Coordination modes, transition metal

Fig. 5.20. Modes of coordination of transition metal ions with /3-lactam antibiotics. Complex A In penicillins, the metal ion coordinates with the carboxylate group and the /3-lactam N-atom. This complex stabilizes the tetrahedral intermediate and facilitates the attack of HO-ions from the bulk solution. Complex B In benzylpenicillin Cu11 binds to the deprotonated N-atom of the amide side chain. The hydrolysis involves an intramolecular attack by a Cu-coordinated HO- species on the carbonyl group. Complex C In cephalosporins, coordination of the metal ion is by the carbonyl O-atom and the carboxylate group. Because the transition state is less stabilized than in A, the acceleration factor of metal ions for the hydrolysis of cephalosporins is lower than for penicillins. Complex D /3-Lactams with a basic side chain bind the metal ion to the carbonyl and the amino group in their side chain. This binding mode does not stabilize the tetrahedral transition complex and, therefore, does not affect the rate of... 

Modes of Isocyanide Coordination in Transition Metal Complexes 13.2.1... 

Heterocycles can coordinate to transition metals by either electron donation from ligand to metal. The former most commonly occurs via the heteroatom, although binding through a ring carbon atom has often been observed, most notably in cyclometallated complexes (84MI7). 7r-coordination almost always requires the metal ion to be in a low oxidation state and can utilize either all or part of the 77-system. Thus any given heterocycle will have a number of coordination modes open to it and synthesis of the metal complex can result in isolation of the kinetic product. 

Figure 2 Known modes of thiophene coordination in transition metal complexes...

Acrylonitrile appears to coordinate to transition metal ions primarily through its carbon-carbon double bond (highest occupied molecular orbital of that molecule ). This mode of coordination has been deduced from the absence of the stretching frequency and, in the cases of Fe(CO)4(CH2=C-CN) and... 

A great deal of effort has gone into determining the factors that influence the geometries of diatomic and related polyatomic molecules when coordinated to transition metal centers. A bending distortion changes the coordination mode from linear (19) to bent (20) and, further, through a kinked conformation (21) to side-on rj -coordination (22). The process culminates in oxidative addition in which the X—Y bond is broken (23). 

In this section the reactivity of metal nitrosyl complexes is discussed and related to the NO coordination mode. A considerable difference between the chemistry of nitric oxide and carbon monoxide complexes has already been noted. The reactivity of nitric oxide coordinated to transition metal centers, and of nitrosyl clusters, were thoroughly reviewed in 1979 by McCleverty (7) and in 1985 by Gladfelter 11), respectively therefore only a summary is presented here. Nucleophilic reactions of linear nitrosyl groups will not be considered. 

The structure of the catalysts was characterized by X-Ray diffraction (Philips PW 1710) and infrared spectroscopy (Perkin Elmer 580). The nature and coordination of transition metal cations was studied by U.V-Vis (Perkin Elmer Lambda 9) and EPR (Varian E9, X-Band mode) spectroscopies. 

A number of transition-metal complexes of RNSO ligands have been structurally characterized. Three bonding modes, r(A,5), o-(5)-trigonal and o (5 )-pyramidal, have been observed (Scheme 9.1). Side-on (N,S) coordination is favoured by electron-rich (et or j °) metal centers, while the ff(S)-trigonal mode is preferred for less electron-rich metal centres (or those with competitive strong r-acid co-ligands). As expected ti (N,S)... 

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

The combination of hard (A) and soft (5) coordination in the 1,5-P2N4S2 ring system leads to a diversity of coordination modes in complexes with transition metals (Lig. 13.1). In some cases these complexes may be prepared by the reaction of the dianion [Ph4P2N4S2] with a metal halide complex, but these reactions frequently result in redox to regenerate 13.3 (L = S, R = Ph). A more versatile approach is the oxidative addition of the neutral ligand 13.3 (L = S) to the metal centre. 

The coordination chemistry of SO2 has been extensively studied during the past two decades and at least 9 different bonding modes have been established.These are illustrated schematically in Fig. 15.26 and typical examples are given in Table 15.17.1 It is clear that nearly all the transition-metal complexes involve the metals in oxidation state zero or -bl. Moreover, SO2 in the pyramidal >7 -dusters tends to be reversibly bound (being eliminated when... 

---