Mechanism for the Reaction of White Phosphorus with Cp2Cr2(CO)6 Leading Ultimately to the Triple-Decker Sandwich Cp2Cr2(μ-η5,η5-P5): A Theoretical Study

The experimentally known reaction of Cp2Cr2(CO)6 with white phosphorus (P4) to give CpCr­(CO)2(η3-P3), Cp2Cr2(CO)4(μ-η,2η2-P2), and the triple-decker sandwich Cp2Cr2(μ-η,5η5-P5) is of interest since the P4 reactant having a tetrahedral cluster of four phosphorus atoms is converted to products having P2, P3, and P5 ligands. The mechanism of this obviously complicated reaction can be dissected into three stages using a coupled cluster theoretical method that has been benchmarked with the P2, Mn­(CO)5, and CpCr­(CO)3 dimerization processes. The first stage of the Cp2Cr2(CO)6/P4 reaction mechanism generates the unsaturated singlet intermediate Cp2Cr2(CO)5 that combines with the P4 reactant. Decarbonylation of the resulting Cp2Cr2(CO)5(P4) complex provides a singlet tetracarbonyl readily fragmenting into the stable triphosphacyclopropenyl complex CpCr­(CO)2(η3-P3) and the chromium phosphide CpCr­(CO)2(P). The isomeric triplet tetracarbonyl Cp2Cr2(CO)4(P4), readily fragments into CpCr­(CO)2(η2-P2), which can generate the stable diphosphaacetylene complex Cp2Cr2(CO)4(η,2η2-P2) as well as the pentamer [CpCr­(CO)2]5(P10). Combination of the coordinately unsaturated CpCr­(CO)­(η3-P3) with CpCr­(CO)2(η2-P2) can lead to a ring expansion. This generates the P5 pentagonal ligand in a Cp2Cr2(CO)3(P5) precursor to the experimentally observed carbonyl-free triple-decker sandwich Cp2Cr2(μ-η,5η5-P5) after three successive decarbonylations.