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Dimethylformamide‐Exchange Mechanism on Some First‐Row Divalent Transition‐Metal Ions
Author(s) -
Cossy Cédric,
Helm Lothar,
Merbach André E.
Publication year - 1987
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19870700608
Subject(s) - chemistry , steric effects , divalent , dimethylformamide , transition metal , metal , crystallography , inorganic chemistry , medicinal chemistry , stereochemistry , organic chemistry , catalysis , solvent
The effect of temperature on the dimethylformamide exchange on Mn(DMF) 2+ 6and Fe(DMF) 2+ 6has been studied by 13 C‐ and 17 O‐NMR, respectively, yielding the following kinetic parameters: k 298 equals; (2.2±0.2). 10 6 S −1 , Δ H ≠ = 34.6 ± 1.3 kJ mol −1 , Δ S ≠ = −7.4 ± 4.8 J K −1 mol −1 for Mn 2+ and K 298 = (9.7 ± 0.2).10 5 S −1 , Delta; H ≠ = 43.0 ± 0.9 kJ mol −1 , Δ S ≠ = + 13.8 ± 2.8 J K −1 mol −1 for Fe 2+ . The volumes of activation, Δ V ≠ in cm 3 mol −1 , derived from high‐pressure NMR on these metal ions, together with the previously published activation volumes for Co 2+ and Ni 2+ (+2.4 ± 0.2 (Mn 2+ ), +8.5 ± 0.4 (Fe 2+ ) +9.2 ± 0.3 (Co 2+ ), + 9.1 ± 0.3 (Ni 2+ )) give evidence for a dissociative activation mode for DMF exchange on these high‐spin first‐row transition‐metal divalent ions. The small positive Δ V ≠ value observed for DMF exchange on Mn 2+ seems to indicate that a mechanistic changeover also occurs along the series, (probably from I d to D), as for the other solvents previously studied ( I a to I d , for H 2 O, MeOH, MeCN). This changeover is shifted to the earlier elements of the series, due to more pronounced steric crowding for dimethylformamide hexasolvates.