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 l 3σ(l), 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.