Conformational Isomerism in Triosmium Clusters:  Structures of Yellow and Red Os3(CO)11[P(p-C6H4F)3] and Os3(CO)11(PBut3)

The clusters Os3(CO)11(PR3) (R = p-C6H4F, 1; But, 2) have been prepared by the addition of PR3 to Os3(CO)11(CH3CN), and their structures determined. There are yellow and red forms of 1 (1y and 1r, respectively). The structure of 1y has the typical structure for Os3(CO)11(PR3) (PR3 = phosphine or phosphite) complexes with the axial carbonyls perpendicular to the Os3 triangle, whereas in 1r the individual Os(CO)3(L) (L = CO, PR3) units are twisted with respect to each other such that neighboring carbonyls on adjacent Os atoms are in a staggered configuration. The reason for the twisting in 1r is believed to be due to intermolecular edge-to-face interactions between fluorophenyl rings in the solid. In solution 1 is yellow with spectroscopic properties typical of Os3(CO)11(PR3) compounds. Unlike 1y and its analogues, 2 is deep red both in the solid and in solution. The structure of 2 reveals that like 1r it also has the twisted structure. It is believed that the large size and strong donor properties of the PBut3 ligand destabilize the edge-bridging molecular orbitals in 2 such that the twisted form, which is sterically favored, is adopted. Consistent with this view is that the Os−Os bond cis to the PBut3 in 2 is long, at 2.9416(3) Å; in 1y and 1r the corresponding lengths are 2.872(1)and 2.888(1) (two independent molecules in the unit cell of 1y) and 2.9117(4) Å, respectively.