IR acidity

The m.ps. of the-ir acids v,iry with the rate of heating, and the values given are those obtained on rapid heating in sealed tubes, t Anhydrous. 

Recovery and Purification. The dalbaheptides are present in both the fermentation broth and the mycelial mass, from which they can be extracted with acetone or methanol, or by raising the pH of the harvested material, eg, to a pH of 10.5—11 for A47934 (16) (44) and A41030 (41) and actaplanin (Table 2) (28). A detailed review on the isolation of dalbaheptides has been written (14). Recovery from aqueous solution is made by ion pair (avoparcin) or butanol (teicoplanin) extraction. The described isolation schemes use ion-exchange matrices such as Dowex and Amberlite IR, acidic alumina, cross-linked polymeric adsorbents such as Diaion HP and Amberlite XAD, cation-exchange dextran gel (Sephadex), and polyamides in various sequences. Reverse-phase hplc, ion-exchange, or affinity resins may be used for further purification (14,89). 

Ligands that can interact with a metal center by forming a ir-type bond are commonly called ir-acids and ir-bases. The definitions of ir-acid and ir-base are similar to those used for... 

Lewis acidity. A 77-acid is a ligand that can accept electrons hv using 17-orbitals, and a 17-base donates electrons by using 17-orbitals. On the basis of these definitions, describe each of the following ligands as either a ir-acid, a ir-base, or neither ... 

Ligand Preferences. The LVC s are electron rich and in order to exist in stable compounds they require ligands, such as CO, that are weak donors and good tt acceptors. On the other hand the HVC clusters are not attractive to such ligands nor do they require them for stability. In this respect, there is a direct parallel to mononuclear compounds where M° prefers CO, RNC or similarly ir-acidic ligands while the Mn+ (n = 2-4) ions do not generally form CO complexes. 

Metal-metal bonds in molybdenum and tungsten alkoxides provide a ready source of electrons for oxidative-addition reactions and addition reactions involving ir-acidic ligands. 

In general, the w-accepting ability of a phosphine increases as the electronegativities of its substituents increase. Thus we would expect the ir acidity of... 

Phosphorus-containing macrocycles, 990, 996, 998 macrocychc effect, 1002 ring size, 1003 stereochemistry, 1003 Phosphorus heterocycles transition metal complexes, 1041 Phosphorus ligands, 989-1061 ir-acid series, 1033 bonding, 1030-1041 cone angle concept, 1012 trigonal bipyramidal complexes, 1036 Phosphorus oxychloride metal complexes, 500 Phosphorus ylides transition metal complexes, 1056 Phosphorylation biological, 978 Phosvitin, 975 Photochromism... 

Describe the following ligands as either ir-acids, 7r-bases, or neither (see Exercise 16.73) (a) NH3 ... 

Transition Metal Clusters with ir-Acid Ligands R. D. Johnston... 

Strong ir-acid jIq Strong ir-acid 1 Strong r-acid... 

---