Experimental and computational preference for phosphine regioselectivity and stereoselective tripodal rotation in HOs 3 (CO) 8 (PPh 3 ) 2 (μ-1,2-N,C-η 1 ,κ 1 -C 7 H 4 NS).

The site preference for ligand substitution in the benzothiazolate-bridged cluster HOs 3 (CO) 10 (μ-1,2-N,C-η 1 ,κ 1 -C 7 H 4 NS) (1) has been investigated using PPh 3 . 1 reacts with PPh 3 in the presence of Me 3 NO to afford the mono- and bisphosphine substituted clusters HOs 3 (CO) 9 (PPh 3 )(μ-1,2-N,C-η 1 ,κ 1 -C 7 H 4 NS) (2) and HOs 3 (CO) 8 (PPh 3 ) 2 (μ-1,2-N,C-η 1 ,κ 1 -C 7 H 4 NS) (3), respectively. 2 exists as a pair of non-interconverting isomers where the PPh 3 ligand is situated at one of the equatorial sites syn to the edge-bridging hydride that shares a common Os-Os bond with the metalated heterocycle. The solid-state structure of the major isomer establishes the PPh 3 regiochemistry at the N-substituted osmium center. DFT calculations confirm the thermodynamic preference for this particular isomer relative to the minor isomer whose phosphine ligand is located at the adjacent C-metalated osmium center. 2 also reacts with PPh 3 to give 3. The locus of the second substitution occurs at one of the two equatorial sites at the Os(CO) 4 moiety in 2 and gives rise to a pair of fluxional stereoisomers where the new phosphine ligand is scrambled between the two equatorial sites at the Os(CO) 3 P moiety. The molecular structure of the major isomer has been determined by X-ray diffraction analysis and found to represent the lowest energy structure of the different stereoisomers computed for HOs 3 (CO) 8 (PPh 3 ) 2 (μ-1,2-N,C-η 1 ,κ 1 -C 7 H 4 NS). The fluxional behavior displayed by 3 has been examined by VT NMR spectroscopy, and DFT calculations provide evidence for stereoselective tripodal rotation at the Os(CO) 3 P moiety that serves to equilibrate the second phosphine between the two available equatorial sites.