Hemichelation, a way to stabilize electron-unsaturated complexes: the case of T-shaped Pd and Pt metallacycles

A rational method of synthesis of stable neutral T-shaped 14 electron Pd and Pt complexes is proposed. It takes advantage of the ambiphilic character of the tricarbonyl(η(6)-indenyl)chromium anion, of which the main property is to behave as a hemichelating ligand, that is a nonconventional heteroditopic ligand capable of chelating a metal center by way of covalent and noncovalent bonding, thus preserving its unsaturated valence shell. The reaction of the in situ formed tricarbonyl(η(6)-2-methylindenyl)chromium anion with a series of Pd and Pt metallacycles afforded new air-stable and persistent synfacial heterobimetallic complexes in which the metallacycle binds the indenyl fragment via its metal in an η(1) fashion, leaving the fourth coordination site at the chelated metal virtually vacant. The structures of eight of these novel complexes are disclosed, and their bonding features are investigated by an array of theoretical methods based on the density functional theory (NBO, EDA, ETS-NOCV, AIM, NCI region analysis). Theory shows that the formation of these unusual structures of bimetallic synfacial η(1)-indenyl-Pd/Pt complexes is driven thermodynamically by attractive Coulombic occlusion of the fourth vacant coordination site at Pd/Pt centers by the Cr(CO)3 moiety.