Dithiolenes cobalt complexes

Clearly, from inspection of Table 4.14, there is a good correlation between the steric bulk of R and L and the non-coincidence angle a. Furthermore, analysis of the hyperfine parameters leads to the conclusion that only about 25% of the electron spin resides in Co orbitals (mainly dxz), and crystal structures of the R = CF3, L = PPh3 and P(OPh)3 complexes do indeed show distortions. The difference between iron and cobalt is just one electron, but this electron occupies a dithiolene 7i orbital, which makes the cobalt complexes much more easily distorted. 

We have already mentioned a very strong dyadic association in the formally d5 cobalt complexes such as [Cp Co(dddt)]+ which dimerizes in the solid state to a fully diamagnetic dicationic dyad (Fig. 6a). It represents the extreme situation where the two radicals form a true 2e bond, with the sulfur atom of one dithiolene ligand entering the coordination sphere of the other metal. It should be considered as the consequence of the electron deficiency of these cationic [CpCo(dt)]+ 15-electron complexes. 

Redox potentials for a group of selected bis(dithiolene) complexes are listed in Table II. Neutral iron and cobalt bis(dithiolene) complexes exist in the dimeric form. The dimers stay intact when partially reduced and dissociate into monomers when fully reduced (34, 35). The potentials listed in Table II for Fe and Co complexes are therefore for the redox couples (0/—1, — 1/—2, etc.) of the dimer (Eq. 2). 

Redox Potential for Iron and Cobalt Nitrosyl Dithiolene Complexes, [M(NO)(S—S)2]z ... 

Garner and Joule next adapted the protected dithiolene strategy to target heterocyclic dithiolene complexes. This approach, first reported in 1988 by Larsen et al is illustrated in Scheme 2.25 with the formation of a cobalt quinoxalyldithiolene complex. Reaction of [CoCp(COD)] (COD = 1,5-cyclooc-tadiene) with 4-(quinoxalin-2-yl)-l,3-dithiole-2-thione (26) affords a quinox-aline dithiolene complex (27) that has been structurally characterized. This cobalt complex undergoes extensive proton-coupled electron transfer process (see Section 2.3.4). This system was further elaborated when the pyrazine ring in 4-(quinoxalin-2-yl)-l,3-dithiole-2-thione 26 was selectively... 

Scheme 2.25 Synthesis of cobalt complex coordinated by a single dithiolene ligand.

Formation constants for the adducts of amines to bi (trifluoromethyl)dithiolen cobalt complexes have been reported. ... 

Figure 4.13 Simulation of ESR spectrum of the above cobalt dithiolene complex in frozen toluene at 77 K.

The reduction behaviour of the alkylidene adduct of a cobalt-dithiolene complex (423) has been examined548 and the study has shown that, when the alkylidene-bridged structure (423) is reduced by one electron, it isomerizes rapidly and quantitatively to the ylide form (424). This represents the first example of reversible isomerization of the metal-carbon bond in a cobaltadithiolene complex. A surprising cis- to tra .s-dihydride isomerization which is unprecedented for 18-electron six-coordinate complexes has been observed549 in an octahedral iridium-c7.y-di hydride complex. 

Dance, I. G., R. C. Conrad, and J. E. Cline Mechanism of Cobalt Dithiolene Complex Catalysis of Thiol Autoxidation in Acidic Acetonitrile Solution. Chem. Commun. 1974, 13. 

The rapidity of substitution reactions at a metal surrounded by a porphyrin or corrin group seems to be connected with r-delocalization and a strong in-plane ligand field which is not present in normal complexes. The substitution reactions (4) of five-co-ordinate neutral dithiolen complexes (where M is iron or cobalt and L and [M(S2CaPh2)2X] + L—> [M(S2C2Ph2)2L] + X (4)... 

McNamara WR, Han Z, Alperin PJ, Brennessel WW, Holland PL, Eisenberg R. A cobalt-dithiolene complex for the photocatalytic and electrocatalytic reduction of protons. J Am Chem Soc 2011 133 15368-15371. 

Eaton GR, Holm RH (1971) Bridged binuclear bis-dithiolene complexes of iron and cobalt. Inorg Chem 10 805-811... 

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