Preparation, crystal structures and isomerization kinetics of cis- and trans-[Co(dtc)₂(PHPh₂)₂]⁺: Thermodynamically and kinetically stable cobalt(III)-P bonds through interplay of σ-donicity, π-acidity, and steric bulkiness

Novel cobalt(III)-diphenylphosphine complexes, cis- and trans-[Co(dtc)₂(PHPh)₂)₂]⁺, were synthesized and structurally characterized by X-ray crystallographic analyses and spectroscopic methods. The Co-P bond lengths in both isomers were shorter than those in the analogous cobalt(III)-bis(tertiary phosphine) complexes with sterically less bulky but more basic phosphine ligands: Co-P(1) = 2.2340(6) and Co-P(2) = 2.2258(7) Å for the cis-isomer, and Co-P = 2.276(1) Å for the trans isomer. The title complexes also exhibited a unique dynamic behavior: cis to trans isomerization was induced by irradiation with visible light, while thermal trans to cis isomerization took place at elevated temperatures. The absorbance change for the trans to cis isomerization reaction exhibited multi-exponential kinetic traces when no free PHPh₂ was present in the solution. Such a complicated kinetic behavior was explained either by the slow dissociation of coordinated PHPh₂ or by the abstraction of a P-H proton from coordinated PHPh₂ through an acid-base interaction with trace water in the bulk solvent. By addition of an excess amount of PHPh₂, the dissociation of coordinated PHPh₂ as well as the basicity of impure water was suppressed, and a first-order kinetic trace was observed. Kinetic studies with excess free PHPh₂ in acetonitrile revealed that the isomerization reaction takes place via an intramolecular twist mechanism: ΔH* = 120 ± 1 kJ mol^-1 and ΔS* = 50 ± 18 J mol^-1 K^-1. AOM calculations indicate that the twist mechanism involves a spin state change (¹A_1g to ⁵A₁′) during the activation process. The importance of the π-acidity of PHPh₂ together with the cooperative effect of the spectator ligand (dtc^-) was suggested to explain the thermodynamic and kinetic behaviors of these complexes.