Weak field complexes

Fe( H20)6](N03)2, containing the weak field complex thus should be green, because the weak field would result in the absorption of light of long wavelength, namely, red light. 

The addition of a fifth electron to a weak field complex gives a configuration t e2 and a CFSE of zero. The two electrons in the unfavorable eg level exactly balance the stabilization associated with three in the t2j/ level (Fig. 11.9a). 

Since [Ni(H20)6] is a weak field complex, the low-lying e m.o.s occupied by two impaired electrons contribute to stabilization (Hund s rule). 

Weak-field complexes have small A values and absorb low-energy red or infrared) radiation. 

Octahedral complexes should be used. Cr [Ar]3high-spin (weak-field) complexes have four unpaired electrons and lo -spin (strong-field) Cr + complexes have two unpaired electrons. Ni [Ar]3unpaired electrons, whether high or low spin. Therefore, complexes cannot be used to distinguish weak- from strong-... 

There are also some complexes with more than 18 electrons, such as [Ni(H20)6] which possesses 20 electrons ( 1.1.2.1). Some antibond-ing MO must therefore be occupied, which can happen only if they are sufficiently low in energy as is the case in weak-field complexes. Organometallic complexes, characterized by the presence of one or several metal-carbon bonds, are strong-field complexes. It is therefore rare for them to possess more than 18 electrons. 

For each of the following metals, write the electronic configuration of the atom and its 3-H ion (a) Ru, (b) Mo, (c) Co. Draw the crystal-field energy-level diagram for the d orbitals of an octahedral complex, and show the placement of the d electrons for each 3+ ion, assuming a weak-field complex. How many unpaired electrons are there in each case ... 

If talking in terms of fractions of electrons seems strange, remember that these fractions refer to the probability distribution of the electron density. Physically, because electron repulsion is larger than the crystal field in weak field complexes, this repulsion forces some electron density into the orbitals. 

Fig. 7.23 A modified Orgel diagram for weak field complexes with free ion D terms as ground state (d, d", d and d ).

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