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

The application of Mg[Ph 4 Pn] and Li·K[Ph 4 Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of "half-baguette" anti -[Rh I (L) n ] 2 [μ:η 5 :η 5 Ph 4 Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn -[Rh I (CO) 2 ] 2 [μ:η 5 :η 5 Ph 4 Pn] complexes as well as the related iridium complex anti -[Ir I (COD)] 2 [μ:η 5 :η 5 Ph 4 Pn] . 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 d z 2 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 -[Rh I (CO) 2 ] 2 [μ:η 5 :η 5 Ph 4 Pn] is a rare example of a twinned organometallic where the two metals are held flexibly in close proximity, but the two d 8 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 -[Rh I (CO) 2 ] 2 [μ:η 5 :η 5 Ph 4 Pn] was investigated with a series of electronically and sterically diverse donor ligands (P(OPh) 3 , P(OMe) 3 , PPh 3 , PMe 3 , dppe) yielding new mono- and bis-substituted complexes, with E-syn -[Rh I (CO)(P{OR}) 3 ] 2 [μ:η 5 :η 5 Ph 4 Pn] (R = Me, Ph) characterised by XRD.