Metal complexes, pentacoordinate transition

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... 

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). 

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

British Biotech has described co-crystal structures of both BB-3497 and actinonin bound in the active site of E. coli PDF [24]. The metal centre (Ni ) in both complexes adopts a pentacoordinate geometry, bound by the two oxygen atoms of the hydroxamate along with Cys-90, His-132 and His-136. This coordination pattern is consistent with the mechanism of de-formylation proposed by Becker et al. [56] and Jain et al. [67], in which a pentacoordinated metal centre stabilises the transition state during hydrolysis of the formamide bond. When compared to the co-crystal structure of a substrate hydrolysis product, Met-Ala-Ser, it is clear that the side chains of these two inhibitors bind into the active site pockets similarly to the substrate [56]. 

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 electrochemical behavior of tetracoordinated Cu(i) complexes (i.e., Cu(dpp)2-based cores) is well established.193,941 The reversible redox potential for the Cu(ii)/ Cu(i) transition is around 0.6-0.7 V versus SCE. This relatively high potential underlines the stability of the 4-coordinate Cu(i) complexes relative to their Cu(n) counterparts. The redox potential of pentacoordinated copper complexes 84 86 is observed in a much more cathodic range. For example, for the 5-coordinate complex Cu(l, dap)2+/+ (dap = 2,9-di-p-anisyl-l, 10-phcnanthrolinc), in which the terpy fragment of the ring is bound to the metal, the redox potential is -0.035 V. This potential shift when going from tetracoordinated to pentacoordinated copper systems is due to the better stabilization of the Cu(ii) state, thanks to the presence in the coordination sphere of live donor atoms. 

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