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The complex [RuCl(η 5 -C 5 H 5 ){PPh 2 (2-MeC 6 H 4 )} 2 ] ( 1 ), unlike the triphenylphosphine analog [RuCl(η 5 -C 5 H 5 )(PPh 3 ) 2 ], reacts under mild conditions with CO, N 2 CPh 2 and HCCPh to give the neutral carbonyl, carbene and vinylidene derivatives [RuCl(η 5 -C 5 H 5 )(L){PPh 2 (2-MeC 6 H 4 )}] (L=CO 2 , CPh 2 5 , CCHPh 6 ), respectively, via displacement of one phosphine ligand. The vinylidene complex 6 promptly reacts with benzylamine affording the aminocarbene [RuCl(η 5 -C 5 H 5 ){C(NHCH 2 Ph)CH 2 Ph}{PPh 2 (2-MeC 6 H 4 )}] ( 7 ). Moreover, the cyclometalated derivative [ Ru(η 5 -C 5 H 5 ){PPh 2 (2-C H 2 C 6 H 4 )}{PPh 2 (2-MeC 6 H 4 )}] ( 4 ) has been obtained from the methyl complex [RuMe(η 5 -C 5 H 5 ){PPh 2 (2-MeC 6 H 4 )} 2 ] ( 3 ) by intramolecular C–H bond cleavage and methane elimination. Complex 1 , whose X-ray structure analysis is also reported, has been found to catalyze alkyne coupling reactions.

The metal amide route has been exploited to synthesize η 5 : η 1 - and η 5 : η 5 ansa -metallocenes of pentavalent niobium and tantalum. Tris(dimethylamido)(2,6-diisopropylphenylimido)-niobium(V) 2a and -tantalum(V) 2b were found to be appropriate starting compounds to cleanly react with CH-acidic ligand precursors such as cyclopentadiene and 2,2-bis(cyclopentadienyl)propane. In the latter case, the novel ansa -metallocenes of formula [C 5 H 4 -C(CH 3 ) 2 -C 5 H 4 ]M[=N(2,6- i Pr 2 C 6 H 3 )][N(CH 3 ) 2 ] ( 4a , M = Nb; 4b , M = Ta) result in high yields. While the hydrocarbon ligand adopts an η 1 : η 5 -coordination to the metals in the solid state (X-ray diffraction studies), a rapid η 1 : η 5 - η 5 : η 1 equilibration occurs in solution. Complexes 4a,b react promptly with (CH 3 ) 3 SiCl, thus affording the ansa -metallocenes [C 5 H 4 -C(CH 3 ) 2 -C 5 H 4 ]M[-N(2,6- i Pr 2 C 6 H 3 )]Cl ( 5a , M = Nb; 5b , M = Ta) quantitatively. The crystal structures of the niobium(V) and tantalum(V) chloro derivatives 5a,b reveal a ‘conventional’ η 5 : η 5 -coordination for the chelating bis(cyclopentadienyl) ligand. Protonolysis of 4a,b with [NHMe 3 ][B(C 6 H 5 ) 4 ] afforded the first cationic ansa -metallocenes [C 5 H 4 -C(CH 3 ) 2 -C 5 H 4 ]M[=N(2,6- i Pr 2 C 6 H 3 )][NH(CH 3 ) 2 ][B(C 6 H 5 ) 4 ] ( 6a , M = Nb; 6b , M = Ta) in high yields. The crystal structure of 6a shows an η 5 : η 5 -coordination mode for the bis(cyclopentadienyl) ligand. The 1 J (C,H) coupling constants of the bis(cyclopentadienyl) ligand of the ansa -metallocenes 4a,b , 5a,b and 6a,b are related to its hapticity, which is a result of the electronic properties of the metal center.

Niobium(V) and tantalum(V) amides [η5-C5H4)-Si(CH3)2-NR]M[N(CH3)2]3 (R = C6H5) containing chelate-type C, N-Ligands are available in 45% (M = Nb) and 60% (M = Ta) isolated yield, respectively, from the homoleptic amides M[N(CH3)2]5 and the N-functionalized cyclopentadiene C5H5-Si(CH3)2-NHR by double amine elimination. The niobium and tantalum derivatives undergo an unprecedented photo-induced rearrangement at room temperature, yielding the imido complexes [η5-C5H4)-Si(CH3)2-N(CH3)2]M(=NR)[N(CH3)2]2 as a result of SiN bond cleavage.

First examples of tetravalent molybdenocene-type compounds containing N -functionalized π-aromatic ligands are described. Thus, the “metal amide route” starting from Mo[N(CH 3 ) 2 ] 4 was used to synthesize [Si(CH 3 ) 2 (C 5 H 4 )(NC 6 H 5 )Mo[N(CH 3 ) 2 ] 2 from the ligand Si(CH 3 ) 2 (C 5 H 5 )(NHC 6 H 5 ) in 60% yield. Related Mo(IV) complexes are reported. It appears that the Mo(IV) amides are least susceptible to redox side reactions.