Ions, tris-bidentate complex

The circular dichroism spectra in the region of the first absorption band of the tris-bidentate complex ions having six-membered chelate rings sure known to be particularly sensitive to experimental conditions. For example, the CD spectrum of A-lel -[Co(R,R-ptn) ]C1, in an aqueous solution shows two peaks A = -O.589, 522 nm A = - 0.104, 462.5 nm, whereas that of A-lel,-[Co(R,R-ptn) ](CIO,) in an aqueous solution gives a negative peak (A , = -O.587T at 518 nm (41). The solid state CD differ from the solution CD and the solution CD are sensitive to the temperature of measurement and are affected by the presence of oxo anions (42, 43, 44). Table V lists the lowest frequency CD spectra of tris-diamine cobalt(III) complexes in the CT region. 

Bidentate oxygen ligands form numerous rhodium(III) complexes. Several tris(jS-diketonato) complexes have been prepared from rhodium(III) nitrate (equation 35). The products are extraordinarily stable. They can be resolved into their optical isomers, and survive nitration and formylation reactions (equations 36 and 37). The tris(oxalato)rhodate(III) ion has also been resolved, but the enantiomers undergo slow racemization. Reaction of this complex with refluxing chloric(VII) acid leads to m-[Rh(ox)2(H20)2], which can be converted into cis- or frani -[RhX2(ox)2] complexes. 

The number of monodentate groups such as hydroxide, halide, water molecule, etc., bonded to a central metal ion, usually equates to its coordination number. However, a remarkable feature occurs if two or more of these ligand donor groups are united, possibly by a short chain of hydrocarbons there will be a markedly enhanced preference for forming such bidentate complexes with the transition metal ion, and indeed in the ultimate, it is possible to go through tri- to tetra- to penta-to hexadentate chelate ligands such as EDTA (ethylenediaminetetra-acetic acid) which is widely described in this book. 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

Complexes of lanthanide chlorides 156,173), bromides (256), and iodides 174) with 2,6-DMePyO have also been prepared and characterized. The presence of bridging 2,6- DMePyO molecules has been suggested in the complexes of lanthanide iodides. Vicentini and De Oliveira (2 73) have reported tetrakis-2,6-DMePyO complexes with lanthanide nitrates. However, by changing the method of synthesis, tris-2,6-DMePyO complexes with the lanthanide nitrates could be prepared in this laboratory (252). All the nitrate groups in the tris-2,6-DMePyO complexes are bidentate. In the 2,4,6-TMePyO complexes (252) also the nitrate groups are coordinated to the lanthanide ion in a bidentate fashion. 

---