Tris complexes bonds

Nickel and palladium react with a number of olefins other than ethylene, to afford a wide range of binary complexes. With styrene (11), Ni atoms react at 77 K to form tris(styrene)Ni(0), a red-brown solid that decomposes at -20 °C. The ability of nickel atoms to coordinate three olefins with a bulky phenyl substituent illustrates that the steric and electronic effects (54,141) responsible for the stability of a tris (planar) coordination are not sufficiently great to preclude formation of a tris complex rather than a bis (olefin) species as the highest-stoichiometry complex. In contrast to the nickel-atom reaction, chromium atoms react (11) with styrene, to form both polystyrene and an intractable material in which chromium is bonded to polystyrene. It would be interesting to ascertain whether such a polymeric material might have any catal3dic activity, in view of the current interest in polymer-sup-ported catalysts (51). 

Attempts to prepare adducts directly from [VO(oxme)2] (138) were unsuccessful and a trigonal bipyramidal structure was suggested for this compound as for [VO(2-Me-oxine)2] (139).825 The [VO(oxine)2X] compounds were prepared by addition of V0S04 to an aqueous solution containing 8-hydroxyquinoline and the base X. For the adducts [VO(oxine)2X], a correlation was found between v(V=0) and the pK of the X ligand, except with bases with substituents where steric hindrance may be operating, and no evidence was obtained for the existence of both cis and trans isomers, unlike the case of [VO(acac)2] adducts (Section 33.5.5.4.ii). For the tris complexes [VO(oxine)3] , an octahedral structure in which one oxine ligand is unidentate and bonded to V was proposed.825... 

Camphorate complexes of chromium (III) have been studied. The four possible isomers of the tris complex of (+ )-3-acetylcamphorate (173) were isolated,752,753 and absolute configurations were tentatively assigned. The photoisomerization of these complexes has been investigated 754 quantum yields of the order of 10-3 were obtained with visible or ultraviolet radiation at temperatures around 100 °C. Bond-breaking processes were held to be important in the reactivity of cis isomers. 

Trimethyldipyrromethane (dpm) yields the low-spin six-co-ordinate tris-complex with iron(m). Strong ligand-field bands in this complex are consistent with the presence of a trigonal distortion and highly covalent metal ligand bonds.217 The complex [Fe(pbz), ]Cl, [pbz = 2-(2 -pyridyl)benzimidazole] has also been reported.214... 

Tris-chelate complexes exist in enantiomeric configuration A and D about the metal atom, and when the chelating ligand is unsymmetrical, there are also geometrical isomers, cis and trans. Each geometrical isomer exists in enantiomeric forms thus there are four different molecules. In the case of tris complexes with symmetrical ligands, the process of inversion (interconversion of enantiomers) is important. When the metal ions are of the inert type, it is often possible to resolve the complex then the process of racemisation can be followed by measurement of optical rotation as a function of time. Possible pathways for racemisation fall into two broad classes those without bond rupture and those with bond rupture. 

In contrast with the unstable complexes, the tris complexes of thd and fod, even though the latter are isolated as hydrates, are completely volatilized with no apparent decomposition. In the light of the obvious ease of dehydration of the fod complexes, there is some doubt that the water is coordinated to the metal ion. An alternate possibility is that in the case of the bulky fod complexes the water molecule may instead be hydrogen-bonded to an electronegative site on the ligand shell. If this is true, it would appear that the bulky thd and fod ligands have eflFectively blocked the access to additional coordination sites on the metal ion. 

Figure 12. Differences in destabilization energies for the low- (1-3) and high-spin (4-6) tris-complexes with TP and TAP geometries at a = 35° [1 and 4], 38.5° [2 and 5], 45° [3 and 6] as a function of the d-electron number. The x-bonding effect is not taken into account [93].

Bipyridine is a strong field ligand that forms relatively stable complexes, with the inherent M—N bond strength enhanced by the chelate effect. These factors favor the formation of 4-coordinate bis and 6-coordinate tris complexes. The tris complexes of the first row transition metals in normal oxidation states (+2 or +3) are best prepared by the reaction of a suitable metal salt with an excess of bpy in water, methanol, or other organic solvent. The solid complexes can be obtained by crystallization or by the precipitation of the perchlorate, hexafluorophosphate, tetrafluoroborate, or other salts. Because bpy is a strong field ligand, the lower oxidation states tend to be favored, and reduction of M(III) complexes can occur in these preparations. The M(III) complexes are usually readily obtained by the chemical, aerobic, or electrochemical oxidation of the M(II) species. 

With trivalent metals, acetylacelone thus forms neutral tris complexes such as [Al(acac)3], [Ti(acac)3], [Cr(acac)3], and [Co(acac)3]. As a result of resonance, the two M—O bonds in each of these complexes are equal in length, as are the two C—0 and the two ring C—C bonds, giving a symmetric structure (only one ring shown) ... 

With the unsubstituted Py, the stable tris(N-bonded) adduct [CpRu(r (N)-Py)3] has been obtained. However, when the permethylated ligand Cp is employed in order to increase the electron density around the Ru center more, the complex [Cp Ru(Ti (N)-Pylsl is a kinetic product that thermally converts to the corresponding ti derivative (Eq. 6.12). 

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