Square planar/octahedral interconversions

The octahedral-square planar interconversion of the nickel(n) complex of 1,4,8,11-tetra-azaundecane (2,3,2-tet) has been studied in water. The mechanism is assumed to be a two-step one [equations (30) and (31), in which L represents 2,3,2-tet]. On this basis and applying the steady-state approximation to the intermediate [NiL(H20)] + the authors calculate a value of... 

The Ni(II) complexes l-3g with hexaaza and pentaaza macrocyclic ligands as well as the Ni(II) cyclam complex display a low-spin square-planar and high-spin octahedral complex interconversion in aqueous solution (7, 12-14). However, complexes l-3a favor formation of the high-spin form upon addition of acid, which is in contrast to the cyclam complex. The protonation of tertiary nitrogen atoms at the bridgehead position facilitates the axial coordination of anion or water because of hydrogen-bonding between them 12, 14a). 

It has been reported that interconversion of square-planar to octahedral species for the Ni(II) complexes of the lipophilic macrocycle L-a... 

Tetraaza macrocycles usually coordinate with the N4 donor set approximately coplanar for square planar or trans octahedral arrangements. The more flexible macrocycles can also coordinate in folded arrangements, usually to accommodate an additional chelate ligand in an octahedral arrangement, and planar-folded interconversions are facile. In planar coordination some degree of tetrahedral distortion of the donor set is common, and this can become substantial for larger ring... 

Isomerization of transition metal complexes is frequently observed to occur for octahedral d6 or square-planar d8 systems. Although the interconversions between isomers are commonly achieved thermally, they are also induced photochemically or by oxidation/reduction. In the latter case, the thermodynamically favored isomeric distribution is dependent upon the oxidation state of the metal center of the complex. 

The maintenance of an octahedral geometry within the RhFe3 (PPh)2(C5 Me5)(CO)8/R.hFe3(PPh)2(C5 Me5)(CO)9 interconversion, formally caused by a two-electron addition/removal, can be understood if one considers that, in these complexes, the electrons enter or leave nonbonding frontier orbitals centred on the square-planar tetrametal fragment stabilized by the interaction with the apical phosphorus atoms [89]. 

Some structures are complicated by interconversions between square planar and tetrahedral, or square planar and octahedral coordination as we discuss later. In addition, the potential of some ligands to bridge between metal centres may cause ambiguity. For example, alkali metal salts of [NiF3] , [NiF4] and [NiCl3] crystallize with extended... 

Some macrocyclic Ni(II) complexes exist as equilibrium mixtures of the yellow, diamagnetic, square-planar [Ni(L)] and blue (or violet), paramagnetic, octahedral [Ni(L)(S)2] species in coordinating solvents (S) such as water, acetonitrile, or Me2SO (7, 10, 12-14, 16, 65, 66). Interconversion between these two forms depends on the ligand structure and reflects a balance between endothermic solvent coordination and exothermic Ni—N bond lengthening in octahedral species. 

Three relaxation processes observed during the Q-switched laser photolysis of dibromo-[l,3-bis(diphenylphosphino)propane]nickel(ii) in acetonitrile have been assigned " to the relaxation of coupled equilibria involving planar and tetrahedral isomers of the complex together with ion pairs and free ions. The kinetics of the rapid square-planar-octahedral interconversion accompanying the addition of two unidentate ligands L to (14) in chlorobenzene are consistent with a two-step mechanism,... 

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