X-ray structure of [Nb(η5 -C5H5)2(SCH2CH2CH2S].: ECP quantum chemistry of niobium five and six metallacycle complexes

[Nb(eta(5) -C5H5)(2)PDT] (1) (PDT= propane-1,3-dithiolate), eta(5)-C5H5 = Cp, in the chair form, was characterized by spectroscopic methods as well as by X-ray crystallography, that indicated a tetrahedral geometry around niobium. Quantum chemistry calculations were performed for 1, for the hypothetical complex containing the five member ring metallacycle [NbCp2EDT], (EDT = ethane-1,2-dithiolate), (2), as well for the cationic complex analog of 2, [NbCp2EDT](+), 2(+). The data obtained indicated also a tetrahedral geometry around niobium in 2, with a S-Nb-S angle of 82.1degrees. The five-membered ring adopts a twisted conformation, with the carbon atoms lying +/-12.5degrees above and below the S-Nb-S plane, resulting in the conformers lambda and Delta, that are separated by an ROHF/ECP barrier of 8.6 kcal mol(-1). For 2(+), the S-Nb-S angle was found to increase to 85.6degrees, in agreement with the fact that the single occupied SOMO orbital is expected to keep the L-M -L' angle to values close to 80degrees. For complex 1 the obtained theoretical results are in good agreement with parameters obtained from the X-ray crystallographic studies. (C) 2002 Elsevier Science Ltd. All rights reserved.