Metal centered

PM3/TM is an extension of the PM3 method to include d orbitals for use with transition metals. Unlike the case with many other semiempirical methods, PM3/TM s parameterization is based solely on reproducing geometries from X-ray diffraction results. Results with PM3/TM can be either reasonable or not depending on the coordination of the metal center. Certain transition metals tend to prefer a specific hybridization for which it works well. 

One way that molecular mechanics methods have been adapted to transition metal applications is by including one orbital-based term in the force field to describe the metal center. These terms are typically based on semiempirical methods or even some variation of ligand field theory. 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Polyols. Polyols, such as pentaerythritol [115-77-5], dipentaerythritol [126-58-9], and sorbitol [50-70-4], most likely chelate the active metal centers to reduce their activity toward the undesired dehydrochlotination reaction. These additives are generally iacluded ia the stabilizer formulation, used ia the range of 0.2 to 0.7 phr. 

Dinitrogen-Reducing Systems. The binding of N2 to a metal center is the first step in activating molecular nitrogen toward reduction. 

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

Comparable with the chloride system, complex ions of the form M2 ThX3] (A =Br [44490-064], M = (CH3) N, (C2H3) N X = I [44490-18-8], M = (C2H3) N, (CH3)3C3H3N) are known where the metal center is octahedral. Additional information on thorium bromides and iodides can be found in the hterature (81). 

Multiple polyhedral expansion reactions, carried out either simuntaneously or sequentiaUy, have been used to prepare metaUacarboranes having multiple metal centers (202). An example is given in Figure 18. 

Exopolyhedral Metallacarboranes. Many metaHacarboranes are known that exhibit exopolyhedral bonding to metals. Most commonly metals are bound via M—H—B interactions in which the B—H group can be regarded as a two-electron donor to the metal center. In other cases, M—B,... 

Bond lengths and infrared spectra support the multiple-bond character of the M—CO bonds. Coordination of a CO molecule to a metal center can change the C—O bond order. According to the description of ( - and TT-bonding given herein, increased ( -bonding between a metal and CO results in a... 

The occurrence of stereospecific polymerization in solution has been explained by the stetic restrictions of ligands bonded to the metal center. For example, the following stmcture has been postulated as an intermediate in solution catalysis (68) ... 

A number of transition-metal complexes of RNSO ligands have been structurally characterized. Three bonding modes, r(A,5), o-(5)-trigonal and o (5 )-pyramidal, have been observed (Scheme 9.1). Side-on (N,S) coordination is favoured by electron-rich (et or j °) metal centers, while the ff(S)-trigonal mode is preferred for less electron-rich metal centres (or those with competitive strong r-acid co-ligands). As expected ti (N,S)... 

Fitting a rearranged system to the steric requirements shown above within a molecule is needed for an intramolecular mechanism. Even when the appropriate arrangement cannot be achieved within a single molecule, the required configuration, 29 or 30, can possibly be realized in a dimer or higher associate. For metal-centered migrants, an alternative intermolecu-lar mechanism of the dissociative SnI type is also possible. 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

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