On the Chemical Shifts of Agostic Protons

Agostic hydrogen atoms in planar
d8 transition metal
complexes display a remarkable wide range of chemical shifts from
+5 to −10 ppm in the proton NMR spectra. It is therefore surprising
that a simple recipe can be elaborated to predict the influence of
the local electronic structure of the metal atom on the shielding
of the coordinating protons: In cases where the agostic hydrogen atom
is pointing to a local Lewis acidic center at the metal the 1H NMR signal is shifted upfield relative to the scenario where the
proton is opposing a local charge concentration at the metal. To trace
the physical origin of this empirical relationship, a systematic study
has been performed to understand how the (i) topology of the electron
density and (ii) orientation of the magnetic field vector, B0, control the paratropic or diatropic characteristics
of the induced current density at the metal atom and thus the shielding
or deshielding of the agostic protons.