Steric effects of bulky phosphines on 95Mo NMR shieldings

Abstract

Three sets of substituted molybdenum carbonyl complexes of the type Mo(CO)6−n(PY3)n (n = 1–3) with a wide range of phosphorus ligands (Y = Ar, R, OR, NR2, Cl, H) were synthesized and their respective 95Mo NMR spectral data were collected. The steric effect of PY3 plays an unexpectedly important role in the paramagnetic shielding term determining δ(95Mo), and is even observed in the least crowded M(CO)5(PY3) complexes. It is deduced that the steric requirement reduces the commonly accepted π-ability of phosphites by weakening the M—P bonds, making δ(95Mo) positions move toward higher frequency in going from M(CO)5(PY3) to fac-M(CO)3(PY3)3. A detailed correlation analysis in the case of phosphite ligands has shown that the free cone space to accommodate a PY3 ligand may be as low as 95° in Mo(CO)6−n(PY3)n (n = 1–3) derivatives. The fair linearity of these correlations suggests indirectly that the σ-component in phosphite bonding is relatively less important. These linear correlations of δ(95Mo) against the Tolman cone angle θ provide an empirical estimation of the bulkiness of any phosphite ligand. Key words: metal–phosphorus bonds, 95Mo NMR, steric effects.