Silylimido Complexes of Niobium and Tantalum at the Limit of π‐Bond Saturation

Convenient strategies towards the synthesis of silylimido niobium and ‐tantalum complexes of the metallocene and half‐sandwich type are reported. Bent metallocenes [(η 5 ‐C 5 H 5 ) 2 M(NSiMe 3 )Cl] (M = Nb: 1a ; Ta: 1b ) and [(η 5 ‐C 5 H 5 ) 2 M(NSiMe 3 )(η 1 ‐C 5 H 5 )] (M = Nb: 2a ; Ta: 2b ) are synthesized by reaction of [M(NSiMe 3 )Cl 3 (py) 2 ] with 2 or 3 equiv. NaCp, respectively. Half‐sandwich type silylimido complexes [(η 5 ‐C 5 Me 5 )M(NSiMe 3 )Cl 2 ] (M = Nb: 3a ; Ta: 3b ) cannot selectively be obtained from the (pyridine)(silylimido) precursor complexes. However, their synthesis is accomplished by reaction of [(η 5 ‐C 5 Me 5 )MCl 4 ] with LiN(SiMe 3 ) 2 . On the other hand pyridine silylimido complexes are perfect starting materials for the synthesis of the related hydridotris(pyrazolyl)borato complexes [Tp*M(NSiMe 3 )Cl 2 ] [M = Nb: 4a ; Ta: 4b ; Tp* = κ 3 ‐HB(3,5‐Me 2 pz) 3 ]. A comparative study of the σ,π‐donor capability of isoelectronically related tert ‐butyl‐ and trimethylsilylimido ligands in complexes [(η 5 ‐C 5 H 5 ) 2 Ta(NR)(η 1 ‐C 5 H 5 )] (R = SiMe 3 : 2b ; t Bu: 2c ) is presented. With respect to solid‐state structural investigations and solution variable‐temperature NMR‐spectroscopic studies on the dynamic exchange process of η 5 ‐ and η 1 ‐bonded cyclopentadienyl rings in 2b , c , tert ‐butlyimido ligands are better donors than silylimido ligands, and both are better donors towards a d 0 metal center than an η 5 ‐C 5 H 5 ligand.