Bonding to Transition Metal Ions

The bonding to transition metal ions has been described in the previous section as being predominantly that of dative covalent bonding whereby lone pairs of electrons are donated into vacant orbitals of the transition series metal ions. It was established by Werner that these bonds are directional in nature and have certain preferred angles subtended at the central metal ion. 

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The O- species on the surface will be further stabilized by coulombic interactions with the lattice cations. For the sake of completeness, the review also includes a brief section on oxygen ions which are closely bonded to transition metal ions (M=0) and which may play an important part in selective oxidation [Weiss et al. (4) The coverage of the mononuclear oxygen species is restricted to those papers where there is direct evidence on the nature of the oxygen species concerned. 

Transition elements form complexes by combining with ligands. Ligands bond to transition metal ions by one or more co-ordinate bonds. 

The coordination of oxygen to transition metal ions which occurs mostly in the side-on fashion on surfaces (Section III,A,2 and Appendix B) can be described following the model of acetylene-metal complexes (467). Both 7tu and 7tg orbitals of molecular oxygen have proper symmetry to interact with the bonding set of s, p, and d orbitals on the metal. The bonding orbitals are shown in Fig. 29. 

The orbitals involved in the bonding of dioxygen to transition metal ions. 

The cyanide ion is called a pseudohalide ion because it behaves like Cl- in forming an insoluble, white silver salt, AgCN. In complex ions such as Fe(CN)63-, CN - acts as a Lewis base (Section 15.16), bonding to transition metals through the lone pair of electrons on carbon. In fact, the toxicity of HCN and other cyanides is due to the strong bonding of CN- to iron(III) in cytochrome oxidase, an important enzyme involved in the oxidation of food molecules. With CN attached to the iron, the enzyme is unable to function. Cellular energy production thus comes to a halt, and rapid death follows. 

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. 

Acrylonitrile appears to coordinate to transition metal ions primarily through its carbon-carbon double bond (highest occupied molecular orbital of that molecule ). This mode of coordination has been deduced from the absence of the stretching frequency and, in the cases of Fe(CO)4(CH2=C-CN) and... 

In general, the ions and complexes of elements in the first two transition series absorb broad bands of visible radiation in at least one of their oxidation states and are, as a result, colored (see, for example. Figure 26-2). Here, absorption involves transitions between filled and unfilled d-orbitals with energies that depend on the ligands bonded to the metal ions. The energy differences between these d-orbitals (and thus the position of the corresponding absorption peak) depend on the position of the element in the periodic table, its oxidation state, and the nature of the ligand bonded to it. 

Because of the importance of metal bonding to histidine in metalloproteins, there have been numerous studies of the pAg values of imidazoles tr-bound to transition metal ions. Usually the coordination decreases acid strength substantially (by 4-6 pA units with LM " systems) because of ff-donation from ligand to metal backbonding effects from metal to ligand are minimal... 

Cyanides or nitriles, RC=N, have two regions for electrophilic attack, namely the triple bond TT-electrons and the lone pair on nitrogen. The two possibilities of attack are illustrated by the behaviour of cyanides with respect to transition metal ions. In their gas-phase studies on the interaction between cyanides and transition-metal ions Chen and Millerconcluded that both side-on and end-on interaction is present. In this respect cyanides resemble the isoelectronic dinitrogen. 

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