Twinned versus linked organometallics - bimetallic "half-baguette" pentalenide complexes of Rh(I).

The application of Mg[Ph4Pn] and Li·K[Ph4Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of "half-baguette" anti-[RhI(L)n]2[μ:η5:η5Ph4Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] complexes as well as the related iridium complex anti-[IrI(COD)]2[μ:η5:η5Ph4Pn]. With CO exclusive syn metalation was obtained even when using mono-nuclear Rh(I) precursors, indicating an electronic preference for syn metalation. DFT analysis showed this to be the result of π overlap between the adjacent M(CO)2 units which overcompensates for dz2 repulsion of the metals, an effect which can be overridden by steric clash of the auxiliary ligands to yield anti-configuration as seen in the larger olefin complexes. syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] is a rare example of a twinned organometallic where the two metals are held flexibly in close proximity, but the two d8 Rh(I) centres did not show signs of M-M bonding interactions or exhibit Lewis basic behaviour as in some related mono-nuclear Cp complexes due to the acceptor properties of the ligands. The ligand substitution chemistry of syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] was investigated with a series of electronically and sterically diverse donor ligands (P(OPh)3, P(OMe)3, PPh3, PMe3, dppe) yielding new mono- and bis-substituted complexes, with E-syn-[RhI(CO)(P{OR})3]2[μ:η5:η5Ph4Pn] (R = Me, Ph) characterised by XRD.