Delocalized coordinates

Amongst the complexes with delocalized coordination bonds, the sandwich structures of aromatic (3, 4) [1,2,7,10] and heteroaromatic (5, 6) [11-14] series are widely represented ... 

On the basis of Cp and its heteroanalogues, a wide series of metallocene structures of types 3, 5, 126, 127 [2-5,7,8,13,132,133,192-205] have been obtained. These complexes are typical for Fe, Co, Ni, Mn, V, Cr, and Ru, and are the standard examples of complex compounds with multicenter delocalized coordination bonds formed as a result of combined grouping orbitals of the cyclopentadienyl rings with rf-metal orbitals. 

Numerous complexes with n-delocalized coordination bonds are formed from pen-tadienyl, benzyl, and organic biradicals [224a], Thus, a considerable number of complexes have been synthesized where a transition metal is bound to a chain formed by five unsaturated carbon atoms, united by the name pentadienyr 138 ... 

If a hapto bond is formed, the metallic center is included in the delocalized coordination system, electrons and charge are now spread over the metallic center and the hydrocarbon system. Corresponding diagonal functions must be used. 

Table B3.5.1 Number of cycles to converge for geometry optimizations of some typical organic molecules usmg Cartesian, Z-matrix and delocalized internal coordinates. ...

By combining the Lagrange multiplier method with the highly efficient delocalized internal coordinates, a very powerfiil algoritlun for constrained optimization has been developed [ ]. Given that delocalized internal coordinates are potentially linear combinations of all possible primitive stretches, bends and torsions in the system, cf Z-matrix coordinates which are individual primitives, it would seem very difficult to impose any constraints at all however, as... 

Baker J, KInghorn D and Pulay P 1999 Geometry optimization In delocalized Internal coordinates An efficient quadratically scaling algorithm for large molecules J. Chem. Phys. 110 4986... 

Baker J, KessI A and Delley B 1996 The generation and use of delocalized Internal coordinates In geometry optimization J. Chem. Phys. 105 192... 

Baker J 1997 Constrained optimization in delocalized internal coordinates J. Comput. Chem. 18 1079... 

Ferrocene (Figure 2-61a) has already been mentioned as a prime example of multi-haptic bonds, i.c, the electrons tlrat coordinate tire cyclopcntadicnyl rings with the iron atom are contained in a molecular orbital delocalized over all 11 atom centers [811, for w hich representation by a connection table having bonds between the iron atom and the five carbon atoms of cither cyclopcntadicnyl ring is totally inadequate. 

In the monomeric structure of 10.21 the solvation of both Li" ions by two THF molecules prevents further aggregation. The four S-N bond lengths are equal at ca. 1.60 A indicating that the negative charge is delocalized over the S(N Bu)4 unit. In the dimer 10.22 one [McS(N Bu)3] anion is coordinated to both Li" ions, one of which is bis-solvated by THF, while the other is chelated only to the unsolvated Li" ion. 

Cp(CO)2Re(THF) forms the complex 105 upon reaction with thiophene (89JA8753, 910M2436). Similar species are known for 2- and 3-methyl-, 2,5-dimethyl, and tetramethylthiophene (91IC1417). Thiophene in 105 is S-coordi-nated, and the sulfur atom is pyramidal. Treatment of 105 with Fc2(CO)9 produces 106, where the thiophene ligand is bridge-coordinated via the sulfur atom to rhenium and four carbon atoms of the dienic system with iron (the coordination mode). The pyramidal nature of the sulfur atom is preserved. The -coordination of thiophene separates the dienic and sulfur counterparts of the ligand and decreases the TT-electron delocalization, which leads to the enhanced basicity of the sulfur atom. 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

---