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δ( 31 P) NMR data of the complexes [Nb(Cp)(CO) 3 PZ 3 ] (Cp = η 5 -C 5 H 5 ) and cis- [Nb(Cp)(CO) 2 (PZ 3 ) 2 ] [Z = H, R (alkyl and aryl), OR, NMe 2 , PR 2 ] are interpreted on the basis of varying P(3p σ ) and P(3d π ) contributions to the bond-order density matrix in the paramagnetic deshielding term of the overall shielding constant. For Z = R, shielding of the  31 P nucleus of the coordinated phosphine decreases as the cone angle θ increases, as a result of increasing p contribution. For Z = OR and NMe 2 , P-Nb π interactions dominate, and the decrease of the P(3d π ) occupation with increasing θ leads to an increase of shielding. In the case of alkoxiphosphines, decreasing  31 P shielding is accompanied by an increase of the shielding of the metal nucleus [quantified here by δ( 51 V) of the vanadium complexes], while, for the substantially less powerful π interacting alkylphosphines, shielding trends for the two nuclei parallel each other. Coordination shifts Δδ(P) are somewhat greater for small phosphines but, spanning a range of 80 ± 30 ppm, tend to be comparatively invariant. In [V(Cp)(CO) 3 PZ 3 ], the  31 P nucleus is deshielded by 10 to 30 ppm with respect to the corresponding niobium complexes.

zeitschrift für naturforschung b

Carbonylniobchemie, VII. Die  31 P—NMR-Spektren der Phosphinderivate des [Nb(η 5 -C 5 H 5 )(CO) 4 ] / Carbonyl Niobium Chemistry, VII. The  31 P NMR Spectra of Phosphine Derivatives of [Nb(η 5 -C 5 H 5 )(CO) 4 ]