Five-coordinate Co complex

Crystallographic studies of transition metal hydride complexes Stereochemistry of six-coordination Five-coordinate structures Stereochemistry of five-coordinate Co complexes Absolute stereochemistry of chelate complexes Stereochemistry of optically-active transition metal complexes Electron density distributions in inorganic compounds... 

Figure 24 A five-coordinate Co complex of an N3S2 ligand, a proposed model for the active site of nitrile hydratase...

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

Carbonylation of solutions of K3[CoH(CN)s] yields [Co(CN)3(CO)2] , however attempts to isolate pure salts of this species have been unsuccessful owing to its tendency to decompose and disproportionate. On treatment of the product solutions with phosphines moderately stable five-coordinate Co complexes of the type [Co(CN)(CO)2(PR3)2], [Co(CN)2(CO)(PR3)2] and [Co(CN)2(CO)2 PR3)] are obtained (Scheme 1). An alternative route to [Co(CN)(CO)2(PR3)2] complexes (PR3 = PEt2Ph, PEtPh2, PPh3) is provide by direct carbonylation of [Co(CN)2(PR3)2]. In C2H4CI2 solution addition of CO gives [Co(CN)2(CO)(PR3)2] initially, which is then reduced by CO to the cobalt(I) product. ... 

Costa and co-workers have compared the effect of different equatorial ligands on the relative stability of the five- and six-coordinate forms by studying the tendency of the five-coordinate acetylcobalt complexes to bind... 

The bi- and tridentate phosphines dppp (l,3-bis(diphenylphosphino)propane) and dppep (bis (2-diphenylphosphinoethyl)phenylphosphine) have been complexed with Co and their divalent four- and five-coordinated thiophenolate complexes Co(dppp)(SPh)2 and Co(dppep)(SPh)2 have been isolated and structurally characterized.379 Somewhat related to dppp is the bidentate silane Ph2PCH2SiMe2CH2PPh2, which forms high-spin, pseudo-tetrahedral dihalocobalt(II) complexes.380... 

The P-N chelate (91) (dapdmp) exhibits a variety of coordination geometries in complexes with divalent Co. Pseudotetrahedral Co(dapdmp)X2 (X = C1, Br, I, NCS), low-spin five-coordinate [Co(dapdmp)2X]+ (X = C1, Br, I), planar [Co(dapdmp)2]2+ and pseudo-octahedral Co(dapdmp) (N03)2 were all identified.390 The tetradentate P2N2 Schiff base complex (92) is formed by reacting the free ligand with CoI2. The iodo complex is low spin and square pyramidal.391... 

The yellow Co(II)-cyanide solid complex contains square-pyramidal five-coordinated Co(CN). The green ion in solution is possibly Co(CN)5H20 with very weak H2O coordination in the sixth axial position. It is an important inner-sphere reductant (Prob. 8 of Chap. 5, and Tables 5.3 and 5.7). Generally... 

The reactions of butatrienylidene iridium complexes with CO depend strongly on the trans ligand. The trans chloro complex 11 reversibly adds CO to form a five-coordinate butatrienylidene complex 31 whereas the trans azido complex yields with CO an alkenyl(azido)ethynyl complex (32) and the trans methyl complex the alky-nylalkenyl complex 33 (Scheme 3.31) [3]. 

Com corroles form five- and six-coordinate complexes in the presence of ligands but the second association constant is much lower than the first. A five-coordinate triphenylphosphine complex was isolated and shown to have a distorted square pyramidal structure with the Co atom displaced out of the N4 plane by 0.28 A.245 In pyridine, it is converted to a diamagnetic bis-pyridine complex.239 Isocyanides displace pyridine ligands to form five-coordinate complexes, from which amine complexes may be obtained (Scheme 78). 

The five-coordinate complex then reacts rapidly with external CO to give (CO)5MnCOCH3. Thermodynamically, the progress of the reaction to completion requires a ligand (not necessarily CO PPh3 will do) which can react quickly with the five-coordinate acyl complex to remove it from the system, so that more will be formed by rearrangement of Mn(CO)5CH3. The mechanism for the intramolecular carbonyl insertion is presumably a concerted process ... 

Hydrazine reduction of [Co(CN)2(dpe)2] affords the air-sensitive five-coordinate complex [Co(CN)(dpe)3].19 This process involves displacement of coordinated cyanide ion accompanied by chelation of the dangling dpe ligand. One diphosphine ligand in the product is readily displaced by CO giving five-coordinate [Co(CN)(CO)2(dpe)]. 

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