Titanium, Zirconium, and Hafnium Compounds

Other studies on mixed Ti-Al systems as alkene polymerization catalysts have afforded examples of bridging between these two metals by cyclopentadienyl residues (69), by carbidic carbon atoms (70, 71, 72), 5 and by methyl groups (74). The carbide systems 70, 71, and 72 were prepared by reactions between phosphinimido-titanium methyl compounds and trimethylaluminum. The coordination at the carbide carbon atoms in 70 and 72 is flattened tetrahedral. However, the coordination in 71 is distorted trigonal bipyramidal, with a near-linear Ti—C—Al vector (178.5°) (cf. 65 and 66) and bonding as in 67. 

Bulky alkyl groups CH2R (R = Ph, CMca, SiMe, or CMo Pli), which have no hydrogen atoms attached to the carbon or silicon atom in the p position, have been widely used to probe the alkyl chemistry of transition metals Uieir bulk protects the metal from nucleophilic attack, while the absence of 3 hydrogen reduces the risk of decomposition by metal hydride formation and alkene elimination. The use of such ligands attached to manganese has provided examples of hypercarbon atoms bridging pairs of metal atoms that are worthy of brief mention here. 

The compound [Mn(CH2SiM63)2], for example, is polymeric in the crystal, -with a structure (75) like those of dialkyls of beryllium and magnesium (MR2) (M = Be or Mg R = Me or Et), Each metal atom, tetrahedrally coordinated. 

In compound 76 and in dimesitylmanganese, which crystallizes as the trimer [Mn(mesityl)2]3 (77) the degree of association is limited by the bulk of the substituents. All of these systems show the characteristic features of 3c-2c Mn-C-Mn bridge bonding greater Mn-C interatomic distances to the bridging (hypercoordinated) carbon atoms than to their terminal counterparts sensitivity of the metal-carbon distance to the metal coordination number and acute Mn-C-Mn bond angles at the hypercoordinated carbon atoms. 

As observed for other bridging complexes, the terminal and bridging methyl groups in compound 84 exchange rapidly, generating only a single resonance ( -0.89) in the 11 NMR spectrum at room temperature. However, this could be resolved into three broad resonances at -80°C (8 -0.15,-0.50,-2.10), consistent with the molecular structure in the solid state. Complex 84 is a useful model for intermediates in transmetalation/transalkylation reactions. 

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Thermodynamic data in the area of transition metal chemistry is available, but additional studies would be desirable. One of the early indications that C—Ti bonds are not notoriously weak was obtained from the heats of combustion of Cp2Ti(CH3)2 and Cp2Ti(C6H5)2 with subsequent estimation of the a-bond dissociation energies (250 kJ/mol-1 and 350 kJ/mol 1, respectively)49. From heats of alcoholysis of a number of titanium, zirconium and hafnium compounds, and heats of solution of the products as well as subsidiary data, Lappert estimated heats of formation (AHf°) and thermochemical mean bond energy terms (EM X) of metal—X bondsso> (Table 2). 

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