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Steric and electronic effects on31P and51V shielding in carbonylphosphinevanadium complexes

Journal of Magnetic Resonance (1969)
Publication Date
DOI: 10.1016/0022-2364(80)90324-8


Abstract The shielding of 51V in η 5-C 5H 5V(CO) 3PR 3 complexes decreases with increasing ligand strength (electronegativity of R) and increasing spatial requirement for the PR 3 ligand as quantified by Tolman's cone angle. This correlation is explained by an increase in the paramagnetic contribution to the overall shielding resulting from diminished electronic interaction between vanadium and the phosphine. Chelate complexes of types [V(CO)4L] −, η 5-C 5H 5V(CO) 2L, HV(CO) 4L, and η 3-C 3H 5V(CO) 3L, where L is a ditertiary phosphine Ph 2P(CH 2) n PPh 2 ( n = 1–4) or cis-Ph 2PCHCHPPh 2, exhibit a minimum 51V shielding in four-membered ring systems, paralleled by a maximum 31P shielding (maximum coordination shift); and a maximum 5'V shielding for five-membered chelate rings corresponding to a maximum 31P coordination shift. Ring size effects are correlated with variations in P-V χ overlap (deformation of interbond angles) and π-electron delocalization into the ring system. As PPh 2 is gradually substituted for the weaker AsPh2 in the chelate five-ring systems [V(CO) 4L′] ∼- and η 5-C 5H 5V(CO) 2L′ (L′ = Ph 2ECH 2CH 2E′Ph 2, o-C 6H 4(EPh 2)(E′Ph 2); E, E′= P, As), 51V shielding decreases, but there is an increase in shielding with the ligand o-C 6H 4(AsPh 2)(SbPh 2). This is caused by direct electronic interaction rather than by a “heavy atom effect” (nonlocal diamagnetic contributions), which is considered to be too small to be effective.

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