X-ray Crystallographic and Solution NMR Structural Studies of Binuclear Rhodium and Iridium Isocyanide Complexes

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The structures of two M2(TMB)44+complexes (TMB = 2,5-diisocyano-2,5-dimethylhexane; M = Rh, Ir) have been determined by X-ray crystallography. [Rh2(TMB)4Cl2](PF6)2: orthorhombic, space group Pbcn (No. 60); a = 13.846 (5), b = 24.773 (3), c = 17.068 (4) A; Z = 4; R = 0.075 for 1405 reflections with F0> 3σ(F). [Ir2(TMB)4I2](BPh4)2.1.5(CH3)2CO: orthorhombic, space group Pccn (No. 56); a = 15.141 (8), b = 28.104 (14), c = 23.877 (12) A; Z = 4; R = 0.127 for 2839 reflections with F0> 3σ(F). The metal-metal distances in the two complexes are 2.770 (3) (Rh) and 2.803 (4) A (Ir); C-M-M-C torsion angles are 33 (Rh) and 31° (Ir). Evidence for interconversion of A- and A-type enantiomers of these and related TMB complexes based on variable-temperature solution1H NMR data is presented. Barriers ΔG⋆ to the Δ-A inversion are between 50 and 62 kJ mol-1for four different M2(TMB)4zcomplexes (M = Rh, Ir; z = 2+, 4+). This inversion or a related process with lower activation energy may be responsible for the relatively short3A2uexcited-state lifetime in Rh2(TMB)42+. These data, as well as crystallographic comparisons with a number of other polynuclear isocyanide complexes in the literature, suggest that the relative orientation of the M(CN)4moieties is determined principally by ligand conformation and only slightly by direct electronic interactions. © 1987, American Chemical Society. All rights reserved.

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Inorganic Chemistry

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