Antiferromagnetic complexes with the metalmetal bond. XVI. Synthesis and molecular structure of binuclear dihalogenide cyclopentadienly-t-butylate chromium(III) complexes

Reactions of (CpCrOR) 2 (I) (R = CME 3 ) with CH 2 X 2 in toluene and benzene at room temperature yield the binuclear complexes Cp 2 Cr 2 (μ-OR) 2 X 2 (X = Cl (II); Br (III); I (IV)) with the X ligands in the cis -position with respect to the CPCrCrCp moiety. The X-ray structural study of complexes II–Iv showed that the CrX distances (2.28(1), 2.29(1); 2.478(2); 2.716(2)m 2.711(2) A in II–IV) are markedly shortened compared with the sums of the corresponding covalent radii, owing to additional XCr π-bonding which involves the half-filled orbitals of the Cr atoms. This shortening is accompanied by the elongation of the CrCr bonds from 2.635 A in I up to 2.917(7), 2.971(2) and 2.967(2) A in II–IV and the weakening of the antiferromagnetic exchange interactions (−2 J = 150, 168 and 148 cm−1 for II–IV respectively as compared with −2 J = 246 cm−1 for I). The similarity between the ligand environment of the chromium atom in I and the vanadium atom in Cp 2 V is described.