A theoretical study for quantum chemical calculations of tungsten tetracarbonyl complexes with tetrylone ligands E(PH₃)₂ (E = C - Pb): a quest for understanding

Quantum chemical calculations at the BP86 level with various basis sets (SVP, TZVPP) have been carried out for transition metal complexes of carbodiphosphorane analogues (called tetrylones) E(PH₃)₂ with E = C, Si, Ge, Sn, Pb. The nature of the [W(CO)₄-E(PH₃)₂] (W4-EP₂) bonds was analyzed with charge method. The equilibrium structures of the tetrylone complexes W(CO)₄-E(PH₃)₂ exhibit an interesting trend which possess for E = C, Si a trigonal bipyramidal coordination at tungsten where the tetrylone ligand occupies an equatorial position. The heavier homologues with E = Ge, Sn, Pb exhibit a square pyramidal coordination at tungsten where the tetrylone ligand is at a basal position while one phenyl group is found trans to the apical CO group which yields a hexacoordinated tungsten complex. The calculated equilibrium structures of the complexes [(CO)₄W-{E(PH₃)₂}] show that only the carbon ligand C(PH₃)₂ is bonded head-on to the metal fragment in the adduct W4-CP₂ which the bending angle, α, is 180.0° and the heavier homologues W4-SiP₂ to W4-PbP₂ have side-on bonded ligands E(PH₃)₂ which the bending angle, α become more acute. The theoretically predicted bond dissociation energies (BDEs) of W4-EP₂ system slightly increase from W4-GeP₂ (D_e = 46.1 kcal/mol) to W4-PbP₂ (D_e = 48.5 kcal/mol). The strange trend has been observed for the slighter homologues with E = C, Si which exhibits the largest value BDE 54.1 kcal/mol for W4-CP₂ and 50.1 kcal/mol for W4-SiP₂. Analysis of the bonding situation shows that the W-E bonds have a significant contribution from (CO)₄W←E(PH₃)₂ π-donation. This is because the energy levels of the π-type donor orbitals of W4-CP₂-W4-SnP₂ are higher lying than the σ-type donor orbitals except for W4-PbP₂ where the σ donor HOMO is higher in energy than the π-type donor HOMO-2. All complexes [W(CO)₄-E(PH₃)₂] are suitable targets for synthesis which would open up completely new perspectives for experimental researchers. (author)