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Molecular mechanics methods have been employed to compute energy-minimized structures for a large number of Cr(CO)sL complexes, where L is CO, trialkylor triarylphosphine, mixed trialkyl/arylphosphine, phosphite, alkyl/aryl/alkoxyphosphine, or one of 8 analogous arsenic ligands. The energy-minimized structures are employed to compute a new measure of ligand steric effect, the ligand repulsive energy, £R, obtained by computing the van der Waals repulsive force acting between the ligand and Cr(CO)5 fragments along the Cr-P axis, at the equilibrium Cr-P distance, rt. The repulsive force is multiplied by rc to obtain £R: £r = [d£^w(repulsive)/dr(Cr-P)]re. Values of £R correlate very well with Tolman's cone angle values, 9, for all phosphines.

The aggregations of several alkyllithium compounds in cyclohexane and benzene have been investigated by cryoscopic measurements. The compounds exist in the two solvents as hexameric aggregates unless steric interactions due to branching at either a-or ß-carbon favor formation of tetrameric aggregates. Equilibria between these two species are in some instances markedly affected by changes in concentration or solvent. There is no evidence in any of the systems studied for a degree of association less than four. The effects of added tetrahydrofuran (THF), diethyl ether (DEE), cineole, quinuclidine (Q), or triethylamine (TEA) on hydrocarbon solutions of alkyllithium compounds have been studied by observation of and 7Li nmr spectra, and measurements of freezing point lowerings of cyclohexane and benzene solutions. The results are consistent with other evidence that tetrameric alkyllithium species are present in excess base. It appears that conversion of the «-alkyllithium compounds from hexamer to solvated tetramer occurs at low base concentrations. The relative extent of interaction with «-alkyllithium compounds is in the order THF ~Q > DEE > TEA > cineole. Differences in the steric requirements of the 737 (1957).