1-[(η⁵-C₅R₅)Co]B₄H₈ (R = H Or Me) Sandwich Complexes Containing a Square Cyclic B4H₈ 2- Ligand Analogous to C4H4²⁻: Structural and Spectroscopic Studies


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-C 4H4)] 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-C 4H4)], are only slightly out of the B4 (or C4) plane, in contrast to B5H9 where these hydrogens are substantially bent toward the apex.



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