Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa -Metallocene Hydride Complex, [K(crypt)][(μ-Cp An )Y(μ-H)] 2 (Cp An = Me 2 Si[C 5 H 3 (SiMe 3 )-3] 2 ), with a 3.4 Å Yttrium-Yttrium Distance.

The reduction of a bimetallic yttrium ansa -metallocene hydride was examined to explore the possible formation of Y-Y bonds with 4d 1 Y(II) ions. The precursor [Cp An Y(μ-H)(THF)] 2 (Cp An = Me 2 Si[C 5 H 3 (SiMe 3 )-3] 2 ) was synthesized by hydrogenolysis of the allyl complex Cp An Y(η 3 -C 3 H 5 )(THF), which was prepared from (C 3 H 5 )MgCl and [Cp An Y(μ-Cl)] 2 . Treatment of [Cp An Y(μ-H)(THF)] 2 with excess KC 8 in the presence of one equivalent of 2.2.2-cryptand (crypt) generates an intensely colored red-brown product crystallographically identified as [K(crypt)][(μ-Cp An )Y(μ-H)] 2 . The two rings of each Cp An ligand in the reduced anion [(μ-Cp An )Y(μ-H)] 2 1- are attached to two yttrium centers in a "flyover" configuration. The 3.3992(6) and 3.4022(7) Å Y···Y distances between the equivalent metal centers within two crystallographically independent complexes are the shortest Y···Y distances observed to date. Ultraviolet-visible (UV-visible)/near infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy support the presence of Y(II), and theoretical analysis describes the singly occupied molecular orbital (SOMO) as an Y-Y bonding orbital composed of metal 4d orbitals mixed with metallocene ligand orbitals. A dysprosium analogue, [K(18-crown-6)(THF) 2 ][(μ-Cp An )Dy(μ-H)] 2 , was also synthesized, crystallographically characterized, and studied by variable temperature magnetic susceptibility. The magnetic data are best modeled with the presence of one 4f 9 Dy(III) center and one 4f 9 (5d z 2 ) 1 Dy(II) center with no coupling between them. CASSCF calculations are consistent with magnetic measurements supporting the absence of coupling between the Dy centers.