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(η6-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(η6-2-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 η1 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 η1-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.