Coordinated ligands Masking

Ionizations are evident before ligand ionizations with early transition metals Ti(m), V(III), or Cr(III), not evident at all or masked under ligand ionization bands with late transition metals such as Cu(ll) or Zn(II), and highly intermixed with the first ligand ionizations for intermediate transition metal configurations such as d6 of Co(III) or d6 of Ni(H). (Fig. 11). This type of behavior is probably of general occurrence and is in fact found also in other classes of coordination compounds, especially with sulfur ligands. 

Define monodentate, bidentate, hexadentate, ligand, complex ion, chelate, chelating agent, masking, masking agent, formation constant, coordinate covalent bond, water hardness, aliquot. 

The masking of the normal reaetions of simple ligands, such as the nitro, cyano, and ammonia groups, by coordination to a metal is a phenomenon encountered early by a chemist. One of the first examples of masking in a chelate complex was reported, signifieantly, in biological journals. It involves the protection by copper ion of the a-amino group in ornithine and lysine ... 

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

Such effective masking may be traced to two factors (a) the inertness of the telluric acid complexes and (b) the near identity of the demands of oxidant and tellurate on the coordinating properties of the ligand. The decrease in reaction rate here is consistent with a very slow release of ligand from the tellurate complex as established by Edwards and his co-workers (1J). 

At the present time the paucity of data renders it impossible to give a complete picture of the various ways in which coordination may affect the reactivity of aromatic ligands. It is possible to indicate the effects which the coordination process may have on the aromatic reactivity, however, to show how masking and polarization may affect reactivities, and to show at least a few instances where coordination may provide useful synthetic routes to some aromatic compounds. 

When the ligand consists of both an aromatic system and reactive portions external to the aromatic system, the most interesting changes in reactivity occur in the latter portion of the molecule. These changes may be divided into two classes masking of reactive groups and activation which results from the polarization arising from coordination. 

Another problem related to vibronic masking is that most sharp-line transitions are spin-forbidden, and easily obscured by even the tails of nearby spin-allowed bands. They commonly occur over a wide range of energies, putting greater demands on instrumentation. Low-spin, six-coordinate Os (IV) complexes with five or six chloride ligands, for example, have t2g -> t2g transitions with energies from below 3000 to about 17 000 cm"1 [45]. Six-coordinate V(III) complexes commonly exhibit the spin-allowed bands from t2g - eg transitions in the visible, while the t2g -> t2g lines occur between 1000 and 1200 nm [46,47],... 

Attempts were then made to perform asymmetric catalytic reactions using chiral Lewis acid catalysts [59]. Reaction of the nitrone 73 and the oxazolidinone 76 with 10 mol % of the bis(oxazoline) 12-Mg(II) catalyst, prepared by Corey s method [13], in the presence of 4-A molecular sieves afforded the cycloadduct 77 in high yield (>95 %) and high (> 95 %) endo selectivity and 82 % ee (Sch. 33). The presence of activated powdered 4-A molecular sieves was essential to the endo and enantioselec-tivity of the reaction in their absence they were 65 % and < 2 %, respectively. The reaction proceeded via an intermediate XXIX, proposed by Corey [13], in which the bis(oxazoline) ligand 12 and the oxazolidinone 76 are both bidentately coordinated to the magnesium and addition to the re face is favored because the si face of the bound oxazolidinone is masked by one of the phenyl substituents on the oxazoline rings. 

This masking of the typical ligand reactivity via coordination is a thread which runs throughout early work on these compounds and allowed the concept of the coordination sphere to be put on a definite basis as a result of chemical tests. 

There are many additional instances where coordination masks a ligand toward the action of normally effective reagents. In most of these cases the interference is based primarily upon steric restrictions intro-... 

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