Palladium is a silvery-white, soft noble metal in the platinum group. It is widely used as a catalyst (e.g., automotive catalytic converters, hydrogenation) and can absorb large amounts of hydrogen relative to its volume.
Palladium most commonly adopts 0, +2, and +4 oxidation states in coordination and organometallic chemistry.
Pd cycles between oxidative addition, transmetalation, and reductive elimination, enabling C–C bond formation with broad functional-group tolerance. Examples include Suzuki–Miyaura, Heck, and Sonogashira couplings.
\(\mathrm{Ar{-}X + Ar'B(OH)_2 \xrightarrow[base]{Pd} Ar{-}Ar' + B(OH)_3 + HX}\)
Pd dissolves large amounts of hydrogen into its lattice to form palladium hydride \(\mathrm{PdH_x}\) (\(0 \le x \lesssim 0.7\) at ambient conditions). This is often described by dissociative adsorption followed by diffusion:
\(\mathrm{H_2(g) \rightleftharpoons 2H_{ads} \rightleftharpoons 2H_{lattice}}\)
The \(\alpha\) and \(\beta\) phases differ in hydrogen content; Pd is used in hydrogen purification and sensing.
The accepted ground-state configuration is \([\mathrm{Kr}]\,4d^{10}\,5s^{0}\), not \([\mathrm{Kr}]\,4d^{9}\,5s^{1}\). Completely filled \(4d\) subshell lowers the energy due to exchange and pairing effects, making Pd an oft-cited anomaly in transition-metal configurations.
Pd is a key component for oxidation of CO and unburnt hydrocarbons and assists in NOx control in three-way catalysts (often with Pt/Rh). A simplified pathway is:
\(\mathrm{2\,CO + O_2 \xrightarrow[\text{Pd}]{ } 2\,CO_2}\)
\(\mathrm{C_xH_y + (x + \tfrac{y}{4})O_2 \xrightarrow[\text{Pd}]{ } x\,CO_2 + \tfrac{y}{2}\,H_2O}\)
Pd/C (palladium on carbon) catalyzes the addition of \(\mathrm{H_2}\) to alkenes, alkynes, and nitro groups under mild conditions.
\(\mathrm{R{-}CH{=}CH_2 + H_2 \xrightarrow[\text{Pd/C}]{ } R{-}CH_2{-}CH_3}\)
Selectivity can be tuned with solvents, additives, and catalyst loading.
Pd is typically a by-product of platinum-group metal (PGM) and Ni/Cu sulfide ore refining. Its scarcity, concentrated supply chains, complex refining, and strong demand from auto catalysts and fine chemicals create high and sometimes volatile prices.
Pd dominates cross-coupling and is excellent for selective hydrogenations. Pt excels in hydrosilylation, reforming, and certain oxidation processes. Ni offers a lower-cost alternative in many couplings but may require harsher conditions and shows different functional-group tolerance.
Bulk Pd metal is relatively inert, but finely divided Pd and soluble Pd salts can be harmful if inhaled/ingested. Use fume hoods, gloves, and dust control. In pharma, residual Pd in products is monitored, and catalyst recovery/recycling is practiced to reduce environmental impact and cost.
Yes. A simplified Pd(0)/Pd(II) cycle:
This regenerates \(\mathrm{Pd^{0}}\) and forms the biaryl product.