Chromium complexes bipyridyls

Carbonyl and other Low Oxidation State Compounds.—Tris-(2,2 -bipyridyl)vana-dium(O) solutions in DMF exhibit three one-electron reduction waves corresponding to VL3 - VL3 (L = 2,2 -bipyridyI) VLJ -> VL " and VLf -VL, and two oxidation waves corresponding to VL3-> VL3 and VLJ ->VL3+ whose half-wave potentials differ by only 0.10 V.342 The difference for the isoelectronic chromium complexes is 0.49 V. An explanation has been offered for the small difference between the vanadium potentials in terms ol the possible diamagnetism of the d4VL3 species arising from a substantial splitting of the l2g orbitals. 

Since complexes of 2,2 -bipyridyl and 1,10-phenanthroline with chromium in oxidation states I and 0 can be obtained by reduction (Scheme 64) of the chromium(n) complexes, these oxidation states will be considered together. Oxidation, as shown in Schemes 65 and 68 of Section 35.4.2.5, gives chromium(III) complexes, which are often best prepared in this way. Earlier work has been extensively reviewed, and few complexes of 2,2 6, 2"-terpyridyl are known.32 A chromium(I) phthalocyanine derivative is mentioned in Section 35.4.9.3. 

Chromium(II) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolyl-amine) and 8-aminoquinoline (amq), which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).103 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(II) systems was unsuccessful ([Cr(en)3]X2 are high-spin and [Cr(bipy)3]X3 and [CrX2(bipy)2] low-spin). 

Mixed dinuclear complexes of tartrates and 2,2 -bipyridyl or 1,10-phenanthroline and chromium(III) have been prepared 934,935 the typical structure is illustrated below (213). Stereochemical correlations have been carried out by oxidatively cleaving the tartrate bridge (214,215).936 The crystal structure of sodium hydrogen bis(//-meso-tartrato)bis(2,2 -bipyridyl)dichromate(III) heptahydrate has been reported.937... 

Chromium(III) does become seven-coordinate in two complexes in which the organic ligands are formed by condensation respectively of 2,6-diacetylpyridine and semicarbazide,1125 and 2,6-pyridinedialdehyde and 6,6 -bis(or-2-hydroxyethylhydrazino)-2 2 -bipyridyl.550 Seven-coordinate complexes are formed by chromium in several oxidation states (Table 95). 

Complexation of chromium(III) by nitrilotriacetic add (H2nta) has been investigated. Values of equilibrium constants (/ = 0, 25 °C) are log K = 10.66 and log K2 — 8.73.1234 A large number of mixed complexes with nta/chromium(III) have been prepared, including mixed aminoad-dates,1235 acetylacetonates, catecholates, oxalates and complexes with 1,10-phenanthroline and 2,2 -bipyridyl.1236 1238... 

The reaction between chromium(II) acetate and bipyridyl has been extensively studied. If a metal-to-ligand ratio of 2 3 is employed, disproportionation occurs and Cr(bipy)3 is the product (359). A 1 2 ratio of reactants affords the neutral monomeric complex Cr(bipy)2(OAc)2... 

The polarographic reduction of chromium bipyridyl complexes has been investigated frequently 26,420.693,694, 707). At 25°C and pH = 4, the initial processes involved in the reduction of Cr(bipy)3 are believed to be 25)... 

The peroxo-complexes provide further examples of the ability of oxygen to coordinate to the metals in their high oxidation states. The production of blue solutions when acidified dichromates are treated with H2O2 is a qualitative test for chromium. The colour arises from the unstable CrO. which can, however, be stabilized by extraction into ether, and blue solid adducts such as tCrOs(py)] can be isolated. This is more correctly formulated as lCr0(02>2py] and has an approximately pentagonal pyramidal structure (Fig. 23.6a). Bidenlate ligands, such as phenanlhroline and bipyridyl produce pentagonal bipyramidal complexes in which the second N-donor atom is loosely bonded irons to the =0 (Fig. 23.6b). This 7-coordinate structure is favoured in numerous peroxo-complexes of Mo and W, and the dark-red peroxo anion [Mo(02)4] is 8-coordinate, with Mo-O 197 pm and 0-0 155 pm. 

Chromium(I) complexes, 702 alkyl isocyanides, 704-709 aryl isocyanides, 704-709 2,2 -bipyridyl, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 cyanides, 703 fluorophosphine, 716 isocyanides crystallography, 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 1,10-phenanthroline, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 tertiary phosphines dinitrogen, 713 tris(bipyridyl)... 

---