Metal complexes, pentacoordinate transition metals

Rapid Intramolecular Rearrangements in Pentacoordinate Transition Metal Complexes. VI. The Coupling of Olefin Rotation and Berry Pseudorotation in Tetracarbonyliron-Olefin Complexes, S. T. Wilson, N. J. Covllle, J. R. Shadely, and J. A. Osborne, J. Amer. Chem. Soc., 96,4038 (1974). 

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

Orbital symmetry arguments and EHT calculations have also provided a way of discriminating between axial and apical substitution in the above mentioned case of pentacoordinate phosphorus. This analysis leads the way to more complex problems of coordination around transition metal atoms. 

Ziegler-Natta catalysts are primarily complexes of a transition metal halide and an organometallic compound whose structure is not completely understood for all cases. Let us use as an example TiCU and R3AI. The mechanism of the polymerization catalysis is somewhat understood. This is shown in Fig. 14.6. The titanium salt and the organometallic compound react to give a pentacoordinated titanium complex with a sixth empty site of... 

The reactions of dihydrobilin (1,19-dideoxybiladiene-a, c) with transition metals are strongly influenced by the nature of the metal ion. Thus with Mn(OAc)3 or FeClj the corresponding metallocorrolates have been obtained in high yield, in the presence of chromium or ruthenium salts the reaction product isolated has been the metal free macrocycle, while coordination of rhodium requires the presence of an axial ligand such as a phosphine, arsine or amine [21]. Neutral pentacoordinated rhodium complexes have thus been obtained. Although analysis of the electronic spectra of the reaction mixtures demonstrated that cyclization of the open-chain precursor and formation of metallocorrolates occur even in the absence of extra ligands, no axially unsubstituted rhodium derivative has been reported. 

The t)1-BH4 coordination is much less common than the other two coordination modes. Most of the r l complexes are constituted by late transition metals and a large variety of phosphino ligands. Only tetrahedral (1.4/T1.4) and octahedral (1.6) structures have been crystallographically characterised (see Scheme 2). Complexes adopting tetrahedral geometries are d10 and contain three phosphorus atoms coordinated to the metal centre. Although some examples of pentacoordinated complexes are present in the literature, these... 

The dominant theme since the mid-1980s in the chemistry of thiophenes has undoubtedly been the application of transition metal catalysis. There are two distinct aspects of this work the first is the use of Pd or Ni as catalysts for bond formation. The other is the study of transition metal-mediated ring opening of thiophene by cleavage of S—C bonds. The former is discussed in Section 2.10.4.7. In addition, some mechanistically interesting results have also been reported during this period. These include the preparation of the first fully characterized, stable pentacoordinated sulfur compound with four C—S bonds (Section 2.10.3.1) and the first clear evidence for an organosulfur ate complex (Section 2.10.2.6.4). 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

Figure 12 The Sn2 (a) and Berry (b) pseudo-rotation distortions that dominate the TBP to SQP transition in pentacoordinate metal complexes...

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