Crowded coordination spheres

In the case of the Ru(III)-chelates, the crowded coordination sphere around the metal center prevents the coordination of 02. Thus, the corresponding kinetic model postulates that ascorbic acid is oxidized by Ru(III) in two subsequent redox steps and Ru(II) is reoxidized by 02 ... 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

It is not clear either how the Ru center can accommodate four ligands simultaneously. The crowded coordination sphere around the metal center in the Ru(EDTA)-ascorbate complex is expected to hinder the coordination of other ligands as was proposed earlier (24,25). The contradiction between the two sets of results reported in Refs. (24,25) and (148) is obvious. While the Ru(EDTA)(H2A)(02) complex was not considered in the kinetic model proposed for the oxidation of ascorbic acid,... 

Recently, tris-p-diketiminate lanthanide complexes [LnL3 ] (X = Cl, L Ln = Pr 111, Nd 112, Sm 113 X = H, L Ln = Nd 114 X = Me, L Ln = Nd 115) (Scheme 12) displaying a high activity in producing PLAs under mild conditions via ROP of L-lactide have been reported. This reactivity may be attributed to the crowded coordination sphere around the central metal, which incidentally affords an activated Ln-N(p-diketiminate) bond. The activity depends on the central metals, and the active trend of Sm < Nd < Pr is consistent with the sequence of the ionic radii [113]. 

The importance of steric hindrance was smdied by Bergman et al. [21] in the computational analysis of the C—H activation of methane by [CpRe(CO)2] and [TpRe(CO)J (where Cp=CjHj and Tp=tris(l-pyrazolyl)borate). The seven-coordinate product [TpRe(CO)2(H)(Me)] is not favorable as it suffers from closer intramolecular contacts, while the [CpRe(CO)2(H)(Me)] complex is less crowded. The author s obsawations suggest that nonbulky ligands will favor a less crowded coordination sphere and promote an exothamic C—H activation via an oxidative addition intamediate. 

The replacement of five waters in [Cr(H20)6]3+ by NH3 in [Cr(NH3)5H20]3+ causes a 20-fold increase in the lability of the single remaining water (which is quite small by comparison with such replacement in [Ni(H20)6]2 +, as seen from Table V) and appears to decrease the importance of the a-activation mode in the approach to the transition state (see Section III,G). Assuming that the free radii (222) of H20 (138 pm) and NH3 (169 pm) reflect their relative electrostricted coordinated radii, it is anticipated that the first coordination sphere of [Cr(H20)6]3+ will be less crowded than that of [Cr(NH3)5H20]3+ and that the extent of a-activation character in the substitution of water in the latter species will decrease. 

Few scandium alkoxides have been studied in detail, but monomeric 3-coordination exists in [(2,6-Bu2-4-MeC6H20)3Sc] where the bulky ligand enforces steric crowding. It will, however, expand its coordination sphere to form 4-coordinate adducts with PhsPO and THF - unlike the corresponding three-coordinate silylamide. Sc(OSiBu3)3.L (L = THF,... 

In 2009, Organ and co-workers studied the mechanism of the alkyl-alkyl Negishi reaction and, through DFT computations, revealed transmetalation to be the rate-limiting step and not oxidative addition A key Pd-Zn interaction was also identified in the mechanism to persist beyond reductive elimination. This interaction, in combination with the NHC ligand was shown to help facilitate reductive elimination by creating a highly sterically crowded environment in the Pd coordination sphere. 

The pathway chosen depends on the geometry of the disilametallacycle intermediate (%), the electronic properties (e.g., hardness) of the central metal and the steric effects of the substituents on the 1,3-diene. The disilacyclopentene products (e.g., (93)) generally are favored by the use of dienes with at least one end unsubstituted (to allow 1,1-addition), steric crowding in the coordination sphere around the metal in the metallacycle intermediate (preventing 1,4-addition) and use of a soft metal (such as Fe) to assist in the migration of the softer base, H (rather than the harder F ). 

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