Thermal stability of Group 6 bis(cyclopentadienyl) and ethylene bridged bis(cyclopentadienyl) monocarbonyl complexes; a theoretical study

Abstract

Density functional calculations have been used to estimate the energy change involved in dissociation of CO from [M(η-C 5 H 5 ) 2 (CO)] (M = Cr, Mo or W) and [Cr{C 2 H 4 (η-C 5 H 4 ) 2 }(CO)] to give, in each case, free CO and a triplet metallocene product. The dissociation energy for [Mη(-C 5 H 5 ) 2 (CO)] is calculated to be 99 (Cr), 186 (Mo) and 194 kJ mol Ϫ1 (W) showing [Cr(η-C 5 H 5 ) 2 (CO)] to be significantly less thermally stable than the Mo or W analogues consistent with the experimental findings. For the ansa-bridged compound [Cr{C 2 H 4 (η-C 5 H 4 ) 2 }(CO)] the calculated dissociation energy was 112 kJ mol Ϫ1 , significantly more endothermic than for [Cr(η-C 5 H 5 ) 2 (CO)], consistent with the enhanced thermal stability found for [Cr{C 2 Me 4 (η-C 5 H 4 ) 2 }(CO)] as a result of introduction of an ansa-bridge. Theoretical analysis shows that the cause of the differences in thermal stability lies in the stability of the triplet metallocene product. The high spin-pairing energy for Cr favours formation of a triplet state more than is the case for Mo and W. Formation of a triplet state is of less advantage in the ansa-bridged free chromocene as the rings are unable to relax to a near parallel structure, thereby lowering the energy of the d electrons. Such relaxation is prevented by the presence of an ansa-bridge.

Ansa‐Cycloheptatrienyl–Cyclopentadienyl Complexes

“…Further experiments are required to quantify the interaction between 1 and CO and to produce kinetic and thermodynamic data for this equilibrium reaction in a similar fashion as described for instance for the reaction of CO with decamethylcalciocene, [(η‐C 5 Me 5 ) 2 Ca],20 and chromocene, [(η‐C 5 H 5 ) 2 Cr] 21. 22…”

Formation and reactivity of CrIIcarbonyls hosted in polar and non polar supports

“…[1][2][3][4][5][6][7] Metallocenes are organometallic substrates of interest to investigate potential reactivity enhancement; among them, chromocene (Cp 2 Cr) is a challenging candidate, both for applicative (being precursor of the well known Union Carbide olefin catalyst 8 ) and for theoretical reasons (because the modeling of open shell systems is still a challenge). [9][10][11][12][13] The experimental approaches adopted so far to investigate the properties and the reactivity of these systems (such as matrix isolation technique 14 and dissolution in a solvent 15,16 ), suffered of some disadvantages. Among the major problems we cite the air sensitivity of Cp 2 Cr (that forced us to work in controlled atmosphere) and its tendency to form clusters (so that most of the techniques give information about the clusters and not about the isolated molecules).…”

Ring‐Borylated 15‐Electron and 17‐Electron ansa‐Chromocene Complexes, their Physical Properties and Molecular Structures

“…The bonding in 18 e chromocene-carbonyl complexes has been analyzed in detail before, [23] and will not be discussed here. The carbonyl complexes have much more stable singlet states than the 16 e chromocenes.…”