Ligand held states

Within an octahedral E term, two parameters are required to describe the splitting of the electronic states due to a low symmetry ligand held,... 

Simple ligand-held arguments, which will be elaborated when M ions of the Ni, IM, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinatc, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation. state and the requirements of the 18-eiectron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j+, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Ir are predominantly square planar, although 5-coordination does also occur. 

CrOJ-) reasonably stable the Cr(l) oxidaiion stale is practically unknown. For both Cu2 + and Cr3 + (as well as many other transition metal ions) ligand held effects in their complexes (see Chapter 11) are much more important in determining stable oxidation states than are electron configurations. 

A ligand-held interpretation of the quantum yields of photosubstitution reactions of metal complexes has appeared. Similarly, a molecular orbital approach, based on the relative changes in metal-ligand overlap populations between ground and excited states, has been described to determine the relative ligand photolabilizations in the excited states of co-ordination compounds. The results complement the predictions of Adamson s qualitative rules. 

Another, indirect, influence of dd-states on photophysics and photoreactivity is through thermal population of a dd-state from a lower-lying excited state. The formation of dd-states is associated with the large structural distortions due to the Jahn-Teller effect [25]. Consequently, these states provide an eflftcient channel for non-radiative decay, leading to extremely short excited state hfetimes. For example, thermal population of dd-states is held to be responsible for the lack of emission and extremely short (sub-nanoseconds) excited state hfetimes for the lowest metal-to-ligand or ligand-to-ligand excited states (see below) in Pt(diimine)L2 complexes (L = Hal, PhS or RO ). 

FIGURE 8.14 Activation of nonreceptor PTKs. Lck is held in an inaccessible compact state through phosphorylation of residue Y505, which interacts with the intramolecular SH2 domain. Upon activation of CD45 (ligand unknown), the pY505 is removed and the molecule opens up. Autophosphorylation at theY-394 residue in the catalytic domain constitutes the activation of the PTK. 

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