Stabilization of Pentaphospholes as η -Coordinating Ligands.

Electrophilic functionalisation of [CpFe(η -P )] (1) yields the first transition-metal complexes of pentaphospholes (cyclo-P R). Silylation of 1 with [(Et Si) (μ-H)][B(C F ) ] leads to the ionic species [CpFe(η -P SiEt )][B(C F ) ] (2), whose subsequent reaction with H O yields the parent compound [CpFe(η -P H)][B(C F ) ] (3). The synthesis of a carbon-substituted derivative [CpFe(η -P Me)][X] ([X] =[FB(C F ) ] (4 a), [B(C F ) ] (4 b)) is achieved by methylation of 1 employing [Me O][BF ] and B(C F ) or a combination of MeOTf and [Li(OEt ) ][B(C F ) ]. The structural characterisation of these compounds reveals a slight envelope structure for the cyclo-P R ligand. Detailed NMR-spectroscopic studies suggest a highly dynamic behaviour and thus a distinct lability for 2 and 3 in solution. DFT calculations shed light on the electronic structure and bonding situation of this unprecedented class of compounds.