Np(COT)* and PU(COT)~ (COT = C8Ha2-) were prepared by the reaction in THF solution of KzCOT with NpC14 and [(CzHS)4N]2P~C16, respectively. Infrared spectra of both were consistent with D8h molecular symmetry ("sandwich" compounds), and X-ray diffraction patterns showed both to be isomorphous with U(COT)*. The isomer shift of the Mossbauer spectrum of Np(COT)* was unusually positive for a Np4+ compound, indicating an electron contribution from the ligand to the bonding. The magnetic moments measured between 4.2 and 44'K were 2.43 BM for U(COT)z and 1.81 BM for Np(C0T)z; Pu(C0T)z was diamagnetic. A crystal-field model with an orbital reduction factor, k, of 0.8 is in reasonable agreement with the experimental results.he reduction of cyclooctatetraene in tetrahydro-T furan (THF) solution produces the planar CsHs2anion (COT),4s5 which can react with the anhydrous metal chlorides of transition metals to form M(COT), M2(COT)3, or M(COT)2 complexes.6 The preparation of U(COT)2 extended these complexes to actinide-Crystallographic studiess show that the U(COT)2 molecule has a "sandwich" structure, with planar eight-member COT rings above and below the U4f ion in D8h molecular symmetry. An electronic configuration has been proposed' for U(COT)2 that involves participation of the 5f orbitals in the bonding of the complex, with the orbitals of the two 5f electrons of the U4+ ion mixing with the degenerate Esu orbitals of the ligands in essentially a nonbonding combination.NP(COT)~ and PU(COT)~ were prepared to allow a more thorough investigation of the nature of the actinide(1V)-COT compounds. Mossbauer measurements on Np(COT)* provide information on the shielding of s orbitals by outer electrons, and thus on the participation of ligand electrons in the bonding. Magnetic susceptibility measurements on U(COT)z, Np-(COT)2, and Pu(COT)2 (U4+, 5f2; Np4+, 5f3; Pu4+, 5f4) narrow the choice of applicable molecular models. Experimental SectionReagents. The preparation of UCL, NpC14, and [(CZH~)~N]Y PuCl6 has been previously r e p ~r t e d . ~ Cyclooctatetraene, benzene, and toluene were of reagent grade, and were further dried by passage through 4-A molecular sieve. Dissolved oxygen was removed by repeatedly evacuating to about 0.3 atm. Tetrahydrofuran (THF) was purified by distillation from LiAlH4 in a nitrogen atmosphere. The quality of THF was found to be a major factor in the success of preparations; addition of LiAlH4 as a solution in