Comparison Of Binuclear Phospholyl And 1,3-Diphosphacyclobutadiene Sandwich Compounds Chromium Carbonyl Derivatives With Their Cyclopentadienyl And Cyclobutadiene Sandwich Analogues:Role Of Phosphorus Lone Pair

Recently,transition metal carbonyl compounds have been a highlight in organometallic chemistry and other relecant fields.In this paper,BP86 and M06-L,two common and efficient density functional（DFT）theory methods,are used to study the effect of lone electrons of phosphorus atom on the structures of binuclear phospholyl chromium carbonyl derivatives（（C₄H₄P）₂Cr₂（CO）_n（n=6,5,4,3））and diphosphacyclobutadiene sandwich compounds（（Me₂C₂P₂）₂Cr₂（CO）_n（n=6,5,4,3））.The main results are as follows:The low energy structures of（C₄H₄P）₂Cr₂（CO）_n（n=6,5,4,3）are studied with two DFT methods（BP86 and M06-L）.It is found that the lowest energy structure 6S-1,4S-1 and 3T-1of（C₄H₄P）₂Cr₂（CO）_n（n=6,4,3）have two five-electron donor terminalη⁵-C₄H₄P rings.For（C₄H₄P）₂Cr₂（CO）₅,apart from the similar one five-electron donor terminalη⁵-C₄H₄P ring,there is another seven-electron donor bridgingη⁵,η¹-C₄H₄P ring in the lowest energy structure5S-1 state.The bridgingη⁵,η¹-C₄H₄P ring is a seven-electron donor through fiveÏ€electrons giving to one chromium atom and two-electrons giving to the other chromium atom through a P→Cr coordination bond from the phosphorus lone pair.However,the relative energies of the structures without bridgingη⁵,η¹-C₄H₄P ring among all low energy structures of（C₄H₄P）₂Cr₂（CO）₅ are slightly higher than that of 5S-1.This means that compared withÏ€bond,the phosphorus lone pair has a greater influence on the stability of the dichromic carbonyl complex of（C₄H₄P）₂Cr₂（CO）_n（n=6,5,4,3）.For the low energy structures of（Me₂C₂P₂）₂Cr₂（CO）_n（n=6,5,4,3）,there is no two-electrons giving to the two chromium atoms through a P→Cr coordiantion bond from the phosphorus lone pair in the lowest energy structures 3T-1 and 4T-1 of（Me₂C₂P₂）₂Cr₂（CO）_n（n=4,3）.As for（Me₂C₂P₂）₂Cr₂（CO）_n（n=6,5）,however,the structures of the lowest energetic6S-1 and 5T-1 of（Me₂C₂P₂）₂Cr₂（CO）_n（n=6,5）is very interesting due to the phosphorus lone pair.The two lowest energy structures have two bridgingη⁴,η¹-Me₂C₂P₂ rings,bonding to one of the two Cr atoms through the ringÏ€electrons and the other Cr through the lone pair of a phosphorus atom.Moreover,both structures do not have bonding between the metal atoms due to the very long Cr-Cr distance.This reveals that Cr atoms with 4 terminal CO groups in 6S-1 and 5T-1 have the favorite 18-electron configurations.