As it is known, Chromium trivalent has d3 electronic configurations which make its reactions to take place with slow rates ( Inert kinetics)
But GTF which is a Cr(III) complex, activates insulin in vivo readily (labile).
It is important to state here that Cr(III) is generally not very labile IF the geometry around the metal is octahedral (that means six-coordinate). If the Cr(III) is tetrahedral for example then the complex is labile. In biological systems some metals change coordination number during the enzymatic cycle. In many cases, this change activates them allow them to bind a substrate, myoglobin is an example. In it's resting state the Fe metal is 5-coordinate allowing it bind O2.
As it is reported in literature, the geometry of Cr(III) is octahedral... even for the GTF in vivo ... it is octahedral
Yes but even a small distortion from octahedral geometry would lead to orbital splitting and a more labile environment. You must remember that metal complexes are not static, especially in vivo. Water for example often comes on and off fairly rapidly. You should also remember that the term labile is relative. Even 'non-labile' Cr(III) is far more labile than Ru(II).
Exactly, and there is also trans effect for some electron pushing ligands on the opposite group. Well, the term labile as know is used to the kinetics of reactions that proceeds in less than 1 min. More than 1 min is said to be inert
Cr(en)3 is much more labile than Cr(pic)3,even they are octahedral. this has relationship with the ligand. The stability is influenced by the size of chelate ring.
The five-member ligands have bigger chelate effect than the sixmember ones. But when the chelate ring contains a double bond (unsaturated), the six-member metal complex is stable.
Additionally, one should keep the famous textbook example in mind: CrCl3 is completely insoluble in water. You can dissolve it by adding traces of a reductant (such as Zn powder). The small fraction of Cr(II) on the surface of the CrCl3 solid will now easily dissolve into the solution but at the same time these soluble Cr(II) species are reductants to the CrCl3 surface. At the end of the story CrCl3 has dissolved yielding the Cr(III) aqua complex [Cr(H2O)6]3+. So, traces of Cr(II) might be responsible for fast kinetics of Cr(III) complexes. The redoxpotentials Cr3+/2+ often lie in the physiological range.
How about Cr (VI)?
Is this form of Chromium also kinetically inert?
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