Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η 5-C 5H 5) 2Cr 2(CS) 2(CO) 3 in contrast to (η 5-C 5H 5) 2Cr 2(CO) 5

Abstract

The cyclopentadienylchromium carbonyl thiocarbonyls Cp 2Cr 2(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 Cp 2Cr 2(CS) 2(CO) 4 structure can be derived from the experimentally characterized unbridged Cp 2Cr 2(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 Cp 2Cr 2(CS) 2(CO) 3 structures have a single four-electron donor η 2-μ-CS group and a formal Cr–Cr single bond of length ∼3.1 Å. In contrast to the carbonyl analogue Cp 2Cr 2(CO) 5 these Cp 2Cr 2(CS) 2(CO) 3 structures are viable with respect to disproportionation into Cp 2Cr 2(CS) 2(CO) 4 and Cp 2Cr 2(CS) 2(CO) 2 and thus are promising synthetic targets. The lowest energy Cp 2Cr 2(CS) 2(CO) 2 structures have all two-electron donor CO and CS groups and short Cr Cr distances around ∼2.3 Å suggesting the formal triple bonds required to give the chromium atoms the favored 18-electron configurations. These Cp 2Cr 2(CS) 2(CO) 2 structures are closely related to the known structure for Cp 2Cr 2(CO) 4. In addition, several doubly bridged structures with four-electron donor η 2-μ-CS bridges are found for Cp 2Cr 2(CS) 2(CO) 2 at higher energies. The global minimum Cp 2Cr 2(CS) 2(CO) structure is a triply bridged triplet with a Cr Cr triple bond (2.299 Å by BP86). A higher energy singlet Cp 2Cr 2(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.