We recently reported the existence of direct bonding interactions between uranium atoms and noble-gas (Ng) atoms [1] in the matrix-isolated neutral CUO molecule from the reaction of laser-ablated U atoms with CO and isolated in solid noble-gas matrices. [2] We have since demonstrated that CUO forms complexes with four or more noble-gas atoms bonded to the uranium atom of CUO. [3,4] The discovery of U À Ng bonding depended on the remarkable energetic closeness of two very different electronic states of CUO with distinct vibrational frequencies, and we have demonstrated that the binding of three or four Ar, Kr, or Xe atoms to CUO causes a changeover in the electronic ground state of the CUO(Ng) n complexes. [5,6] The discovery of actinide-noble-gas bonding adds to the recent renaissance in noble-gas chemistry, [7,8] including the HArF molecule of Räsänen and co-workers, [9] which is kinetically stable in solid argon, and the [AuXe 4 ] 2+ ion of Seidel and Seppelt, [10] which is stable enough to be characterized by X-ray crystallography. as 57 kcal mol À1 per bond. [11,12] The UÀNg bonds in our systems and the AuÀXe bonds in [AuXe 4 ] 2+ are formed from Lewis acid-base interactions in which electron density in the Ng lone pairs is donated into vacant orbitals on the metal center. We anticipated that part of the strong interaction between the Au and Xe atoms is due to charge polarization effects facilitated by the positive charge of Au 2+ , and we predicted that the positively charged UO 2 2+ ion, which is isoelectronic with CUO, might form stronger UÀNg bonds.[1] Herein we report that U V monocation, UO 2 + , forms a variety of noble-gas complexes in solid noble-gas matrices, and our calculations indicate that the UÀNg bonding in these complexes is stronger than the UÀNg bonds that involve neutral CUO. The UO 2 + ion has previously been isolated and characterized by a 980.1 cm À1 antisymmetric UÀO stretching frequency in solid neon.[13]The laser-ablation matrix-isolation method used to prepare uranium dioxide molecules has been described previously. [13][14][15] Laser-ablated uranium atoms insert into dioxygen to form UO 2 during condensation with excess noble gas at 4 K. Other product molecules include UO and UO 3 . In addition laser-ablation produces U + cations, which react in a similar fashion to form the linear, centrosymmetric UO 2 + cation.[13] These uranium-bearing species are identified and characterized from matrix-infrared spectra by using 16 [16] and to frequencies calculated by using relativistic density functional theory (DFT).The UO 2 + cation exhibits a strong, sharp UÀO stretching mode absorption at 980.1 cm À1 in solid neon, at 952.3 cm À1 in solid argon, at 940.6 cm À1 in solid krypton, and at 929.0 cm À1 in solid xenon. [13,14,17] These bands reveal the 1.0527 AE 0.0001 16 O/ 18 O isotopic frequency ratio characteristic of the antisymmetric stretching mode of a linear O-U-O linkage. Our calculated antisymmetric stretching frequency of isolated UO 2 + is 1007 cm À1 at the CCSD(T) level. The matrix...