Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η5-C5H5)2Cr2(CS)2(CO)3 in contrast to (η5-C5H5)2Cr2(CO)5

The cyclopentadienylchromium carbonyl thiocarbonyls Cp2Cr2(CS)2(CO)n (n = 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The lowest energy Cp2Cr2(CS)2(CO)4 structure can be derived from the experimentally characterized unbridged Cp2Cr2(CO)6 structure by replacing the two terminal carbonyl groups furthest from the Cr–Cr bond with two terminal CS groups. The two lowest energy Cp2Cr2(CS)2(CO)3 structures have a single four-electron donor η2-μ-CS group and a formal Cr–Cr single bond of length ∼3.1 Å. In contrast to the carbonyl analogue Cp2Cr2(CO)5 these Cp2Cr2(CS)2(CO)3 structures are viable with respect to disproportionation into Cp2Cr2(CS)2(CO)4 and Cp2Cr2(CS)2(CO)2 and thus are promising synthetic targets. The lowest energy Cp2Cr2(CS)2(CO)2 structures have all two-electron donor CO and CS groups and short Crtriple bond; length of mdashCr distances around ∼2.3 Å suggesting the formal triple bonds required to give the chromium atoms the favored 18-electron configurations. These Cp2Cr2(CS)2(CO)2 structures are closely related to the known structure for Cp2Cr2(CO)4. In addition, several doubly bridged structures with four-electron donor η2-μ-CS bridges are found for Cp2Cr2(CS)2(CO)2 at higher energies. The global minimum Cp2Cr2(CS)2(CO) structure is a triply bridged triplet with a Crtriple bond; length of mdashCr triple bond (2.299 Å by BP86). A higher energy singlet Cp2Cr2(CS)2(CO) structure has a shorter Cr–Cr distance of 2.197 Å (BP86) suggesting the formal quadruple bond required to give each chromium atom the favored 18-electron configuration.