Structural and spectroscopic comparisons in two series of cobalt(III)–monodentate phosphine complexes: trans-[Co(acac)2(PMe3−nPhn)(H2O)]PF6 and trans-[Co(acac)2(PMe3−nPhn)2]PF6 (acac=pentane-2,4-dionate; n=0, 1, 2 and 3)

The X-ray crystal structures and spectroscopic properties of two series of cobalt(III)–monodentate phosphine complexes, trans-[Co(acac)2(PMe3−nPhn)(H2O)]PF6 (na) and trans-[Co(acac)2(PMe3−nPhn)2]PF6 (nb), have been compared in variation of methyl/phenyl substituents of phosphines. The CoP bond lengths in complexes nb are found to be remarkably longer (>0.095 Å) than those in the corresponding complexes na. Complex trans-[Co(acac)2(PPh3)(H2O)]PF6 (3a) shows an unusually long CoP bond (2.243(1) Å) and an anomalous distortion of the equatorial acac planes, owing to a severe steric interaction associated with three phenyl rings of PPh3. It appears that the PPh3 in 3a is bound more closely to cobalt(III) in solution than in the solid state, as suggested by comparison of the absorption spectra of the solution and the solid. The ligand spectral parameters of phosphines, dCo(PMe3−nPhn), are estimated by using the energy of the a1Eg←1A1g transition, which is observed as an absorption band for complexes na or a broad shoulder for complexes nb, and evaluated in relation to the 59Co NMR chemical shifts. The estimated dCo(PMe3−nPhn) values indicate that the ligand-field strengths of the phosphines become considerably reduced as the number of phenyl substituents increases. Also, the ligand-field strengths of the phosphines in complexes nb are smaller than those in the corresponding complexes na. Furthermore, the low-lying (18500–25000 cm−1) LMCT band of complexes nb can be explained by the estimated parameters.