Can the cyclo-P5 ligand introduce basicity at the transition metal center in metallocenes?: A hybrid density functional study on the cyclo-P5 analogues of metallocenes of Fe, Ru and Os

Density functional theory (DFT) calculations and natural population analysis (NPA) performed on metallocenes of Fe, Ru and Os triad reveal that the cyclo-P-5 ligand introduces basicity at the metal center. All-electron calculations at the B3LYP/6-311+G * level yield NPA charges of 0.28, -0.28 and -0.91 at the iron center in [FeCP2], [FeCp(eta(5)-P-5)] and [Fe(eta(5)P(5))(2)], respectively. The same trend in the charges is observed even when the core electrons of the metal are replaced by effective core potentials (ECPs). The accumulation of negative charge at the metal center is found to be similar in the cyclo-P-5 analogues of ruthenocene and osmocene. Thus, for example, the net NPA charges on Ru and Os in the decaphos-phametallocenes are predicted to be -0.6 to -0.9, respectively, at different levels of calculations involving ECPs. The natural orbital populations show a small transfer of electron density from each phosphorus atom to the metal center, leading to a significant build-up of negative charge on the metal in [M(eta(5)-P-5)(2)]; this is a new feature observed in the transition metals bound to CYC10_P5 ring. This finding is indeed supported by the indirect evidence observed by Scherer and coworkers from the diamagnetic behavior in an iridium derivative of Fe(P-5), that the Fe atom is negatively charged while one of the P atoms is positively charged. Natural bond orbital analysis reveals that the extent of covalent bond formation between the metal and the ligand is more pronounced for the cyclo-P-5 than the cyclopentadienyl ligand. While Cp retains 56-68% of aromaticity, the P-5 ring exhibits 26-51% aromaticity as compared to the free anionic rings. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).