Mononuclear palladium(I) halides are proposed intermediates in many palladium-catalyzed reactions. However, isolable examples of such species remain scarce. Herein, we report that 9,9-dimethyl-4,5-bis(di-tert-butylphosphino)xanthene (^tBuXantphos) ligand can effectively stabilize mononuclear palladium(I) halides. Mononuclear palladium(I) chloride [(^tBuXantphos)PdCl] (1) was synthesized in 73% yield via the reduction of [(^tBuXantphos)PdCl₂] using excess potassium graphite (KC₈) in THF under strictly controlled reaction time. Similarly, mononuclear palladium(I) bromide and iodide, [(^tBuXantphos)PdBr] (2) and [(^tBuXantphos)PdI] (3), were obtained with the yields of 68% and 42% respectively, through reduction of the reaction mixtures of [(^tBuXantphos)PdCl₂] with NaX (X=Br, I) with excess KC₈. These three complexes are stable under nitrogen atmosphere and sensitive to air and moisture. Single-crystal X-ray diffraction studies revealed that all three complexes adopt a distorted T-shaped geometry, and the Pd—X bond lengths in [(^tBuXantphos)PdX] (0.2469(2) nm, 0.2591(1) nm, and 0.2739(1) nm, for 1~3, respectively) are longer than those in the three-coordinate mononuclear palladium(I) halides [(dtbpf)PdX] (dtbpf=1,1'-bis-(di-tert-butylphosphino)ferrocene) (J. Am. Chem. Soc. 2025, 147, 44552-44561.). The ¹H NMR spectra of these three complexes exhibit similar paramagnetically shifted signals as six broad peaks fall in the range of δ=0~20, while no ³¹P NMR signal was observed. Electron paramagnetic resonance (EPR) spectroscopy recorded at 100 K indicated that g-factors of [(^tBuXantphos)PdX] are g=[2.231, 2.079, 2.034] (X=Cl), [2.195, 2.081, 2.035] (X=Br), [2.201, 2.129, 2.074] (X=I), with hyperfine splitting constants attributed to the halogen atoms of A(X)=[33, 27, 90] MHz (X=Cl), [312, 75, 425] MHz (X=Br), [360, 150, 600] MHz (X=I). The g-values of [(^tBuXantphos)PdX] are larger than those of [(dtbpf)PdX], whereas the hyperfine splitting constants A(³¹P) is smaller and A(X) is larger. Density functional theory (DFT) calculations demonstrated that the singly occupied molecular orbitals (SOMOs) of 1~3 correspond to the antibonding interactions from the overlap of the d_x2_-y2 orbitals of palladium and the np orbitals of halogen. The calculated spin density on the palladium center decreases sequentially in complexes 1~3 (0.63, 0.61, and 0.57, respectively), which is larger than [(dtbpf)PdX]. On the other hand, the spin density on the halogen atoms increases correspondingly (0.17, 0.19, and 0.22, respectively), which is smaller than [(dtbpf)PdX]. Recently, Mirica and coworkers reported the mononuclear palladium(I) chloride [(^tBuXantphos)PdCl] and bromide [(^tBuXantphos)PdBr] (J. Am. Chem. Soc. 2025, 147, 41882−41896). While the structure and spectroscopic features of the two complexes in their report are nearly identical to what we observed here, they use cobaltocene as the reductant for the synthetic reactions and noted these compounds are unstable, which differ from our findings. Additionally, they did not achieve the synthesis of the iodide [(^tBuXantphos)PdI] (3).

Key words: mononuclear, palladium(I), open-shell, phosphine, halide