR: v˜(C≡O) 2137 cm−1) very similar to the free carbon monoxide molecule.

Treatment of the reagent [C5H4B(C6F5)3]Na,Li·Et2O (4) with zirconocene dichloride gave the neutral tris(cyclopentadienyl)Zr−betaine-type complex (η5-Cp)2[η5-C5H4B(C6F5)3]Zr (1). In the crystal complex 1 contains the three η-cyclopentadienide ligands in a nearly trigonal-planar coordination around zirconium with a pronounced Zr−F−C(aryl) coordination perpendicular to it. The Zr−F bond length is 2.310(3) Å. The Zr−F−C(aryl) coordination is persistent in solution. The activation energy of the reversible cleavage of the Zr−F linkage of 1 was determined as ΔG ⧧ (Zr - F)diss(253 K) = 10.2 ± 0.2 kcal mol-1 by dynamic 19F NMR spectroscopy in toluene-d 8.

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