Tris(cyclopentadienyl)praseodymium is an organopraseodymium compound with the formula Pr(C5H5)3, commonly abbreviated PrCp3. It is a pale-green organometallic solid containing praseodymium in the +3 oxidation state and three cyclopentadienyl ligands. In the solid state it forms a polymeric chain structure rather than discrete PrCp3 molecules.
Preparation Tris(cyclopentadienyl)praseodymium was among the first organolanthanide cyclopentadienyl compounds reported by Wilkinson and Birmingham in 1954.
It can be prepared by reacting anhydrous praseodymium(III) chloride with sodium cyclopentadienide in tetrahydrofuran:
PrCl3 + 3 NaC5H5 → Pr(C5H5)3 + 3 NaCl
The crude product commonly contains coordinated THF and can be purified by sublimation under high vacuum. Preparative and spectroscopic studies have used sublimation at about 200–250 °C under pressures of approximately 10−2–10−4 mmHg.
Properties Structure and bonding Sublimed PrCp3 crystallizes in the monoclinic space group P21, with lattice parameters a = 8.314 Å, b = 9.714 Å, c = 8.372 Å and β = 116.06°. The solid forms polymeric chains. Each Pr3+ ion is η5-coordinated by three nearby cyclopentadienyl ligands, while one of these Cp rings additionally bridges to a neighboring praseodymium ion through η2 coordination. This gives a formal coordination number of 11 around praseodymium. This behaviour reflects the large ionic radius and high preferred coordination number of early lanthanides. The bonding in lanthanide cyclopentadienyl complexes is substantially more ionic than in many transition-metal metallocenes, although covalent contributions from metal–ligand orbital interactions are also present.
Volatility and thermal behaviour PrCp3 is sufficiently volatile to be purified by vacuum sublimation. Thermochemical measurements give a standard enthalpy of sublimation of about 125.5 kJ mol−1 near room temperature. The compound melts only at high temperature and begins to decompose on melting. Reported melting/decomposition temperatures are around 420–430 °C, while sublimation is practical at much lower temperature under vacuum.
Electronic properties Photoelectron spectroscopy of gaseous PrCp3 has been used to investigate the electronic structure of the lanthanide–cyclopentadienyl bond. The spectra show ionizations associated with the cyclopentadienyl π orbitals together with localized Pr 4f-derived states. Comparison across Pr, Nd and Sm tris(cyclopentadienyl) complexes shows that the ligand-based ionization energies change relatively little across the series, while the 4f states shift more substantially.
Coordination chemistry PrCp3 acts as a Lewis-acidic organolanthanide fragment and can form adducts with donor ligands. For example, THF adducts of the type PrCp3(THF) are known, and related tris(cyclopentadienyl)lanthanide complexes form adducts with nitriles and isocyanides. PrCp3 can also participate in formation of anionic cyclopentadienyl complexes. In the presence of 1,10-phenanthroline, reaction of PrCl3 with sodium cyclopentadienide gives the ion-pair complex [Na(phen)3][Pr(C5H5)3Cl]·phen.
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