M-L bonds

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 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. 

The EA/CA ratio was proposed as a measure of hardness of the Lewis acid, and EB/CB as hardness of the Lewis base in aqueous solution (17). It now seems that the E/C ratio is not a measure of hardness in the sense in which Pearson (5,5a) defined hardness. Rather, the E/C ratio for a Lewis acid or base is a measure of the tendency to ionicity in the M-L bonds formed. The EAICA ratio should rather be called IA, and the EbICb ratio IB, the tendency to ionic bonding in forming the M-L bond. Acids and bases in Tables I and II are placed in order of increasing tendency towards ionicity in the M-L bond, according to the E/C ratios IA and 7b. A justification for this interpretation is that the order of IA values for metal ions in aqueous solution strongly resembles the order of hardness derived by Pearson (19) from enthalpies of complex forma-... 

An observation regarding HSAB theory here is that metal ions that are soft in aqueous solution must not only be able to form covalent M-L bonds, but also must have a loose enough coordination sphere to tolerate potentially adverse steric effects such as bulky donor atoms or substituents on a ligand. For a soft Zn(II) ion to be produced in zinc metalloenzymes, its coordination number must drop from six in the... 

Rotation around the M-L bonds requires no spatial considerations for the linear N-bonded SCNT but structure II shows a different situation for S-bonded SCN-. In that case, which is shown in Figure 16.7, a volume represented as a cone (sometimes referred to as a cone of revolution) is swept out as the rotation occurs. 

Furthermore, according to (4.9a) the total number of covalent M—L bonds (n + m, both oml and 7TMl) is 10 — k. Because main-group ligands generally are more electronegative than the central transition-metal atom, each such covalent M—L bond can be termed a formal one-electron oxidation of M. The duodectet-rule-conforming oxidation number ( ox) of M is therefore... 

Figure 4.2 Idealized molecular shapes of ML2 (left, sd) and ML3 (right, sd2) Lewis-like species (cf. (4.45) and (4.46) in the text), with optimal L—M—L bond angles a = 90° for all ligands.

Figure 4.4 Idealized ML5 molecular shapes having (limited) similarity with optimal L—M—L bond angles cWe = 65.91° and a0btuse = 114.09° of equivalent sd4 hybridization.

Figure 4-5 Idealized MLg molecular shapes for equivalent sd5 hybridization (cf. (A 49a)—(4 49d)in 1116 text) wittl °Ptimal L—M—L bond angles a acute = 63.43°...

Bent s rule for d-block elements. Increased metal s character tends to go to the M—L bonds of higher covalent character, and increased d character to the bonds of higher ionic character. 

As described in Section 3.5, any polar M—L bond is susceptible to backside attack by a Lewis base I. to form a linear (or near-linear) 3c/4e /ryperbonded L i- M -i L triad, equivalent to strong resonance mixing of the form... 

Enzyme/ of Cu centers, Type, Ligands (M-L Bond Length, A)... 

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... 

Table 7.6 lists the theoretical BDEs of the M-L bonds in the group-10 Ni(CO)3L, Pd(CO)3L and Pt(CO)3L complexes calculated at the MP2/II and CCSD(T)/II levels of theory [49, 50], The only experimental value known for those compounds is an estimate of ca. 10 kcal/mol obtained for the (CO)3Ni-N2 bond energy at 298 K [59], This estimate is based on kinetic measurements of nitrogen extrusion from the complex. Thermal corrections to the CCSD(T)/II value of D0 = 4.6 kcal/mol yield a theoretical prediction of 6.7 kcal/mol, which is in a reasonable agreement with experiment [49]. The MP2/II BDEs listed in... 

This is expected to be favoured for metal-centred excited states for example, in d-d states of d or d complexes, where excitation often involves promotion of an electron from an essentially non-bonding orbital to one with appreciable sigma antibonding M-L character (e.g. in CrfNHalsCl Eq. 3). The net effect is lengthening of the M-L bond, which predisposes the complex to dissociation or associative substitution. The incoming ligand is often the solvent (e.g. as in Eq. 3) or counterion of an ion pair (Eq. 4). 

This empiricism (which concerns the concept of trans-influence) [93] is derived from theory and the Fermi contact term. Since the s-component of the M-L bond determines the magnitude of ij(M,L), bonding considerations which decrease the s-component, e. g., a relatively strong a-bond in the trans-position, decrease j(M,L). In Wilkinson s catalyst, RhCl(PPh3)3, (and the p-tolyl analog) the two j( ° Rh,3ip) values are quite different 189 Hz (P trans to Cl) and 142 Hz (P trans to P) [94]. The larger i [-value arises from the P-atom trans to the weaker donor. 

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