ses. Their relative total energies are 0, 0.4, 4.4 and -3.6 kcalmol-I , respectively.1111 Although the difference in energy between the (q2-0,C-HNCO)(q2-C,N-HNCO) and (q2-C,N-HNCO), isomers is small, the calculations are inconsistent with the observed structure.
Next, we used molecular mechanics calculations to examine the steric factor responsible for discriminating the two functionalities of PhNC0.'14] The energy-minimized structure found for 1, which agrees well with the X-ray structure,'"] is 12.3 kcalmol-' lower in total strain energy than that calculated for 1'. The high strain energy of the latter seems to stem mainly form stereochemical repulsion between the axial hydrogen atoms of ring CH, groups and the q2- C,N-PhNCO ligand at the congested axial site. To reduce these steric repulsions as much as possible, both of the trans S-MoS vectors should bend toward the uncongested axial site. In fact, the S(5kMo-S( 13) angle (161.7') in 1' is more acute and the S(l )-Mo-S( 9) angle (1 75.0') more obtuse than the corresponding angles of 1 discussed above. Thus, the novel discrimination of the C=O and C=N bonds of PhNCO at the axial sites can be explained in terms of steric control rather than electronic effects.
Since the space available for an axial ligand at the congested site can be adjusted by the ring size of syn-crown thioethers,r2I the present result may provide a basis for further development of crown thioether complexes as hosts in molecular recognition through coordination at the metal center.