Weak field ligand

The simplest iron-sulfur centers, which were first discovered in ru-bredoxins, consist of one iron ion coordinated by a distorted tetrahedron of cysteinyl sulfur atoms. This environment provides a weak ligand field giving a spin equal to and 2 when the ion is Fe(III) and Fe(II), respectively. It also determines the splitting of the ground spin manifold, and consequently the characteristics of the EPR spectrum. This splitting is generally described in the framework of the spin Hamiltonian ... 

As a consequence of the weak ligand field of (L19)2-, the Mn11, Fe11, Co11, and Ni11 ions in the acetato-bridged complexes 61-63... 

No Ligand Field Weak Ligand Field Strong Ligand Field... 

We observed that the d—d absorption maxima of the DBQP-Cu chelates were red-shifted from 660 nm toward 700 nm with C%, which could be caused by the relatively weak ligand field introduced by the crosslinking46 The ESR spectra of DBQP-Cu are shown in Fig. 1246 The parameters,gj = 2.3,g = 2.1 and... 

The size of the macrocycle coordination hole has a profound influence on its coordination chemistry thus in the weak ligand field of hydroporphyrinoid systems Ni2 + assumes the high spin state and binds extra axial ligands, while in the stronger ligand field of the corrin macrocycle the coordination sphere of the metal is saturated and diamagnetic complexes are generated [82]. 

Weak Ligand Field (Spin Free) Strong Ligand Field (Spin Paired) ... 

Figure 2. Excited-state spectral features ofD -CuCl/-. A Energy level diagram showing the ligand-field (d - d) and charge-transfer (CT) optical transitions. The intensity of the transitions is approximated by the thickness of the arrow with the very weak ligand-field transitions represented as a dotted arrow. B Electronic absorption spectrum for D4h-CuCl42 (12). C Schematic of the a and tt bonding modes between the Cu 3dx2 y2 and Cl 3p orbitals.

Catalase clearly remains high-spin to a more marked degree than any other haem-protein, for example the cyanide is about 50% high-spin, and this observation too is in keeping with a weak ligand field group in the fifth coordination position. 

Contributions to Ag can arise from many atoms in the molecule. Studies of minerals commonly involve metal ions in relatively weak ligand fields, where the major contribution is from the metal. The calculation of Ag in this case involves the Hamiltonian ... 

Electron configuration High-spin complexes (weak ligand field) Low-spin complexes (strong ligand field)... 

SO there is no difference in the net spin of the electrons for strong- and weak-field cases. On the other hand, the through ions exhibit high-spin and low-spin states, as shown in Table 10-5. Strong ligand fields lead to low-spin complexes, and weak ligand fields lead to high-spin complexes. 

Strong ligand field = large = low spin Weak ligand field = small A = high spin... 

The Cu complex ions CuCl4 and CuBr42- have distorted tetrahedral structures. In the weak ligand field exerted by the halogen ions (p. 134), the metal has four electrons in d orbitals and only five in the three d orbitals which point in the direction of the ligands the ligands are thus able to approach more closely and form stronger bonds. 

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