Mechanism for the Reaction of White Phosphorus with CpCr(CO) Leading Ultimately to the Triple-Decker Sandwich CpCr(μ-η,η-P): A Theoretical Study.

The experimentally known reaction of CpCr(CO) with white phosphorus (P) to give CpCr(CO)(η-P), CpCr(CO)(μ-η,η-P), and the triple-decker sandwich CpCr(μ-η,η-P) is of interest since the P reactant having a tetrahedral cluster of four phosphorus atoms is converted to products having P, P, and P 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 P, Mn(CO), and CpCr(CO) dimerization processes. The first stage of the CpCr(CO)/P reaction mechanism generates the unsaturated singlet intermediate CpCr(CO) that combines with the P reactant. Decarbonylation of the resulting CpCr(CO)(P) complex provides a singlet tetracarbonyl readily fragmenting into the stable triphosphacyclopropenyl complex CpCr(CO)(η-P) and the chromium phosphide CpCr(CO)(P). The isomeric triplet tetracarbonyl CpCr(CO)(P), readily fragments into CpCr(CO)(η-P), which can generate the stable diphosphaacetylene complex CpCr(CO)(η,η-P) as well as the pentamer [CpCr(CO)](P). Combination of the coordinately unsaturated CpCr(CO)(η-P) with CpCr(CO)(η-P) can lead to a ring expansion. This generates the P pentagonal ligand in a CpCr(CO)(P) precursor to the experimentally observed carbonyl-free triple-decker sandwich CpCr(μ-η,η-P) after three successive decarbonylations.