Octahedral modes

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

Q3e(e) Qha(a2)> Qhe(e)> Q5a(al) Qse(e)> Q6a(al)> Q6e(e)-If the trigonal distortion is assumed small, then we can expect the trigonal modes to reflect strong octahedral parentage. Thus, for example, the Q3a(a2) and Q3e(e) trigonal modes are expected to correlate strongly with the Q3(t]u) octahedral mode. 

Trigonal e-type modes (derived, for example, from the t2g or eg octahedral modes) can effect the following "inter-state" couplings of trigonal electronic components (Ea + Eh) e, (Ea + Ai) e, and (A2 + Eh) e. This has the consequence that vibronic components of the Ai(Aig) - Ai(T2g) transition can exhibit a nonvanishing CD which has been "borrowed" from the Ai (Aig)- -Ea(Tig) transition. Furthermore, vibronic components of the Ai(Aig) - Eh (T2g) transition may exhibit CD "borrowed" from both the A - Ea and Ai A2 trigonal components of the Aig - Tig transition. Heretofore, the appearance of multiple components in the Aig - ... 

Table 4.3 indicates that octahedral coordination is a common mode for Li. Less usual is planar 6-fold coordination (Fig. 4.8a), pentagonal pyramidal coordination (Fig. 4.8i) or irregular 6-fold coordination (Fig. 4.9a). Examples of 7-fold coordination are in Fig. 4.9b and c. Lithium has cubic 8-fold coordination in the metallic form and in several of its alloys with metals of large radius. It is also 8-coordinate in the dilithionaphthalene complex shown in Fig. 4.9d here the aromatic... 

While it remains true that tetrahedral and octahedral coordination modes are the predominant stereochemistries adopted by the group 13 metals, nevertheless increasing diversity is being achieved by carefully selecting appropriate electronic and geometric features to enhance the stabilization of unusual stereochemistries. Some representative examples follow. 

To this point the complexes considered have shared the coordination number six and approximate octahedral geometry. It has been argued that they also share the dissociative reaction mode. There are examples of reactions both with and without intermediates of reduced (that is, 5) coordination, but the insensitivity to entering ligands is a consistent feature. It will be interesting, shortly, to see if the dissociative pattern persists in more or less organometallic octahedral systems but first we shall give some attention to the non-labile square planar systems. 

As the trans effect theory indicates, there should be some relationship between lability of a ligand and its role as a labilizing group in another position in a complex. In an octahedral complex reacting via a dissociative mode of activation, the transition state has five strongly bound ligands. This state will be stabilized... 

An alternative mechanism that has been suggested [93, 118] for the intersystem crossing process is based on a twist movement of the octahedral ligand arrangement. Two modes designated by M 3 and Ml and illustrated in Fig. 13... 

The rate constants for the processes considered in this section are determined by Eq. (75) and Eq. (79). However, all the complexes for which measurements of rates are available deviate significantly from strict octahedral symmetry. In order to determine the coupling parameter S, the contributions of all modes which are characterized by a displacement Ar O have to be summed up ... 

Owing to the almost octahedral environment of the iron center, three out of six Fe-N stretching modes are invisible in NIS and IR spectra. Those modes that transform according to and Eg representations of the ideal octahedron do not contribute to the msd of the iron nucleus or to the variation of the electric dipole moment. Only the remaining three modes that transform according to Tiu representations can be observed in NIS- and IR spectra. These three modes, as obtained... 

Although the bulk of work with NOz as a ligand features Co111, nitrite can also be O- or N-bonded to Co11, and both modes have been observed in complexes formed by the reaction of several PR3 compounds with Co(N03)2, as well as in mixed nitro/nitroso complexes.375 The product Co(NO)(N02)(PEt3)2 reacts with dioxygen to form Co(N03)2 (OPEt3)2, a distorted octahedral structure with bidentate nitrate and cis phosphine oxides (102). 

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