D-transition metal complexes

Scheme 6.1 Elementary steps for the hydrogenation of olefins with d° transition-metal complexes.

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

The UV/visible absorption spectra of [MoO S4 ]2 (n = 0-2) complexes are dominated by the sulfur-to-metal charge transfer transitions of the anion and these bands often serve to characterize the particular system, even though these absorptions may be correlated with those of the parent anion.4 The reduction in symmetry due to complex formation often leads to a broadening or splitting of the bands and, in the majority of the d transition metal complexes, the lowest energy band is red-shifted by an amount which is dependent both upon the nature of the transition metal and the number of such metals bound per MoviO S4 group. 

At present there is consensus on the fact that the observed nephelauxetic effect in the spectra of lanthanide compounds is analogous to the phenomenon observed in the spectra of d-transition metal complexes. The nephelauxetic effect cannot be quantitatively interpreted by excluding the covalent interaction of lanthanide ions with surrounding ligands [34]. Jorgensen has proposed [38] two possible mechanisms of interaction for the observed nephelauxetic effect, namely (i) direct participation of lanthanide 4f orbitals in the formation of molecular orbitals also known as symmetry restricted covalency , (ii) transfer of some part of the ligand electron density to the unfilled 6s and 6p orbitals of the lanthanide also known as central field covalency . 

The photophysics and photochemistry of d transition metal complexes, such as Re(I), Ru(II), Os(II), and Ir(III), with a-diimines as electron acceptor ligands have been widely investigated during past four decades. These complexes have been frequently used as photofunctional molecules because of the following common properties (1-16) ... 

High-valent d transition metal complexes, e.g. complexes of Mo , V and Ti, catalyze numerous oxidations of organic substrates by alkyl hydroperoxides, such as epoxidation of alkenes, oxidation of tertiary amines to the corresponding N-oxides, of sulfides to sulfoxides. 

According to the first information on the catalysis of hydrosilylation by orga-noactinide complexes Cp 2A Me2 (where A = Th, U), they are efficient for hydrosilylation of terminal alkynes [82]. All catalytic and kinetic examinations of catalysis by early (d°) transition metal complexes (also by metal complexes with non-Cp ligands, e.g., [83]) support the generally accepted mechanism involving rapid olefin (acetylene) insertion into an M-H bond followed by a... 

There are many transition metal containing examples which may be phrased in similar terms. Reference 21 contains a discussion of the preferred orientation of trans carbene ligands. It is also noteworthy that in d transition metal complexes which contain CO, a good 31 acceptor ligand, the CO groups arrange themselves to be either cis or mer to each other rather than as the trans or fac alternatives. 

Lowe, N.D. Garner, C.D. Transition-metal complexes of crown ether benzodithiolenes. Part 2. The effects of alkali-metal cation binding. J. Chem. Soc., Dalton Trans. 1993, 3333-3340 and references therein. 

Rogers, R.D. Transition metal complexes of p-sulfonato- 28. calix[5]arene. Inorg. Chem. 1996. 35. 2602-2610. 

Unusual behavior results from d7r-p7r interactions between open-shell metal ions and organic radicals as illustrated by the family of 3-D metals [Cu(R R -DCNQI)2] (DCNQI = 7V,A -dicyanoquinonediimine) (Figure 14). The halogen-bridged binuclear-metal mixed-valence complex, [Pt2(dta)4l] (dta = dithioacetate) (1-D linear chain Scheme 5a), exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged 1-D transition-metal complex. ... 

---