1-[(η5-C5R5)Co]B4H8(R = H or Me) sandwich complexes containing a square cyclic B4H82– ligand analogous to C4H42–: structural and spectroscopic studies

Abstract

The title complexes, prepared in moderate yields via an improved procedure, have been investigated by X-ray crystallography, n.m.r., u.v., and i.r. spectroscopy, and compared with their structural relatives B₅H₉, 1-[(CO)₃Fe]B₄H₈, and [Co(η⁵-C₅H₅)(η⁴-C₄H₄)] as well as their 2-[(η⁵-C₅R₅)Co]B₄H₈(R = H or Me) isomers. Disorder of the C₅Me₅ ring prevented satisfactory refinement of the crystallographic data for 1-[(η⁵-C₅R₅)Co]B₄H₈(R = Me); however, for the parent (R = H) molecule, a model having four-fold disorder in the C₅H₅ ring was found to refine satisfactorily. The complex has a sandwich structure in agreement with the geometry previously assigned from n.m.r. data. The molecular bond lengths and angles are close to those of their counterparts in B₅H₉, and the unusually short Co–B distance [1.977(2)Å] suggests a strong covalent metal–B₄H₈^2– linkage; this value is similar to the mean Co–C(C₄H₄) distance in [Co(η⁵-C₅H₅)(η⁴-C₄H₄)][1.965(1)Å] despite the smaller covalent radius of carbon versus boron. The terminal H atoms on the B₄H₈^2– ligand, like those in [Co(η⁵-C₅H₅)(η⁴-C₄H₄)], are only slightly out of the B₄(or C₄) plane, in contrast to B₅H₉ where these hydrogens are substantially bent toward the apex.