Ionic bonds transition-metal ions

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. 

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. 

In an ionic compound, the partial covalence of a bond formed between a transition metal ion and its ligand modifies the magnetic properties of the cation. It can be seen, for example, that if electrons were... 

Interpretations of the crystal chemistry of transition metal ions by crystal field theory are based on a predominatly ionic model of the chemical bond. As... 

Boreskov (18) has proposed a model for transition metal compounds in which the rate of oxidation is assumed to be determined by the rate of electron transfer between oxygen and the transition metal ion. This process is further assumed to be facilitated with increasing degree of covalency of the metal-oxygen bond. Thus the more covalent transition metal oxides are more active than the rather ionic metal ion-exchanged zeolites. The oxygen-bridged species as described above is considered to be more covalent in character, and hence more active for oxidation catalysis than the transition... 

Let us look first for transition-metal compounds that arc truly covalent in the sense of tetrahedral structures and two-electron bonds, which we di.scu.sscd earlier. There are only a few examples. NbN and TaN both form in the wurtzite structure. We presume that bond orbitals of sp hybrids must be present to stabilize the structure this requires three electrons from each transition-metal ion. Both ions are found in column D5 of the Solid State Table, so we anticipate that the remaining two electrons would form a multiplet (as in the ground stale of Ti " ). Thus the effects of the d state are simply added onto an otherwise simple covalent system, just as they were added to a simple ionic system in the monoxides. MnS, MnSe, and MnTe also form a wurtzite structure and presumably may be understood in just the same way. This class of compounds is apparently too small to have been studied extensively. 

The directionality in the bonding between a d-block metal ion and attached groups such as ammonia or chloride can now be understood in terms of the directional quality of the d orbitals. In 1929, Bethe described the crystal field theory (CFT) model to account for the spectroscopic properties of transition metal ions in crystals. Later, in the 1950s, this theory formed the basis of a widely used bonding model for molecular transition metal compounds. The CFT ionic bonding model has since been superseded by ligand field theory (LFT) and the molecular orbital (MO) theory, which make allowance for covalency in the bonding to the metal ion. However, CFT is still widely used as it provides a simple conceptual model which explains many of the properties of transition metal ions. 

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