Formation and molecular structure of (η5-cyclopentadienyl)-(η4-cyclobuta-[l]phenanthrene)rhodium: a cyclobutadiene–metal complex derived from the intramolecular cyclodimerization of an acyclic diacetylene

Interactions of the diacetylene 2,2′-bis(phenylethynyl)biphenyl with the metal carbonyls [(η5-C5H5)-Co(CO)2], [(η5-C5H5)Rh(CO)2], and [Fe(CO)5] proceed via intramolecular cyclodimerization to form the respective η4-1,2-diphenylcyclobuta[l]phenanthrene complexes in each case; for the rhodium analogue, a cyclobutadiene-type structure containing a unique trapezoidal η4-C4-ring has been unequivocally established by means of X-ray diffraction techniques.