Each glowing sphere at the hub is a Pd(0)/ligand active site. A cross-coupling catalytic cycle has three core steps, shown here as colour changes: oxidative addition — the C(sp²)–X bond of the aryl (or vinyl) halide inserts into Pd(0), giving Pd(II); transmetalation (Suzuki: boronic acid + base, Negishi: organozinc) or, for Heck, migratory insertion into the alkene followed by β-hydride elimination — swaps the halide for the second carbon fragment; and reductive elimination, which forges the new C–C bond and regenerates Pd(0) for another turn.
Pd(0)L_n + R–X → R–Pd(II)–X (oxidative addition)
R–Pd–X + R'–[B/Zn] → R–Pd–R' + X–[B/Zn] (transmetalation)
R–Pd–R' → R–R' + Pd(0)L_n (reductive elimination)
- Catalyst loading — how many independent Pd centres run in parallel; more sites process more substrate per unit time (higher effective mol%).
- Ligand bulk — bulkier, more electron-rich phosphine ligands (large cone angle, e.g. P(t-Bu)₃) speed up oxidative addition into the strong Ar–X bond but crowd the metal, slowing transmetalation; small ligands do the opposite.
- Temperature — every step is thermally activated; raising it speeds the whole cycle roughly exponentially, up to the point real catalysts would decompose.
Turnover number (TON) counts finished cycles per site; turnover frequency (TOF) is TON per minute of simulated time — the practical measure of how "good" a catalyst is at a given temperature and ligand choice.