Periodic Trends and Ligand Effects on the Substitution Labilities of Tetrametal Cluster Complexes of the Cobalt Triad. Molecular Structure of Rh4(CO)83>

Article abstract of DOI:10.1021/ja00192a020

The substitution kinetics of M4(CO)9 cluster (M = Co, Rh, and Ir) with a variety phosphorus donor ligands, L, has been explored.In each case CO substitution occurs at the apical metal atom to form M4(CO)8L.Crystals of Rh4(CO)8*CH2Cl2 were examined by X-ray crystallography and found to belong to the monoclinic space group P2₁/n with a = 14.482(6) Angstroem, b = 18.747(7) Angstroem, c = 20.92(1) Angstroem, β = 98.90(3) deg, V = 5611(3) Angstroem³, and Z = 4.Refinement of 563 least-squares variables for 4648 uniquereflections with F₀ > 4.5?(F₀) led to R_F = 0.0636 and R_wF = 0.0706.The compound adopts a tetrahedral metal atom structure with three P of the HC(PPh2)3 ligand bound in axial positions.Each edge of the basal triangle of Rh atoms is bridged by a CO ligand (Rh-Rh of 2.744(1) Angstroem, 2.771(2) Angstroem, and 2.760(2) Angstroem).The P(OEt)3 ligand substitutes on the unique apical Rh atom (Rh-P = 2.253(5) Angstroem).The most unusual aspect of the structure is a highly asymmetric bridging carbonyl group connecting the apical Rh atom and a basal Rh (Rh^ap-C = 1.77(2) Angstroem, Rh^ba-C = 2.08(2) Angstroem).The Rh-Rh bond between these bridged atoms of 2.923(2) Angstroem significantly exceeds the other Rh^ap-Rh^ba distances (2.676(2) Angstroem and 2.685(2) Angstroem).Kinetic studies for the M4(CO)9 cluster family show that two term rate laws (k₁ + k₂)> are observed for M = Co and Rh, whereas for Ir the second-order term dominates for all L except perhaps (13)CO.Activation parameters for the k₁ terms (ΔH₁(excit.) = 22 to 26 kcal/mol and ΔS₁(excit.) = 0-23 kcal/mol) areconsistent with a CO dissociative mechanism for the k₁ process.Activation parametres for the k₂ terms (ΔH₂(excit.) = 5 to 14 kcal/mol, ΔS₂(excit.) = -20 to -45 cal/(mol*K)) and the dependence of rate on incoming nucleophile suggest an associative mechanism for CO substitution, where L attacks an apical metal atom of the cluster.Rates of CO substitution in M4(CO)9 are generally less than in corresponding M4(CO)12 clusters since the HC(PPh2)3 ligand donates electron density to the cluster (as compared to CO).This results in enhanced ?-back-bonding to CO groups in the cluster (a raised barrier to CO dissociation) and a decreased susceptibility to nucleophilic attack.An anomalous high reactivity of Ir4(CO)9(PPh3)3 as compared to Ir4(CO)9 is shown to arise from an enhanced rate of CO dissociation, which can be attributed to the presence of bridging CO groups in the former complex.A unique aspect of substitution in the cluster compounds as compared to mononuclear carbonyl complexes is the presence of rapid associative paths for CO substitution.This may result from the presence of low-lying acceptor orbitals for...