Ligand dissociation: planar or pyramidal intermediates?

In organometallic chemistry, ligand dissociation is a key intermediate step in many useful processes. The dissociation of halide from an 18-electron half-sandwich complex (that is, with a single cyclopentadienyl ligand) of the type [CpRu(P-P')Hal] leaves an unsaturated 16-electron intermediate [CpRu(P-P')](+), which is then ready for subsequent addition reactions. Does the intermediate maintain its structure with a vacant site in place of the dissociated ligand, or does it rearrange, either concurrent with its formation or subsequently? In other words, are the 16-electron species planar or pyramidal? The outcome is relevant for chiral-at-metal compounds [CpML(1)L(2)L(3)] because an intermediate [CpML(1)L(2)] retains its chirality with respect to the metal atom as long as it is pyramidal, whereas it loses its chiral information if it becomes planar. In this Account, we address experimental results and theoretical calculations that help illuminate the energetics of structural rearrangements after halide dissociation. The rate-determining step in the halide exchange and racemization reactions of (R(Ru))- and (S(Ru))-[CpRu(P-P')Cl] is the cleavage of the Ru-Cl bond to give pyramidal intermediates (R(Ru))- and (S(Ru))-[CpRu(P-P')](+), which have kept the original metal configurations. These unsaturated intermediates can react with added ligands, such as Br(-) or I(-) (k(2) paths). The substitution products form with retention of the metal configuration. However, the pyramidal intermediates (R(Ru))- and (S(Ru))-[CpRu(P-P')](+) can also invert to their mirror images (k(3) paths). For (R(Ru))- and (S(Ru))-[CpRu(P-P')](+), the barrier of the pyramidal inversion (k(3)) is much higher than that of the halide addition (k(2)). The competition ratio k(3)/k(2) determines how much racemization occurs in a ligand exchange reaction. The competition ratio k(3)/k(2) can be determined from the ratio of the (R(Ru))- and (S(Ru))-products, which is constant throughout the course of the reaction. For compounds like [CpRu(P-P')Hal], k(3)/k(2) is much smaller than 1, resulting in an energy profile that resembles a basilica. These results, established with chiral-at-metal compounds, are supported by calculations that show that 16-electron half-sandwich intermediates with sigma-donating ligands, such as [CpM(NH(3))(2)], adopt planar structures, whereas strongly pi-bonding ligands, as in [CpM(CO)(2)], lead to pyramidal intermediates. The computed activation energies for the pyramidal inversion are on the order of 10 kcal/mol, with the planar species being transition states. The ligand dissociation behavior of 18-electron transition metal complexes is compared with nucleophile dissociation in main group compounds with octet configurations; here we include new computational results. Without exception, unsaturated main group intermediates, such as the carbenium ions formed in S(N)1 reactions, are planar. Our results and analysis help put transition metal chemistry on a firmer mechanistic foundation, and chiral-at-metal compounds are invaluable to this end.