Co in complexes

It is well established that increased backbonding from the metal to coordinated CO results in a decrease of the CO stretching frequency. The extent of Zr backbonding to CO in complexes of the type (C5H5B-R)2Zr(CO)2 can therefore be used as a relative measure of electron density at zirconium as a function of R. 

Co(in) complexes promote similar reactions. When four of the six octahedral positions are occupied by amine ligands and two cis positions are available for further reactions, it is possible to study not only the hydrolysis itself, but the steric preferences of the complexes. In general, these compounds catalyze the hydrolysis of N-terminal amino acids from peptides, and the amino acid that is removed remains as part of the complex. The reactions apparently proceed by coordination of the free amine to cobalt, followed either by coordination of the carbonyl to cobalt and subsequent reaction with OH or H2O from the solution (path A in Figure 12-15) or reaction of the carbonyl carbon with coordinated hydroxide (path B). As a result, the N-terminal amino acid is removed from the peptide and left as part of the cobalt complex in which the a-amino nitrogen and the carbonyl oxygen are bonded to the cobalt. Esters and amides are also hydrolyzed by the same mechanism, with the relative importance of the two pathways dependent on the specific compoimds used. 

With Pd/C catalysis, one CO in complex 1 can be selectively substituted for an isonitrile R-NC to give 21. ° When 21 is reacted with dmad at — 60°C, three types of product, 22-24, are obtained in 60-95% total yields after column separation. 

In preparing the table, the total coordination number for any ligand (column 3) is calculated as (tic) (number of ligand units), and c obtained from Table 8.2. Thus for CO in complex 1, the value is (1 x 2) = 2. The number of electrons for any ligand (column 4) is calculated as (the number of electrons for each unit of a ligand) (the number of ligand units). Thus for CO the value is (2x2) = 4. When all of the d electrons are paired, the number of electrons is twice the coordination number. Recall that our treatment is restricted to such systems. Column 4 gives directly the number of d electrons in outer shell. 

A great deal of mechanistic information has been obtained from studies on Co(in) complexes such as those described in Scheme 3.6. An effective catalyst requires a coordinated OH" cis to the phosphate ester, and O-tracer studies have identified the bonds being made Md broken d suggest that the hydrolysis proceeds through the five-coord species thought to be typical for phosphate ester hydrolysis. An study from Co(III)... 

In preparing the table, the total coordination number for any ligand (column 3) is calculated as (n (number of ligand units), being obtained from Table 15.2. Thus, for CO in complex 1,... 

The Co(in) complex formed in reaction 29.4 can be considered to contain coordinated [02] , but the presence of the axial base, L, is cmcial to the formation of the monomeric product. In its absence, a dicobalt species with a Co—0—0—Co peroxido-bridge (i.e. analogous to those discussed in Sectirm 21.10) is formed. 

Insertion of CO in complexes of Pt(II) is sometimes accelerated by the presence of SnC. A study to investigate the role of tin chloride has appeared. In chloroform under CO, complex 28 is rapidly converted to the ionic product 29 by dissociation of SnClJ. In the absence of CO, complex 29 is attacked by the SnClJ to give a five-coordinate intermediate that inserts CO to give the final product 30. With excess CO, 29 also... 

Jaeger, D.A. Reddy, V.B. Bohle, D.S. Cleavable double-chain surfactant Co(in) complexes. Tetrahedron Lett. 1999, 40, 649-652. 

Cobalt. - Crystal structures are reported for the phosphine substituted complex [Co2(C0)g(PMe3)2] and for the alkynyliron bridged species [Co2((t-PhCsCFp) (CO)g].240 studies of the radical anion [CO2(C0)2 (PF2)2NMe>3r indicate that the unpaired electron is in an intermetallic a molecular orbital mainly composed of cobalt orbitals.241 The coupling of alkylIdene with CO in complexes [Co2 /t-CHR)(jx-L)( -dppm)(CO)4] to yield ketenes has been studied under CO or S02. ... 

The coupling of u-alkylidene groups with CO in complexes of the type CC02(w-R)(u-R )(CO) (n-dppm)] (R = CH2,CHMe,CHC02Et R CO,... 

The vibrational frequencies of carbonato complexes have been used to assign some of the frequencies experimentally observed by Iwasita et al. [7] in an electrochemical environment. These are hexacoordinated Co (in) complexes with five or six ligands, one carbonato interacting with the metal either through one or two oxygen atoms and either five or four NH3 molecules respectively, see Fig. 2. 

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