Structural and H-1 NMR data have been obtained for cobaloximes with the bulkiest substituted pyridines reported so far. We have isolated in noncoordinating solvents the complexes CH3Co(DH)(2)L (methylcobaloxime, where DH = the monoanion of dimethylglyoxime) with L = sterically hindered N-donor ligands: quinoline, 4-CH(3)quinoline, 2,4-(CH3)(2)pyridine, and 2-R-pyridine (R = CH3, OCH3, CH2CH3, CH=CH2)-We have found that the Co-N-ax bond is very long in the structurally characterized complexes. In particular, CH3Co(DH)(2)(4-CH(3)quinoline) has a longer Co-N-ax bond (2.193(3) angstrom) than any reported for methylcobaloximes. The main cause of the long bonds is unambiguously identified as the steric bulk of L by the fairly linear relationship found for Co-Nax distance vs CCA (calculated cone angle, CCA, a computed measure of bulk) over an extensive series of methylcobaloximes. The linear relationship improves if L basicity (quantified by pK(a)) is taken into account. In anhydrous CDCl3 at 25 degrees C, all complexes except the 2-aminopyridine adduct exhibit H-1 NMR spectra consistent with partial dissociation of L to form the methylcobaloxime dimer. H-1 NMR experiments at -20 degrees C allowed us to assess qualitatively the relative binding ability of L as follows: 2,4-(CH3)2pyridine > 4-CH(3)quinoline approximate to quinoline approximate to 2-CH(3)pyridine > 2-CH(3)Opyridine > 2-CH(3)CH(2)pyridine > 2-CH2=CHpyridine. The broadness of the H-1 NMR signals at 25 degrees C suggests a similar order for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecular hydrogen bond between the NH2 group and an oxime O atom. The weaker than expected binding of 2-vinylpyridine relative to the Co-N-ax bond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bind to the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.