Axial coordination compounds

Thermodynamic and Kinetic Stabiiity of Axial Coordination Compounds... 

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

M = Os) has not been established. This is very remarkable because a variety of other d6 metalloporphyrin moieties show an unusual lability at their axial coordination sites (see Sect. 3). The stability of the species [Os(OEP)L2]n (n = 0 or 1) representing the osmochrome/osmichrome system has allowed a complete physical and chemical characterization of the compounds [29a-k] listed in Table 9 and their cationic derivatives [Os(OEP)L2]PF6 (78, 80). 

The interpretation of observations on penta- or hexa- coordinated compounds is not so clear as CF3 groups may occupy axial positions, even when competing with fluorine substituents as in (44).<1968,41)... 

With a view to examining whether bulky substituents at the amidine nitrogen atoms can prevent axial coordination in quadruply bonded dimetal compounds, Cotton and coworkers have studied the reactions of (Bu4N)2Re2Cl8 with PhN(CMe)NHPh and MeN(CPh)NHMe.83 The structures of the crystalline products can be represented as (34) and (35) respectively. 

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