Electronic and Steric Manipulation of the Preagostic Interaction in Isoquinoline Complexes of Rh I

Structures and properties were obtained by density functional calculations for the complexes [RhCl(CO)2(L)] (L = isoquinoline ligands) containing close Rh····H separations. Electrostatic repulsion keeps the atoms apart and Rh····C separations are prevented from developing much further by ligand inflexibility. Natural bond orbital analysis shows small components of C–Hσ to Rh (agostic) donation and Rh to C–Hσ* backdonation. Both σ- and π-electron-withdrawing substituents produce an increase in the Rh····H separation, while ligand inflexibility prevents σ- and π-electron-donating substituents from decreasing the Rh····C separation. Substitution at C2 by Me closes up the Rh····H separation and CHMe2 and CMe3 groups cause a structural change which minimises the interaction. Substitution at C2 by a Ph group gives the shortest Rh····H separation (2.187 A) and largest Rh to C–Hσ* backdonation. Electron donation at C7 or at C7 and C9 increases the Rh····H separation, as the ligand flexes substantially to accommodate a decrease in the Rh····C separation. On the basis of the minimal C–Hσ to Rh (agostic) donation involved, the complexes are best described as preagostic.