Complexation behavior of two-coordinated carbon compounds containing fluorenyl ligands

DFT calculations using BP86 in conjunction with the SVP and TZVPP basis sets as well as ab initio calculations at SCS-MP2 have been carried out for six dicoordinated carbon molecules CLL′ where L is a fluorenyl carbene while L′ is a phosphine PH₃ (1) or PPh₃ (2) or a carbene, i.e. NHC_Me (3), benzannulated NHC_Me (4), cycloheptatrienylidene (5) and benzannulated cycloheptatrienylidene (6). The complexes of these compounds with one and two AuCl moieties were also calculated. The monoaurated adducts of 1-4 have the AuCl fragment η¹ coordinated to the central carbon atom. The complexes 5(AuCl) and 6(AuCl) have AuCl η² bonded across a CC double bond. Three different bonding modes are found as energy minima for the diaurated species LL′C-(AuCl)₂. The AuCl fragments are found to be either both coordinated η¹, both coordinated η² across double bonds, or a combination of the two. According to the electronic structure analysis of the free compounds, 1 and 2 might best be classified as carbenes, 3 and 4 as bent allenes while 5 and 6 are typical allenes. The complexation with AuCl reveals that 1-4 may exhibit chemical behaviour which is typical for carbones and thus, they may be termed "hidden carbones". The AuCl complexes show that compounds 5 and 6 are classical allenes.