Hello Victor, your system seems very interesting indeed. With 11 lines you refer to the fine structure of the dimer, or some hyperfine structure? Because the hyperfine pattern for a Co(II) dimer should be composed of 15 lines in a 1:2:3:4...7:8:7:6...2:1 intensity pattern, but I haven´t been able to find one single example of an EPR spectrum as this anywhere. I observed some dimer features in a Co(II) doped Zn(II) hosts, but there was a mixture of monomer and dimer signals and therefore not all the dimer resonances could be seen.

The broadening could be due to the enhancement of the already fast relaxation rate of Co(II) ions by the interaction with each other.

Could I see some of the spectra? Perhaps there is something else I can add.

I was calculating multiplicity of two I: 7/2 coupled

hyperfine pattern for a Co(II) dimer should be composed of 15 lines in a 1:2:3:4...7:8:7:6...2:1 intensity pattern.

Kinda hard to decide the intensity because I am deconvoluting the spectrum.

At 20H the lines at higher field are broader (Intensity you mean hight?) and smaller.

Anyhow, I get a RT spectrum with two broad bands. I think I better send you the pictures. You don't happend to have Quattro (Corel)?

I don't have Corel, if you can export them as jpg or something similar it's better.

The room temperature spectra puzzles me. As far as I know high spin cobalt(ii) cannot be measured at temperatures higher than 40-60 K, due to very fast relaxation times, but then again the number of not yet measured compounds is infinite, so maybe it is possible.

The intensity pattern would be proportional to the resonance heights if all the lines had the same linewidth, but usually due to strains and sometimes even quadrupolar interaction not all the hyperfine lines have the same linewidth and so some could be observable and others barely.

Enclosed are the three spectra I love most, father (in CH2Cl2 at RT), son (Co2RCOO5  at 3.8K) and the holly crap (Co2RCOO5 at RT). The sharp line is DPPH.

They are in Xcell format (export them)

I've seen the spectra.

Here are is what I think (of course I can be wrong in any of these)

The first one puzzles me. At room temperature a high spin Co(II) signal should not be observable. I've taken EPR spectra of low spin Co(II) corroles, and in some cases they show really large g- and A- values (for a low spin Co(II)) that look similar to that spectrum. It is possible that the many lines between ~1800 and 4000 G are partially resolved hyperfine structure.

The third one is really broad, but could perfectly arise from a Co(II) dimer.

The second one with the hyperfina pattern, is very narrow and is centered very near g = 2, so it is probably not Co(II) centered. It seems a radical, but the hyperfine splitting with many lines could be compatible with two I = 7/2 Cobalt nuclei. Actually in the same kind of compounds in which I saw very large g- and A- Co(II) values, I also had a fraction of Co(III)O2*- (superoxo) which comes from reaction of Co(II) with O2. This guy has an anisotropic spectrum with 8 hyperfine lines but g values typical of C,O,N centered radicals. It actually has been observed many times. So it is possible that a small amount of O2 binds to two cobalts (or maybe some ligand can become a radical), giving rise to a 15 line pattern. In the spectrum 11 lines are clearly seen, but actually there are a few more. It is easy to smooth that spectrum to see better the resonance lines, and also if integrated the S/N ratio will be better.

I don't know the chemistry of your complex. Do you think that some of the things I suggested could happen? Is it also possible that there is a high-spin to low spin transition at some temperature? or that there are populations of high spin and low spin cobalt ions in your sample?

I hope some of this is useful.

Best regards

Included is the 4K spectrum, solid. I fitted it to 0,95 Gaussian + 0.05 Lorentzian, see picture.

First peak at 4K (g: 1.98) second peak (g: 7.0), enclosed picture.

Multiplicity at 20K, could arise from S.I coupling,  (Co)  I: 7/2.

The point group of each Co(II) is distorted Td with a possible coordination of a second O, see enclosed picture.

The Co-. is actually what forms initially in CH2Cl2, very sharp lines. The Co-, anion radical are d10 species, very stable then. I enclose the XRD (solid), see Co2(AcO)5 XRD simplified structure. We know that they are Co(II). 

Air is carefully excluded during synthesis and crystallization because the (CO)9Co3C.COO- is sensitive to moisture (Co-CO + H2O = Co(COOH)...) but Proton Sponge may be able to stabilize a radical. The initial radical species observed in CH2Cl2 comes from (CO)9Co3C-COO which dispropotionates into Co(II) and (CO)6Co2C2-COO. These are possible radical species in solution, with sharp EPR lines in solution, fast relaxation rate. Lines are represented by spin-spin coupling. 

At room temperature (solid) the spectra is a single broad band with a Voigt function shape. The DC susceptibility is that of an ANTIFERRO-magnetic exchange coupled dimer. A rapid lattice relaxation  is in order at low temperature (sharp because). No lattice relaxation of course. At low T the bands are broad, being the low field the less broader one. I guess that means a high degree of anisotropy. I can only see 9 lines. I wonder if this is the characteristic low field octet of a single Co(II) ion overlaped with the high field signal. I don't have access to this journal but it looks important Co(II)-CO

mu at (4700 Gauss) does NOT decrease until 4K. It remains constant all the way up to 298K. My Japanese theoretician just asked me if it was a dipole or it was a Single Molecule Magnet? I don't know, yet. Before measuring Xi,I ran an EPR spectra up to 6000Gauss at 298K. Then I ran further spectra at 280K, 20K, 8K and 4K. Sample could have been magnetized. SMM. Sorry, I have never been good at History lessons.