FULL PAPER Can the cyclo-P 5 Ligand Introduce Basicity at the Transition Metal Center in Metallocenes? A Hybrid Density Functional Study on the cyclo-P 5 Analogues of Metallocenes of Fe, Ru and Os E. J. Padma Malar [a] Keywords: Basicity / Density functional calculations / Electronic structure / Metallocenes / Natural charge / Transition metals 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 [FeCp 2 ], [FeCp(η 5 -P 5 )] and [Fe(η 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 nega- tive 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, respect- ively, at different levels of calculations involving ECPs. The natural orbital populations show a small transfer of electron Introduction Cyclopentadienyl, C 5 H 5 (Cp), is a very common and ver- satile ligand in the field of organometallic chemistry. [1-4] The inorganic ring cyclo-P 5 , also known as pentaphospholyl or pentaphosphacyclopentadienyl, is π isoelectronic with Cp and is known to form complexes with several transition metals. [5-7] The first carbon-free entirely inorganic sand- wich complex [Ti(η 5 -P 5 ) 2 ] 2- was synthesized and charac- terized in 2002 by Urnezius et al. [8] The crystal-structure analyses of mixed metallocenes [M(η 5 -C 5 Me 4 R)(η 5 -P 5 )] (M = Fe, Ru; R = Me, Et) and [Ti(η 5 -P 5 ) 2 ] 2- show a sandwich structure with two parallel η 5 - or π-bonded rings. [6a,6b,8] Besides the pentaphospolyl ligand, inorganic rings of the type E n (E = As, P; n = 3-6) are also known to form complexes with transition metals. [7] Numerous tran- sition metal complexes have been studied with heterocyclic ligands of the type P n (CH) 5-n (n = 1-4). [9-11] Phosphorus heterocycles are also known to form sandwich complexes [a] Department of Physical Chemistry, University of Madras, Guindy Campus, Chennai-600025, India E-mail: ejpmalar@vsnl.net Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. Eur. J. Inorg. Chem. 2004, 2723-2732 DOI: 10.1002/ejic.200400058  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2723 density from each phosphorus atom to the metal center, lead- ing to a significant build-up of negative charge on the metal in [M(η 5 -P 5 ) 2 ]; this is a new feature observed in the transition metals bound to cyclo-P 5 ring. This finding is indeed sup- ported by the indirect evidence observed by Scherer and co- workers from the diamagnetic behavior in an iridium derivat- ive 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% aro- maticity as compared to the free anionic rings. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) with some lanthanides. [10] The similarity between cyclo-P 5 and the Cp ligands towards complex formation is a mani- festation of the ‘‘diagonal relationship’’ of the elements car- bon and phosphorus in the Periodic Table; they have similar electronegativity values. [7,11] In recent years the role of planar aromatic heterocycles possessing novel structural and bonding features in catalysis has been recognised. [12] Although experimental studies illustrate the potential of the P 5 ring as a ligand and can lead to a wealth of interest- ing complexes, there is still not much known about the structure and bonding in complexes containing a P 5 ring. Semiempirical and ab initio molecular orbital studies show that cyclo-P 5 - is aromatic in nature. [13-15] Early investi- gations [16] on bonding and stabilities in the pentaphospho- lyl complexes of iron were based on the extended Hückel theory. A number of studies document the importance of electron-correlation methods in obtaining reliable predic- tions in transition metal compounds. [17-19] Recent studies show that density functional theory (DFT) [20] calculations are remarkably successful in predicting a variety of proper- ties to an accuracy which rivals that of more expensive cor- related ab initio methods. Several recent works reveal that the DFT method provides reliable results in the case of transition metal and lanthanide complexes. [21] DFT analysis