title: Nickelacyclopentadienylchromium tricarbonyl unit as a bulky pseudohalogen in cyclopentadienylchromium complexes leading to low-energy high-spin structures.
abstract: Recent studies, particularly from the laboratory of Buchalski and co-workers, have resulted in the syntheses of nickelacyclopentadienyl and nickelafluorenyl metallacycles that can function as pentahapto ligands in transition-metal complexes, similar to the ubiquitous cyclopentadienyl ligand. The structures and energetics of the neutral binuclear chromium carbonyls (CpNiC4H4)2Cr2(CO)n (n = 6, 5, 4, 3; Cp = η(5)-C5H5) containing the unsubstituted nickelacyclopentadienyl ligand have been investigated by density functional theory. The lowest energy (CpNiC4H4)2Cr2(CO)n (n = 6, 4) structures are similar to the corresponding experimentally characterized Cp2Cr2(CO)n structures with predicted Cr-Cr distances of ∼3.22 and ∼2.27 Å corresponding to the formal single and triple bonds, respectively. This gives the chromium atoms, as well as the nickel atoms, the favored 18-electron configuration. These species appear to be promising synthetic targets. However, the lowest energy (CpNiC4H4)2Cr2(CO)n (n = 5, 3) structures, as well as two (CpNiC4H4)2Cr2(CO)4 structures ∼10 to 12 kcal/mol in energy above the global minimum, can be dissected into a discrete pseudohalogen (CpNiC4H4)Cr(CO)3 unit and a (CpNiC4H4)Cr(CO)n-3 unit linked by a Cr-Cr bond flanked by one to three generally weakly semibridging CO groups. In most cases, the chromium atoms in the (CpNiC4H4)Cr(CO)n-3 units of these structures have 14-16-electron configurations rather than the favored 18-electron configuration. This leads to triplet and even quintet spin states in the lowest energy structures.
authors.affiliation: †School of Science, Research Center for Advanced Computation, Xihua University, Chengdu, China 610039.
authors.forename: Yi
authors.initials: Y
authors.lastname: Zeng
authors.affiliation: †School of Science, Research Center for Advanced Computation, Xihua University, Chengdu, China 610039.
authors.forename: Ying
authors.initials: Y
authors.lastname: Yu
authors.affiliation: †School of Science, Research Center for Advanced Computation, Xihua University, Chengdu, China 610039.
authors.forename: Hao
authors.initials: H
authors.lastname: Feng
authors.affiliation: ‡Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States.
authors.forename: R Bruce
authors.initials: RB
authors.lastname: King
authors.affiliation: ‡Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States.
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer