Complexes with coordination number five

The interconversion shown in Fig. 3.10 is called a Berry pseudorotation— Berry because this is the name of the person who first suggested the mechanism and pseudorotation for the reason shown in Fig. 3.10(c,d). Whereas the top half of the figure shows the motions required to turn a 

The small energy difference between the two modes of five-coordination is demonstrated in the crystal structure of the compound [Cr(en)3][Ni(CN)5]1.5H20, where there are two distinct types of [Ni(CN)5] anions, one square pyramidal and the other approximately trigonal bipyramidal. Were one form to be appreciably more stable than the other, then that would be the only one present in the crystal. Although it was not evident from the crystal structure work, it seems that the I.5H2O play a key role presumably by hydrogen bonding to the anions on dehydration of the compound there is spectroscopic evidence that all the anions become square pyramidal. 

Examples of trigonal bipyramidal structures are the [Co(NCCH3)5] and [Cu(bpy)2l]cations. In the latter, one nitrogen of each bipyridyl is in an axial position. Anionic examples are [CuClg] , [SnClg] and 

A feature of square pyramidal structures is that there is the possibility of an additional ligand occupying the vacant axial site to produce a six-coordinate complex. Some of the small variations that have been observed in the electronic spectrum of [VO(acac)2] in different solvents are believed to be caused by a solvent molecule being weakly bound at the sixth coordination position. There is evidence that good donor solvents sometimes also introduce a ligating atom cis to the vanadyl oxygen. 

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All known Au compounds are diamagnetic ([Xe]4f " 5d low-spin configuration), and the large majority have square-planar geometry. However, Au complexes with coordination number five or six are also known. 

Various adducts of T1X3 have been noted earlier in this chapter, and a number of anionic complexes with coordination numbers of four, five or six have also been prepared. Salts of TlCl " and T1C13X (X = Br, I) are readily formed from T1X3 and (say) R NCl in non-aqueous... 

Most of the nickel compounds in the solid state and almost all in aqueous solution contain the metal in the oxidation state +2, which, by consequence, can be considered the ordinary oxidation state for nickel in its compounds. The electronic structure and stereochemistry of nickel(II) were reviewed in 1968.6 The most stable electronic configuration of the free Ni ion is [Ar]3d8 which is also the ground state configuration in its complexes. The overwhelming majority of nickel(II) complexes have coordination numbers of four, five and six. Complexes with coordination numbers of three, seven and eight are still quite rare. 

Again a compound CoC12.3 CsCl is not a complex compound Cs3[CoCl5] with coordination number five but on the basis of its crystal structure it is a double salt Cs2[CoCl4].CsCl. 

In a metal ion surrounded by other atoms, the d orbitals are at higher energy than they are in an isolated metal ion. If the surrounding electrons were uniformly distributed about the metal ion, the energies of all five d orbitals would increase by the same amount (a spherical crystal field). Because the ligands approach the metal ion from different directions, they affect different d orbitals in different ways. Here we illustrate the application of these ideas to complexes with coordination number 6 octahedral crystal field). 

The most common coordination numbers for transition metal complexes of relevance in catalysis are four, five, and six. However, a few metal complexes with coordination numbers two, three, and seven are also of relevance. Structures 2.3-2.8 show the common coordination geometries for coordination numbers four, five, and six. 

Iridium(III) complexes (d6) are diamagnetic and low-spin ((f,). The coordination number of most known iridium(III) complexes is six, though a few five- and seven-coordinate complexes have been reported. Iridium(III) forms complexes with soft ligands. The [Ir(H20)6]3+ ion is unknown no solid iridium(III) complexes with coordinated H20 groups are known with the exception of a few aquachloro complexes.197... 

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