Distortions induced by electronic and steric properties of axial ligands in pseudo-octahedral cobaloximes. Crystal and molecular structures of trans-bis(dimethylglyoximato)n-pentyl(triphenylphosphine)cobalt(III) and trans-aquabis(dimethylglyoximato)n-pentylcobalt(III)

The crystal structures of two cobaloximes, [Co(Hdmg)₂(CH₂CMe₃)(OH₂)] (1) (Hdmg = dimethylglyoximate monoanion) and [Co(Hdmg)₂(CH₂CMe₃)(PPh₃)] (2), containing a neutral ligand of widely differing bulk trans to the same alkyl group, are reported and discussed. Both compounds crystallize in the space group Pbca with cell parameters a = 26.91(1), b = 14.449(8), and c = 9.535(6) Å, and Z = 8, for (1) and a = 19.120(8), b = 15.998(8), c = 20.62(1) Å, and Z = 8 for (2). Both structures were solved by Patterson and Fourier methods and refined by anisotropic least-squares methods to a final R 0.045 for (1) and 0.035 for (2) using respectively 1 405 and 2 245 independent reflections having I ≥ 3σ(I), using MO-K_α radiation to a maximum 2θ = 56°. The H₂O-Co-CH₂CMe₃ fragment in (1) is characterized by an O-Co-C angle of 172.4(3)° and Co-C and Co-O bond lengths of 2.044(7) and 2.056(5) Å. The Co-CH₂-CMe₃ angle is 126.9(5)°. The Ph₃P-Co-CH₂CMe₃ fragment in (2) is characterized by a Co-P distance of 2.460(1) Å which is the longest so far reported for Co^III-PPh₃ compounds. The n-pentyl group is disordered with mean Co-C bond lengths of 2.117(9) Å and P-Co-C bond angles of 166.6(2)°. The corresponding Co-CH₂-CMe₃ angles average to 129.2(7)°. Comparative steric effects are discussed on the basis of the increasing bulk of R and L in these and related complexes of formula [Co(Hdmg)₂(CH₂R)L]. It is suggested that distortions involving the metal centre and arising from the interaction of the axial CH₂R group and the two equatorial Hdmg ligands affect the co-ordination C-Co-N-(equatorial) bond angles, the Co-CH₂-R bond angle, and the Co-C bond length.