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Interpretation of 31 P‐NMR Coordination Shifts for Phosphane Ligands. Ab Initio ECP/DFT Study of Chemical Shift Tensors in M(CO) 5 L [M = Cr, Mo, W; L = PH 3 , P(CH 3 ) 3 , PF 3 , PCl 3 ]
Author(s) -
Kaupp Martin
Publication year - 1996
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19961290510
Subject(s) - chemistry , chemical shift , paramagnetism , ab initio , density functional theory , crystallography , atomic orbital , protonation , lone pair , metal , ab initio quantum chemistry methods , molecular orbital , computational chemistry , stereochemistry , molecule , ion , physics , organic chemistry , quantum mechanics , electron
The 31 P chemical shift tensors of the transition‐metal phosphane complexes M(CO) 5 PX 3 (M = Cr, Mo, W; X = H, CH 3 , F, Cl) were studied using a combination of density functional theory and ab initio effective‐core potentials. The calculated isotropic shifts agree well with experimental results both for the free ligands and for the complexes, with the largest deviations occuring for the tungsten complexes. A breakdown of the computed phosphorus shielding tensors into contributions from localized molecular orbitals (LMOs) indicates that the positive coordination shift of PH 3 and P(CH 3 ) 3 is due to increased paramagnetic contributions from the phosphorus lone pair (P‐M s̀ bonding) LMO to δ ⊥ . A similar increase of this contribution is found for PF 3 and PCl 3 . However, for PCl 3 complexes these terms are overcompensated by a reduction in the paramagnetic contributions from the P‐Cl bonds and by shielding contributions from metal‐centered orbitals. This results in a negative overall coordiantion shift. A partial cancellation is found with P(CH 3 ) 3 and with PF 3 . The changes in the 31 P‐shift tensors of the same phosphane ligands upon protonation are qualitatively and quantitatively very different from the coordination shifts and do not provide good models for the latter.