Bridged and Unbridged Nickel–Nickel Bonds Supported by Cyclopentadienyl and Phosphine Ligand Sets

A series of Ni complexes [Ni­(Cl)2(PR3)2] with R = Me (1-Me), nPr (1-nPr), and nBu (1-nBu) and nickelocenes [Ni­(η5-C5H4R′)2] with R′ = H (2-H), Me (2-Me), and SiMe3 (2-SiMe3) were synthesized and characterized. From these complexes, the synthesis of the Ni complexes [NiCl­(PR3)­(η5-C5H4R′)] R = Me, R′ = H (3-Me), R= nBu, R′ = H (3-nBu), R = nPr, R′ = H (3-nPr), R = Et, R′ = Me (4), and R = Et, R′ = SiMe3 (5) was achieved. All complexes were fully characterized, including single crystal X-ray crystallography. Complexes 3-R, 4, and 5 were then used to obtain homobimetallic Ni complexes with rare examples of unbridged Ni–Ni bonds [{Ni­(η5-C5H5)­(PR3)}2], with R = Me (7-Me) and R = nPr (7-nPr) being structurally characterized by single crystal X-ray diffraction. In order to probe the effect of bridging ligands on the Ni–Ni bond, the bridged complex [{Ni­(μ:η5-C5H4CH2CH2P­(tBu)2)}2] (8) was synthesized from the monomeric precursor [Ni­(Cl)­(κ1-η5-C5H4CH2CH2PtBu2)] (6). The Ni–Ni distances in 7-Me, 7-nPr, and 8 were found to be 2.407(1), 2.3931(6), and 2.6027(17) Å, respectively, the latter seemingly lengthened compared to the other two due to the tethered nature of the bridging ligand. DFT calculations confirm the presence of unbridged σ-bonds between the Ni atoms in 7-Me and 7-nPr and show that the bridging ligand in 8 has a minimal effect on the character of the Ni–Ni bond.