Transition element complexes octahedral

Octahedral complexes of Cr (d ), and of the low-spin d species Co , Rh , Ii, and Pt, are numerous and well studied. They are kinetically inert, that is, they undergo ligand-substitution reactions relatively slowly, and the bulk of kinetic and mechanistic stndies on transition element complexes have been performed... 

Polyatomic molecules cover such a wide range of different types that it is not possible here to discuss the MOs and electron configurations of more than a very few. The molecules that we shall discuss are those of the general type AFI2, where A is a first-row element, formaldehyde (FI2CO), benzene and some regular octahedral transition metal complexes. 

This is the most common coordination number for complexes of transition elements. It can be seen by inspection that, for compounds of the type (Ma4b2), the three symmetrical structures (Fig. 19.6) can give rise to 3, 3 and 2 isomers respectively. Exactly the same is true for compounds of the type [Mayby]. In order to determine the stereochemistry of 6-coordinate complexes very many examples of such compounds were prepared, particularly with M = Cr and Co , and in no case was more than 2 isomers found. This, of course, was only negative evidence for the octahedral structure, though the... 

For elements adjacent to the noble gases the principal orbitals used in bond formation are those formed by hybridisation of the s and p orbitals. For the transition elements there are nine stable orbitals to be taken into consideration, which in general are hybrids of five d orbitals, one s orbital, and three p orbitals. An especially important set of six bond orbitals, directed toward the comers of a regular octahedron, are the d2sps orbitals, which are involved in most of the Werner octahedral complexes formed by the transition elements. 

The extension to non-octahedral complexes is possible, but must be carried out with great care The orbitals chosen can be expressed as a linear combination of the usual orbitals for an octahedron, and electron-electron repulsions can then be calculated from those for the octahedral case. It is not necessarily adequate for tetrahedral complexes of the first row transition elements, to use ligand field theory in the strong field limit, even for powerful ligands in V(mesityl)4 the ligand field splitting is only 9250 cm ... 

There are three kinds of electronic structures that may be expected for the octahedral complexes MXfl of the iron-group transition elements (and also for those of the palladium and platinum groups). -... 

Table 5-3.—Observed Magnetic Moments of Octahedral Complexes of Transition Elements ...

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

NMR studies of the fluxionality of octahedral transition element dithiocarbamates were performed by Pignolet and coworkers.65 The synthesis and X-ray structure of the pseudo-octahedral complex ion (40) were described by McCleverty et al. in their investigations of the inorganic aspects of rubber vulcanization.66 Our investigations point to the existence of a barium analogue.67... 

Octahedral six-coordination is especially favoured by the low-spin d6 configuration. This can be understood in terms of simple CFSE considerations. For M(III) (M = Co, Rh, Ir) and Pt(IV), hardly any complexes other than octahedral ones are known. These complexes are kinetically fairly inert, in the sense that they undergo ligand exchange reactions slowly. For this reason, much of our knowledge of kinetics and mechanism in transition element chemistry has come from studies of low-spin d6 octahedral complexes (see Sections 9.4 and 9.5). 

It should be noted the coordination number, N for trivalent lanthanides does not bear the same relevance and context as the transition elements, Fe(3d), Pd(4d) and Pt(5d). The 4/" electron population does not influence N in moving along the lanthanides series while the d electron population has considerable influence on N in the transition metals. Taking nickel as an example we have compounds of Ni(IV), Ni(III), Ni(II) and Ni(0). The majority of the compounds are of Ni(II) such as Ni(H20) +, NiiNH- ) 4" which have N = 6 and an octahedral disposition. The compound KNiF3 is a cubic perovskite with N = 6 and also paramagnetic. When Ni(II) forms diamagnetic complexes N = 4 with a square planar disposition. Tetrahedral NiCl - with N = 4 tetragonal-pyramidal Ni(CN)j- with N = 5 are also known. 

Octahedral six-coordination is the most common chemical environment for the transition elements (see Coordination Numbers Geometries). Apart from the great number of octahedral complexes, the formally ionic oxides and halides usually adopt stmctmes in which the cation is in an octahedral... 

Qualitative information about the rate of ligand substitution in octahedral complexes is summarized in Table 18.1. Quantitative data are obtained by application of ligand field theory. The rate constants may vary by many orders of magnitude. Thus, for the exchange of water molecules in the first coordination sphere of 3d transition elements the following values have been measured Cr (d" ) 7 10 s Cr (d ) 5-10- s-i Mn2+(d5) 3-lO s- Fe2+(d ) 3-10 - Co + d ) 1 lOS Ni2+(d ) 3 104s-i. 

Table 18.1. Relative rates of ligand exchange with octahedral complexes of transition elements.

Excluding cases where the complex interacts strongly with solvents or with itself, solution spectroscopic data generally have corresponded well with structural data, except as indicated for Mn(AA)3. The tris chelate complexes of the transition elements appear to remain approximately octahedral in solution. In fact. Fay and Piper (20) showed conclusively by NMR and optical activity studies that the trigonal prismatic configuration is not even an intermediate in the isomerization of the unsymmetrical M(A)3 complexes of Al(III), Ga(III), and Cr(III). 

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