Phosphoraneiminato complexes of transition metals containing the NPR 3 À group are known in many coordinationchemical variations [1] as a result of varying bonding modes. In complexes with metal ions in low oxidation states (i, ii) the NPR 3
À group acts as m 3 -N bridging ligand, in medium oxidation states (iii, iv) preferably as m 2 -N bridging ligand, and in high oxidation states (v to viii) exclusively as terminal ligand. In the latter mode short metal ± nitrogen distances and linear or almost stretched M-N-P axes are observed, which can be described with the resonance structures A and B. As first deduced for imido complexes (NR 2À ) [2] style B requires a s, 2p orbital set for the M N bond that proves the NPR 3 À ligand to be y-isolobal to the h 5 -cyclopentadienide(Cp) ligand C [3] with respect to equal charge. Recent ab initio calculations confirm this interpretation. [4] Since we surprisingly found the bonding mode A/B also in the mixed-ligand complexes [Ln 2 Cp 3 (NPPh 3 ) 3 ] (Ln Y, Dy, Er, Yb) containing Ln 3 ions, [5] we examined the opportunity to synthesize homoleptic compounds [Ln(NPPh 3 ) 3 ], too, and to compare them with the y-isolobal complexes [LnCp* 3 ] (Cp* C 5 Me 5 ) that are known as initiators for ring-opening polymerization (ROP) of lactones. [6±8] Compounds 1 are formed in almost quantitative yields by reacting the alkaline phosphoraneiminates MNPPh 3 (M Na, K, Rb, Cs) [9] with the anhydrous metal trichlorides or anhydrous metal triflates in THF [Eq. ( 1)]. Interestingly, only the lanthanum compound crystallizes as a THF-solvate, that is 1 a ´2 THF, whereas the ytterbium compound 1 b forms a solvate-free complex due to the ionic radius of the Yb 3 ion which is 16 pm smaller than that of the La 3 ion. [10] Kinetic analysis of the 12 clones: The clones were labeled at the 5'-end with g-32 P-ATP and incubated at 1.0 nm in SB8 mm MgCl 2 at 37 8C. Aliquots were taken at different times, quenched with urea and EDTA on ice, and loaded onto a denaturating PA gel. Bands corresponding to the uncleaved and cleaved fractions were quantified by phosphorImaging (Molecular Dynamics). The k obs values were then determined by using an exponential curve fitting of the measured values of the uncleaved ribozyme fraction during the first 3 min.