M-L bond formation

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

In this equation, kjy is the rate constant for the diffusion-limited formation of the encounter complex, d is the rate constant for diffusion apart, and ka is that for the activation step, i.e. M-L bond formation. Based on the steady-state approximation for the encounter complex concentration, the apparent rate constant for the on reaction is kon = k kj (k - ,+ka), and the activation volume is defined as... 

The thermodynamically controlled synthesis gives rise to another advantage of metal-directed assembly the capability to correct mistakes within the assembly until the final product is formed. The reversibility of M-L bond formation means that large assemblies and the individual components are in equilibrium until a stable product is formed. Many small assemblies form in the reaction mixture, some of which continue to grow towards the final product while others fall apart and their components are recycled. 

In forming the chelate complex, there is a high probability of the seeond donor atom Y forming a bond to the metal whereas, with monodentate ligands, the probability is much lower. In other words, once the first M-L bond is formed, the second donor atom is held close to the position required for the formation of the second bond. 

M—L bonds in d° systems may be authentic (e.g. equation 19). Elimination could still take place from an t]1-H2 complex because H2 can add in this way without requiring metal d electrons to be available. Molecular hydrogen can also give metal hydrides by more complicated processes, e.g. equation (20), involving ligand hydrogenation and multiple H2 additions. The formation of hydrides from H2 is involved in the catalysis of hydrogenation, hydroformylation and isomerization by metal... 

The dipole moments of a variety of coordination compounds show that the bond dipole moments of the M-L bonds of most a-donor ligands are about 4 D, with the donor atom positive. In contrast, metal carbonyls show an M-C bond moment which is essentially zero because the M L back donation compensates for L M direct donation plus the enhanced polarization of CO on binding. Formation of the M-CO bond weakens the C-O bond relative to free CO because a n orbital on CO is now partly filled by back donation. This will still lead to a stable complex as long as the energy gained from the bond exceeds the loss in C-O. Bond weakening within L on binding to a metal is a common feature in many M L systems. 

Carbonylation metal salt reduction by CO at high pressure with M-CO bond formation Polyhydride complex of the type L MHj (x > 3) Semibridging CO unsymmetrically bridged CO... 

In the absence of ion pairing and rate limitation by solvent dynamics, the volume of activation for adiabatic outer-sphere electron transfer in couples of the type j (z+i)+/z ju principle, be calculated as in equation 2 from an adaptation of Marcus-Hush theory. In equation 2, the subscripts refer respectively to volume contributions from internal (primarily M-L bond length) and solvent reorganization that are prerequisites for electron transfer, medium (Debye-Huckel) effects, the Coulombic work of bringing the reactants together, and the formation of the precursor complex. 

Reaction types O denotes oxidation of metal center, R denotes reduction of metal center, and M—L denotes formation of an alkyl metal bond. 

Another example of M-L bond dissociation following LF excitation is provided by the photochemistry of CpFe( -arene). LF excitation of this molecule labilizes one bond to the arene and the initial product is CpFe( -arene) (Equation (2)). Subsequent reactions lead to cleavage of the other Fe-arene bonds and the formation of CpFe(solvent)3. The utility of this photoreaction is discussed later. 

Besides the splitting of terms observed in the electronic transitions of coordination compounds, other forms of experimental verification of the Jahn-Teller effect include the ESR spectra of coordination compounds. X-ray crystallographic data showing different M-L bond lengths, and thermodynamic data. As an example, consider the stepwise formation constants in Table 16.28 for [Cu(NH3)6] +, which... 

A molecular orbital scheme for the interaction of Mn(CO)5 with L (Figure 5.3) shows, to a first approximation, that half of an M-L bond is gained by formation of the 19-electron species. This partial bond is formed because two electrons (the odd electron of the starting... 

Figure 13.2. Orbitals determining migration reactions (a) allowed migration to the carbonyl group (the d metal orbital participates in the formation of the M —L bond), (b) allowed migration to alkene, (c) external nucleophilic addition to alkene, (d) forbidden migration to alkene (the s orbital contributes to the formation of the M —L bond), (e) allowed migration to alkene (the p metal orbitals participate in the formation of the M —L bond).

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