Relation between cobalt–carbon bond lengthening and conformational changes in octahedral neopentylcobaloximes. Crystal and molecular structure of bis(dimethylglyoximato)neopentyl(trimethylphosphine)cobalt(III)

Abstract

The crystal structure of a cobaloxime, [Co(Hdmg)₂(CH₂CMe₃)(PMe₃)](Hdmg = monoanion of dimethylglyoxime), is reported and discussed. The crystals are monoclinic, space group P2₁/c with cell parameters a= 11.807(7), b= 12.136(7), c= 15.225(9)Å, β= 95.42(8)°, and Z= 4. The structure was solved by Patterson and Fourier methods and refined by block-diagonal anisotropic least-squares methods to a final R of 0.028, using 2 580 independent reflections. The Me₃P–Co–CH₂CMe₃ fragment is characterized by a C–Co–P angle of 173.68(9)° and Co–P and Co–C bond lengths of 2.316(1) and 2.084(3)Å respectively. The Co–CH₂–C angle is greatly widened to 130.2(2)°. The P–C bond lengths and the C–P–C bond angles average 1.813(4)Å and 103.1(2)° respectively, being significantly different from those in free PMe₃[1.846(3)Å and 98.6(3)°]. The two Hdmg units make an interplanar angle (α) of 5° bending towards the phosphine ligand, and the Co atom lies approximately in the N₄ equatorial donor plane. The Co–C bond lengths, α angles, and the displacement of Co out of the N₄ donor plane (d) are compared in the series [Co(Hdmg)₂(CH₂CMe₃)L](L = H₂O, pyridine, PMe₃, and PPh₃). This shows that the lengthening of the Co–C bond is accompanied by movement of the Hdmg ligands from downward- to the upward-pointing (with respect to the alkyl group), which appears to be determined by the increasing bulk of L. Comparison with the analogous methyl derivatives shows also that the Co–L and Co–C bond lengths are influenced by the bulk of both the axial ligands.