Nickelacyclopentadienylchromium Tricarbonyl Unit as a Bulky Pseudohalogen in Cyclopentadienylchromium Complexes Leading to Low-Energy High-Spin Structures

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)_<i>n</i> (<i>n</i> = 6, 5, 4, 3; Cp = η⁵-C₅H₅) containing the unsubstituted nickelacyclopentadienyl ligand have been investigated by density functional theory. The lowest energy (CpNiC₄H₄)₂­Cr₂(CO)_<i>n</i> (<i>n</i> = 6, 4) structures are similar to the corresponding experimentally characterized Cp₂­Cr₂(CO)_<i>n</i> 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)_<i>n</i> (<i>n</i> = 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)_<i>n</i>−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)_<i>n</i>−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