Metal valence and geometry: synthesis, structure and solution behaviour of the isomers of [Os(S₂CNEt₂)₂(PPh₃)₂]^0,+

Abstract

The reaction of [Os(PPh₃)₃Br₂] with Na(S₂CNEt₂) affords cis-[OsL₂(PPh₃)₂](L = S₂CNEt₂) from which trans-[OsL₂(PPh₃)₂]PF₆ is isolated via cerium(IV) oxidation. Reduction of the latter complex by hydrazine hydrate furnishes trans-[OsL₂(PPh₃)₂]. The X-ray structures of all the three complexes have been determined and their spectra and magnetic properties are reported. Solution equilibrium data show that the metal oxidation states strongly differentiate the isomeric co-ordination spheres—the more stable geometry is cis for the bivalent and trans for the trivalent metal. The metastable trans-bivalent complex (d⁶) can still be isolated because isomerisation is slow but the cis-trivalent complex (d⁵) isomerises rapidly and could not be isolated. Variable-temperature isomerisation rates, activation parameters and metal reduction potentials [E½(cis) > E½(trans)] of the isomers are reported. There is a progressive decrease in the Os–P bond length in going from trans-[OsL₂(PPh₃)₂]PF₆ to trans-[OsL₂(PPh₃)₂] to cis-[OsL₂(PPh₃)₂], the net change being a remarkable 0.15 Å. This correlates with the 5dπ–3dπ back-bonding order cis-[OsL₂(PPh₃)₂] > trans-[OsL₂(PPh₃)₂]>trans-[OsL2(PPh₃)_{2]PF6. In cis-[OsL2(PPh3)2}], PPh₃···PPh₃ steric repulsion is present but it is more than offset by the strong back-bonding. In trans-[OsL₂(PPh₃)₂] the steric factor is absent but the back-bonding is also weaker. In equilibrated solution both the isomers are present but the cis form predominates (85%). For the very weakly back-bonding trivalent metal, the steric factor becomes strongly controlling and in solutions very little of the cis isomer is present at equilibrium (≈0.002%).