Chromium complex compounds cations

The chromium(IIl) compound contains a central tridentate oxide ligand (Fig. 2), an arrangement which is typical of other oxo-centred derivatives of chromium(III), such as the carboxylato and sulfato complexes. In the diethylcarbamato derivative, the central oxide ligand deviates by only 0.07 A from the plane of the three chromium atoms. It should be noted that the chromium derivative still retains a chloride ligand in its coordination as the starting material is anhydrous CrCl3, this corresponds to incomplete removal of chloride from the kinetically inert 3d chromium(III) cation. 

It is well known that the tricarbonylchromium-complexed benzylic anions and cations are stabilized due to overlapping between d-orbital of the chromium and p-orbital of the benzylic carbon [1]. Tricarbonylchromium complexes of a-te-tralone and a-indanone having a carbonyl group at the side chain underwent a deprotonation of the exo-benzylic protons by treatment with base to give the stereo-controlled tricyclic compounds (Eqs. 1 and 2) [2]. In these cases, Robinson annulation products were formed in less than 10% yield. Also, base treatment of benzyl ether chromium complex having a chlorine at the side chain 3 gave cyclization product as a diastereomeric mixture (Eq. 3) [3]. 

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