Metallocene Carbene Complexes and Related Compounds of Titanium, Zirconium, and Hafnium

The structure and creativity spectrum of transition‐metal carbene complexes has been widened significantly by the bent metallocenes of titanium, zirconium, and hafnium. Proceeding from metal carbonyls and reactive (butadiene)‐, (aryne)‐, or (olefin) MCp2 complexes, many new Fischer‐type metaloxycarbene complexes of Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Ni, and Rh have been synthesized. The incorporation of titanocene, zirconocene, or hafnocene fragments allows new types of carbene complexes to be prepared. For the (alkylidenamido)metallocene complexes equation image , metallocene ylides equation image , and binuclear (μ‐alkynyl)metallocene compounds, some metalligand π bonding is indicated. Metallocene complexes with metallaoxirane units, equation image , show similar chemical behavior to that of binuclear (μ‐methylene)complexes equation image . The methylene groups of zirconaoxirane complexes, which are derived from carbon monoxide, may be expelled as ethylene by thermally induced CC coupling. With metal hydrides, CH2 transfer with insertion into the metal–hydride bond occurs. In one case, methylene insertion into a metal–carbon bond can even be observed. These reactions of Ti‐group metallaoxiranes could be models for postulated intermediates in the Fischer–Tropsch synthesis.