Stereochemical consequences of the two-electron oxidation of a dirhodium fulvalene complex: the X-ray crystal structures of trans-[Rh2(CO)2(PPh3)2(η5:η′5-C10H8)] and cis-[Rh2(CO)2(PPh3)2(η5:η′5-C10H8)][PF6]2

Abstract

Single-crystal X-ray diffraction studies on the dirhodium fulvalene complexes cis-[Rh₂(CO)₂(PPh₃)₂(η⁵:η′⁵-C₁₀H₈)][PF₆]₂(1) and trans-[Rh₂(CO)₂(PPh₃)₂(η⁵:η′⁵-C₁₀H₈)](2), both as their n-hexane solvates, show the structures to be dramatically different. In the dication of (1) the rhodium atoms lie on the same side of the fulvalene ligand and are bonded to one another with Rh–Rh 2.930(2)Å; in (2) the metal atoms are on opposite sides of the hydrocarbon ligand at a separation of 5.662(1)Å. A detailed comparison of the two structures shows that the oxidation of complex (2) to (1) also results in changes in the geometry of the metal–ligand binding: the Rh–P distances increase by 0.067(4)Å; the ring centroid–Rh–P(CO) unit is planar in (2) but pyramidal in (1). The electronic factors behind the structural changes are examined by the extended-Hückel molecular-orbital method; the fulvalene ligands in complexes (1) and (2) have characteristics (C–C bond orders, ability to rotate about the central C–C bond) more similar to those of the fulvalene dianion than to free fulvalene. These conclusions are in accord with the structural observations, and with the assignment of formal oxidation states II and I to the rhodium atoms of (1) and (2) respectively. In the light of these results, brief comments are made on the apparently anomalous electrochemical behaviour of the complexes.