Cobalt complexes bond lengths

Molecular structures of the complexes (molecules A and B) and coordination about the cobalt ion in molecules A and B of the mugeneic acid-Co(III) complex. Bond lengths in A angles in degrees. See Reference 42. 

Bond length differences between HS and LS isomers have been determined for a number of iron(II), iron(III) and cobalt(II) complexes on the basis of multiple temperature X-ray diffraction structure studies [6]. The available results have been collected in Table 17. Average values for the bond length changes characteristic for a particular transition-metal ion have been extracted from these data and are obtained as AR 0.17 A for iron(II) complexes, AR 0.13 A for iron(III) complexes, and AR = 0.06 A for cobalt(II) complexes. These values may be compared with the differences of ionic radii between the HS and LS forms of iron(II), iron(III) and cobalt(II) which were estimated some time ago [184] as 0.16, 0.095, and 0.085 A, respectively. 

Table 18. Bond lengths R for equatorial and axial bonds in HS and LS Cobalt(II) complexes with salen and salen-type ligands...

The coordination modes of the nitrate ligand in the complexes [TpBut]M(N03) (M = Cu, Ni, Co, Zn) are summarized in Fig. 46. (171, 184). Evidently, the coordination mode varies from unidentate for Zn to symmetric bidentate for Ni and Cu, with the cobalt derivative exhibiting an anisobidentate coordination mode. Moreover, the related cadmium derivative [TpBut,Me]Cd(N03) also exhibits bidentate coordination of the nitrate ligand, with Cd-0 bond lengths of2.272(6) A and 2.295(7) A (91). Such symmetric bidentate coordination contrasts with the significantly different Zn-0 interactions [1.978(3) A and 2.581(3) A] in unidentate [TpBut]Zn(N03). The coordination modes for a variety of [TpRR ]M(N03) complexes are summarized in Table VIII. 

Let us start with the Ni(II) complexes of the already mentioned scorpiand diammac (6,13-diammino-6,13-dimethyl-1,4,8,11-tetraazacy-clotetradecane) in its two cis and trans conformations. In contrast to the previously mentioned chromium-, iron-, and cobalt-diammac complexes, in which the geometry of [M(fra s-diammac)]" + was substantially octahedral and that of the [M(cw-diammac)]" + was substantially trigonal prismatic, in the case of both [Nif/raws-diammac)]2+ and [Ni(m-diammac)]2 + the structural differences are attenuated and both can be viewed as more or less distorted octahedral geometries, with two sets of averaged Ni-N bond lengths of 2.07 A and 2.13 A, respectively.161 162 This is reflected by the fact that both the two complexes exhibit in aqueous solution a chemically reversible Ni(II)/Ni(III) oxidation ([Nif/raws-diammac)]2 + E° = + 0.67 V vs. SHE [Ni(m-diammac)]2 + ... 

Figure 6.3 Illustration of the three-coordinate complex [Co N(SiMe3)2 3]. Cobalt and nitrogen atoms are shown as black spheres, silicon atoms are grey and carbon atoms are given in white shadow. The Co-N bond length is 1.87A...

Mean cobalt-cobalt and nickel-nickel distances observed in these complexes are very close to interatomic distances determined at ambient temperatures in cobalt and nickel metals (Co-Co 2.489(7) A vs. 2.507 A in a-cobalt (33) Ni-Ni 2.469(6) A vs. 2.492 A in the metal (39)). The mean M-H bond lengths, as well as hydride displacements from M3 faces, are less for nickel in H3Ni4(Cp)4 than for cobalt in HFeCo3(CO)9(P(OMe)3)3. Although the differences are marginally significant within error limits (Ni-H 1.691(8) A vs. Co-H 1.734(4) A displacements from plane Ni3 0.90(3) A vs. Co3 0.978(3) A), they are in the expected direction since the covalent radius should vary inversely with atomic number within a transition series. However, other effects such as the number of electrons in the cluster also can influence these dimensions. 

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