Pentamethylcyclopentadienyl-rhodium and -iridium complexes. Part 36. Reaction of [Rh 2 (C 5 Me 5 ) 2 Cl 4 ] with phenylacetylene to give the η 4 -cyclobutadiene complex [Rh(C 5 Me 5 )(C 4 Ph 2 HCPhC 5 Ph 2 H 2 )] and the η 4 -benzocyclobutadiene complex [Rh(C 5 Me 5 )(C 8 Ph 4 H 2 )], identified by X-ray structure determinations

Reaction of [Rh2(C5Me5)2Cl4] with PhCCH in acetonitrile in the presence of Na2CO3 gives two complexes (3) and (4) as well as some isomers of triphenylbenzene and acetophenone. Complexes (3) and (4) were characterised by 13C n.m.r. spectroscopy and by single-crystal X-ray structure determinations. Complex (3) has the rhodium π-bonded η5 to a C5Me5 ring and η4 to the C4 ring of a tetraphenylbenzocyclobutadiene. The benzocyclobutadiene is very close to planar and only a little distorted upon co-ordination; the structure of the ligand is best understood in terms of a ‘1,2-divinylcyclobutadiene’ type of bonding. Complex (4) has the rhodium π-bonded η5 to a C5Me5 ring and η4 to a cyclobutadiene; this cyclobutadiene carries two phenyl substituents (1,3-) and an (unco-ordinated) 6-(1,3,6-triphenylfulvenyl) substituent. Possible routes by which (3) and (4) could be formed are discussed.