Synthesis and properties of cyclopentadienylniobium(III) complexes. A magneto-structural correlation for 16-electron four-legged piano stool complexes

Sodium reduction of (ring)NbCl4 in the presence of the appropriate phosphine ligand affords the 16-electron Nb(III) complexes (ring)NbCl(2)L(2) (ring=C5H5, L(2)=dppe or ring=C(5)H(4)Me, L=PEt(3)). While the previously reported Cp*NbCl2(PMe(3))(2) complex behaves as a Curie-Weiss spin triplet paramagnet, compound (C(5)H(4)Me)NbCl2(PEt(3))(2) has a spin singlet ground state and a thermally populated excited triplet state (2.3 kcal/mol higher in energy) and (C5H5)NbCl2(dppe) is essentially diamagnetic. The X-ray structure of (C(5)H(4)Me)NbCl2(PEt(3))2 shows a four-legged piano stool geometry, with Cl-Nb-Cl and P-Nb-P angles correlating with the experimentally determined singlet ground state. Theoretical calculations with geometry optimization at the MP2 level on both (1)A' and (3)A '' states for the (C5H5)NbCl2(PH3)(2) model system confirm the strong dependence of the angular parameters on the spin state.