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

The application of Mg[Ph4Pn] and Li center dot K[Ph4Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of "half-baguette" anti-[RhI( L)n](2)[mu:.5:.5Ph4Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[RhI( CO)2]2[mu:.5:.5Ph(4)Pn] complexes as well as the related iridium complex anti-[IrI( COD)]2[mu:.5:.5Ph4Pn]. 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 p 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[mu:.5:.5Ph4Pn] 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)[mu:.(5):.(5)Ph4Pn] 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 bissubstituted complexes, with E-syn-[RhI( CO)(P{OR})(3)]2[mu:.(5):.5Ph(4)Pn] (R = Me, Ph) characterised by XRD.