Coordinative saturation, unsaturation

To form gels in this mixed alcohol solvent system, a critical value of (N4> must be exceeded. This observation has been rationahzed on the basis of the kinetic stabihty of B-O-B bonds toward hydrolysis or alcoholysis. Becanse tetrahedrally coordinated borate gronps are coordinatively saturated see Coordinative Saturation Unsaturation), this group is unable to participate in dissociative reactions of the triangular group that hinder polymerization and the formation of gels (equation 2). 

This trend is reasonable in view of the coordinatively unsaturated see Coordinative Saturation Unsaturation) nature of Co(CO)3L+ (16-electron connt) and the much higher concentration of L in solution as compared to that of Co(CO)4. Recombination reactions, for example between Co(CO)3L and Co(CO)4, or of Co(CO)3L with itself, that would lead to Co2(CO)7L and symmetric Co2(CO)6L2, respectively, are kinetically fast (kdim 10 - 10 s ). 

Although other transition metals have been found to catalyze hydroborations with HBcat, early studies have shown that rhodium complexes are the most effective for reactions of simple alkenes. The catalytic cycle proposed resembles one suggested previously for the rhodium-catalyzed addition of carborane B-H bonds to the C=C unit in acrylate esters. The reaction is believed to proceed via initial loss of phosphine and oxidative addition (see Oxidative Addition) of the B-H bond of HBcat to the coordinatively unsaturated (see Coordinative Saturation Unsaturation) rhodium center. Coordination ofthe alkene (seeAlkene Complexes) and subsequent insertion (see Insertion) into the Rh-H bond and reductive elimination (see Reductive Elimination) of the B-C bond then generates the organoboronate ester product(s) (Scheme 1). 

Octahedral hypervalent halides, on the other hand, are much less reactive (see Coordinative Saturation Unsaturation). For example, PFe" is a widely used noncoordinating anion (see Noncoordinating Anion or Cation), SiF4- bipyridine is stable in the presence of moisture and SFe is remarkably inert it can be treated with steam at 500 °C or with molten KOH without any effect. It is nontoxic and used as an insulating gas in high voltage applications. 

Oligomerization and polymerization catalysis by metal complexes comprises three steps initiation, propagation, and termination. Chain growth proceeds at a coordinatively unsaturated see Coordinative Saturation Unsaturation) center having a metal carbon or metal hydride see Hydride Complexes of the Transition Metals) bond, nsuaUy generated by the interaction of a metal complex with an activating species such as an alkylaluntinum cocatalyst. The first insertion of an alkene monomer into the metal carbon or metal hydride bond (chain initiation) is followed by repeated insertions... 

A more recent addition to the half-sandwich chemistry of ruthenium is given by a number of complexes where the central metal obeys a 16 valence electron count. These coordinatively unsaturated (see Coordinative Saturation Unsaturation) metal centers are widely invoked as intermediates or transition states in dissociative substitution processes or in catalytic transformations at transition metal centers. Such complexes are not, however, easily isolated. The most common way to stabilize such species is by coordinating sterically bulky ligands to the metal, thereby preventing further ligand addition. They can also be stabilized in the form of dimeric complexes. 

Metal /3-diketonates are often coordinatively unsaturated see Coordinative Saturation Unsaturation) and will therefore react with Lewis bases to form complexes. For example, the trimer [Ni(acac)2]3 will react with Lewis bases such as water or pyridine to form monomeric trans-[Ni(acac)2L2]. The formation of weak complexes with lanthanide /3-diketonates is critical in their use as NMR shift reagents. 

The insertion of alkynes into Os-H bonds produces vinyl complexes. For example, 0sH(02CCF3)(CO)(PPh3)2 reacts with PhC=CR (R = Me, Ph) to give Os C(Ph)=CHR (02CCF3)(C0)(PPh3)2. These complexes are catalysts for the oligomerization of phenyl acetylene. Terminal alkynes insert into the Os-H bond of OsHCl(CO)(P Pr3)2 to give Os(CH=CHR)Cl(CO)(P Pr3)2. This complex is coordina-tively unsaturated see Coordinative Saturation Unsaturation) and can react with CO to form the six-coordinate complex Os(CH=CHR)Cl(CO)2(P Pr3)2 (Scheme 2). 

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