STABILITIES OF METALLACYCLOBUTADIENE AND METALLATETRAHEDRANE COMPLEXES
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Ab initio molecular orbital theory has been used to study the stabilities of isomeric metallacyclobutadiene and metallatetrahedrane complexes. In the metallacyclobutadienes, the C3R3 ligand is bonded to the transition metal in a ring-opened 2-C3R3 coordination mode. In the metallatetrahedranes, the C3R3 coordinates as 3-C3R3. In this paper, we examine the stability governing factors, such as the ligand environment, number of valence electrons, and type of transition metal. For most L5M(C3R3) complexes, the metallatetrahedrane structures are found to be more stable than the metallacyclobutadienyl ones because the former can be viewed as pseudo-octahedral d6 compounds if one takes the C3R3 ligand as a single [3-C3R3]+ cationic ligand. For most L4M(C3R3) complexes,however, the metallacyclobutadienyl structures are more stable than the metallatetrahedrane ones. If one takes the 2 ligand as a bidentate [2- C3R3]- anion, these metallacyclobutadienyl complexes also can be viewed as octahedral d6 compounds. For most L3M(C3R3) complexes, the metallatetrahedrane structures become more stable. By adopting metallatetrahedrane structures, these 3-C3R3 complexes satisfy the 18-electron rule and can be viewed as late transition meteil tetrahedral d10 complexes. Most metallacyclobutadienes are found to have symmetric bonds within the M(2-C3R3) unit. The short-long-short-long bonds in the experimentally known CpCl2W(2-C3R3) complex are explained as a result of a compromise between steric and electronic factors. The former forces the C3R3 unit to twist and induces bond alternation; the latter favors asymmetric structure. 1994, American Chemical Society. All rights reserved.
