Abstract
Addition of formate on the dicationic cluster [Pd~3~(dppm)~3~(μ~3~‐CO)]^2+^ (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd~3~(dppm)~3~(μ~3~‐CO)(μ~3~‐H)]^+^. This new palladium–hydride cluster has been characterised by ^1^H NMR, ^31^P NMR and UV/Vis spectroscopy and MALDI‐TOF mass spectrometry. The unambiguous identification of the capping hydride was made from ^2^H NMR spectroscopy by using DCO~2~^−^ as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped‐flow methods. The kinetic data are consistent with a two‐step process involving: 1) host–guest interactions between HCO~2~^−^ and [Pd~3~(dppm)~3~(μ~3~‐CO)]^2+^ and 2) a reductive elimination of CO~2~. Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH~4~ to [Pd~3~(dppm)~3~(μ~3~‐CO)]^2+^ and 2) electrochemical reduction of [Pd~3~(dppm)~3~(μ~3~‐CO)]^2+^ to [Pd~3~(dppm)~3~(μ~3~‐CO)]^0^ followed by an addition of one equivalent of H^+^. Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one‐electron transfer), C=chemical (hydride dissociation)) for the two‐electron reduction of [Pd~3~(dppm)~3~(μ~3~‐CO)(μ~3~‐H)]^+^ to [Pd~3~(dppm)~3~(μ~3~‐CO)]^0^ is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd~3~(H~2~PCH~2~PH~2~)~3~(μ~3~‐CO)(μ~3~‐H)]^n^ model clusters were performed by using DFT at the B3 LYP level. Upon one‐electron reductions, the PdPd distance increases from a formal single bond (n=+1), to partially bonding (n=0), to weak metal–metal interactions (n=−1), while the PdH bond length remains relatively the same.