Ligand second coordination sphere

Whereas ligand exchange reactions in aquated complexes of Li+ have been studied extensively, the corresponding ammine complexes have so far received very little attention. The interaction of an ammonia molecule (belonging to the second coordination sphere) with [Li(NH3)4]+ is weaker than in the... 

Figure 2. Dy(P30io)2 is a lanthanide shift reagent commonly used in biological 7Li NMR experiments. The Dy3+ ion has a coordination number of nine with two P3O10 moieties, acting as tetradentate ligands, and one molecule of H2O coordinated in the first coordination sphere up to seven Li+ ions can bind in the second coordination sphere.

The Ia and A mechanisms are mutually exclusive, since the 7 -coordinate transition state has an imaginary mode that either describes a concerted entry/leaving of the ligands and the mechanism is Ia (Fig. 6a) or only the motion of the entering (or leaving ligand) and the mechanism is A (Fig. 6b) (104). The Id and D mechanisms are not mutually exclusive and up to now no Id has been computed, most probably due to the omission of the second coordination sphere (104). 

Considering the [M(H20)6]2+/3+ system as a typical example, Rotzinger has performed calculations on the species [M(H20)fi-(H20)l2T +, where the second coordination sphere is described by 12 water molecules that are hydrogen bound to the six water ligands, an example being shown in Fig. 2. 

Although the cation-anion interaction of metallocenium ions is very weak, the counteranion is likely to remain in proximity with the metal cation to form a contact ion pair in low-permittivity solvents such as toluene (commonly used in polymerization reactions). If the metal cation allows the counteranion to penetrate into the first coordination sphere, it can form an inner-sphere ion pair (ISIP). When the counteranion is relegated to the second coordinating sphere, the ion pair becomes an outer-sphere ion pair (OSIP), which is more ionic in nature than ISIPs. A schematic representation of the relationship between ISIPs and OSIPs is depicted in Scheme 2. This simple scheme shows us the principal elements that affect the cation-anion interactions (e.g., counteranion (Y ), ancillary ligands (L ), transition metal (M), and alkyl ligand (R)), and the subtle balance between these elements in the dynamic equilibria. 

In the 1 M solution it shows a distinct and fairly well-separated peak at 4.5 A, corresponding to about 16 water molecules, which is roughly equal to twice the number of H20 molecules in the first coordination sphere. In the more concentrated perchlorate solution (Fig. 15) this peak has a more structured appearance, but is not resolved from longer distances, presumably due to the presence of perchlorate ions in the second coordination sphere, apparently bonded as bidentate ligands. The many overlapping interactions, however, prevent an unambiguous analysis. 

An important property of crown ethers (also cyclodextrins) is that they can act as second coordination sphere ligands. Thus [Pt(bipy)(NH3)2]2+ gives crystalline (Pt(bipy)(NH3)2[18-C-6] 2+, where there is N—H—O bonding ammonium ions can be similarly coordinated. A cheap ether for catalysis of solid-liquid phase reactions is N(CH2CH2OCH2CH2OMe)3 known as TDA1. 

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