Hexacoordinated compounds, theoretical

In connection with the use of the hypervalent concept as an explanation of mutual cw/fraw5-influence in penta- and hexacoordinated compounds of group 14 elements, the experimental and theoretical works and references cited therein should be consulted. 

Studies of hypervalent phosphorus compounds in biological phosphoryl transfer reactions include the preparation of novel anti-apicophilic penta-coordinated phosphoranes with frozen stereomutation using bulky biden-tate ligands. Kinetic studies have enabled the activation enthalpy of the stereomutation of an 0-equatorial phosphorane to its O-apical stereoisomer to be calculated. The involvement of hexacoordinated phosphoranes in phosphate transfer reactions has been clarified by theoretical calculations as well as in vitro studies. Applications of hexacoordinated compounds as catalysts have also been described. 

The electronic ground state that a particular metal center adopts is a function of the chromophore. In many cases the ground state may be derived from chemical knowledge (e. g., octahedral cobalt(HI) ( Aig) or tetrahedral Ni(II) (3T() complexes). However, based on the molecular mechanics formalism alone, this problem cannot be solved in a general way. Let us consider coordination compounds which are close to the spin-crossover limit (for example hexacoordinate iron(II) (1Aig/5T2g)). In these cases it is not possible to assign the atom type of the metal center without further information (experimental or theoretical). Therefore, molecular mechanics alone is not always able to predict the structural properties. 

The H chemical shifts of phthalocyanine and porphyrine complexes with hexacoordinate siliconas well as theoretical calculations of the contribution to H magnetic shieldingshow a high degree of aromatic nature of the macrocycle compounds of this type can even be used as diamagnetic shift reagents... 

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