Synthesis and characterisation of enantiomerically-pure zirconium complexes containing a linked amido-cyclopentadienyl ligand; X-ray structures of [Zr{η5:η1-C5Me4SiMe2NCH(Me)(Ph)}X2] (X=NMe2 and Cl) and [Zr{η5:η1-C5Me4SiMe2NCH(Me)(Ph)}Cl(μ-Cl)]2