Six-coordinate transition metal complexes

Acetylacetone (acac) and related ) -diketones continue to be extensively used as ligands for transition metal complexes. This section deals with tris(acac) complexes followed by a discussion of the spectra of adducts of bis(acac) complexes. Cramer and Chudyk (75) have studied [Ni(acac)3]2C104. INDO calculations show that the major spin delocalization is into the highest filled ligand orbital which possesses cr-symmetry plus a minor contribution from delocalization into the lowest empty 7i-orbital. This is in contrast to the spin delocalization in Ni(acac)3 and Ni(acac)py2 where the major de-localization is into the highest filled ligand orbital of Ti-symmetry. 

In contrast to pyridine adducts, those of pyridine-iV-oxide produce isotropic shifts which support a dominant -delocalization mechanism. Spin density distributions over the aromatic carbons have been determined from H and spectra of several Ni(acac)2(py-NO)2 complexes. (79) relaxation measurements indicate that Tj values arise mainly from hyperfine dipolar interaction induced by spin density localized on Ni(ii) and on the C-centred 2p orbital. Adducts of Ni(acac)2 with aniline, (80) fluoroanilines, (80,81) alkylanilines, (81-83) aniline derivatives, (541, 542) and nitrogen heterocycles (543) have been extensively studied. The results are consistent with a dominant n-spin delocalization mechanism. 

The shifts for 4-cyclopropylanilines, however, deviate from this relationship and depend on the conformational preference of the group. 

Adducts of bis(benzoyltrifluoroacetonato)M(ii) complexes have been studied by NMR methods. (84-86) Aquo complexes of stoichiometry M(BTFA)2(H20)2 [13] were studied (84) by and NMR. Spin 

A contact shift study (87) of various benzazole [14] adducts of 

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TABLE IV Dq Values of Six-Coordinate Transition Metal Complexes... 

HOMOGENEOUS ACTIVATION OF MOLECULAR HYDROGEN BY FOUR-, FIVE-, AND SIX-COORDINATE TRANSITION METAL COMPLEXES. 

Figure 23 Regions of existence of the geometries of six-coordinate transition-metal complexes (Schane 14a-c) with tetraaza macrocyclic ligands (shaded areas). The two geometrical parameters correspond to the angle between two N3 planes (y) and the X-M-X bond angle involving the nonmacrocycUc donor atoms (a).

J.A.S. Howell, P.M. Buckinshow - Ligand Substitution Reactions at Low-Valent Four-, Five-, and Six-Coordinate Transition-Metal Complexes, Chem. Rev. 83,557,1983. D.J. Darensbourg - Mechanistic Pathway for Ligand Substitution Processes in Metal Carbonyls, Adv. Organomet. Chem. 21, 113,1982. 

Nitrogen ligands, which tend to coordinate to three meridional coordination sites of five- and six-coordinated transition metal centers, form trimethylplatinum complexes with these ligands as bidentates 974 and 975. "" ""... 

This chapter is concerned with the simplest reactions of inert transition metal complexes. Fig. 1 shows a typical compound. This is Co(III) coordinated to six NHj molecules to form a triply positive cation [Co(NH3)6]. It is indicated in Fig. 1 to be in aqueous solution where water molecules occupy positions in what... 

A further simplification of Eq. (24) can be made by considering the symmetry of the crystal field. To see how this happens, we shall consider Vc for octahedral symmetry. This field would be given by six equal charges placed at R along the x, y, and z axes. Since the coordination number six is very common in transition-metal complexes, this type of crystal field occurs often. 

This review, which complements an earlier one (Part I) dealing with transition metal complexes of triazenes, tetrazenes, tetraazadienes, and pentaazadienes, examines the coordination chemistry of related cyclic catenated nitrogen ligands. Six-membered rings containing three, four, or five adjacent nitrogen atoms — 1,2,3-triazines, 1,2,3,4-tetrazines, and pentazines, respectively — are either unknown or are relatively unstable species whose coordination chemistry has yet to be explored. 

The structures of two transition-metal complexes that contain a Mo=Mo quadruple bond are shown in Fig. 19.2.3. In the compound [M02(O2CMe)2(NCMe)6](BF4)2, each Mo atom is six-coordinate, and all nine valence AO s are used for bonding according to the 18-electron rule. In the cation [M02(O2CMe)2(NCMe)6]2+,... 

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