Syntheses and crystal structures of geometrical isomeric pairs: Trans- and cis-(PPh4) [Co(acac)2(CN)2] and trans- and cis-[Co(acac)2(PMe3 or PEt3)2]PF6 (acac = pentane-2,4-dionate)

Article abstract of DOI:10.1016/S0020-1693(98)00146-7

Reactions of trans-[Co(acac)₂(PPh₃)₂]PF₆ (acac = pentane-2,4-dionate) with NaCN and with PR₃ (R = Me or Et) in methanol afforded new complexes, trans-(PPh₄) [Co(acac)₂(CN)₂] and trans-[Co(acac)₂(PR₃)₂]PF₆, respectively. The structures of the new trans-isomers, together with those of the corresponding known cis-isomers, were determined by single-crystal X-ray diffraction. Complexes trans[Co(acac)₂(PR₃)₂]⁺ were stable in dry organic solvents, but in wet methanol easily hydrolysed to give trans-[Co(acac)₂(PR₃) (H₂O)] ⁺ and isomerized to cis-[Co(acac)₂(PR₃)₂]⁺ by the addition of a catalytic amount of active charcoal. In contrast, trans- and cis-[Co(acac)₂(CN)₂]^- and cis-[Co(acac)₂(PR₃)₂]⁺ were stable in both dry and wet solvents. The X-ray structure analyses showed that the Co-C and Co-P bonds in the trans-isomers were longer by about 0.07 and 0.08 A, respectively, than those of the corresponding cis-isomers. The Co-O bonds trans to cyanide and phosphine ligands were also longer than those of the mutually trans Co-O bonds, owing to the strong trans influences of cyanide and phosphine ligands.