  The amido rhenium trioxides of composition (iPr2N)ReO3, (iPrCyN)ReO3 and
(Cy2N)ReO3 (Cy = cyclohexyl) were synthesized in a one pot reaction starting
from Re2O7, Me3SiCl and the corresponding amines (iPr)2NH, (iPr)(Cy)NH, and
(Cy)2NH, respectively. In the solid state the amido ligands of all three
complexes are asymmetrically coordinated to the ReO3 core allowing for one
short Re-H-C contact in each case which might indicate the presence of
beta-agostic interaction. However, analysis of the charge density distribution
provided us clear-cut criteria that beta-agostic interactions are suppressed by
the trans-influence of the oxo-groups. Comparison with structurally related
tetra-coordinated d0 titanium amido complexes highlighted a systematic concept
how the extent of beta-agostic interactions in these complexes can be
controlled by reducing the trans-influence of the co-ligands. We therefore
suggest to employ the expression "hindered agostic interactions" in cases where
covalent M-H-C are in principle supported by vacant coordination sites at
Lewis-acidic metal centers but are actually hindered for electronic reasons. We
further point out that electronic hindrance originating from the transinfluence
might be overruled by changing the s/p-donor characteristics of ligands.
