Complex isoelectronic

Because Mo and W are usually more stable and more reactive than Re, the reactivity of the corresponding isoelectronic complexes of Mo, [( iO)Mo( NAr) (=CH Bu)(CH 2Bu)[ [70], and W, [( iO)W(=NAr)(=CH Bu)(CH)Bu)] [71], have been studied. Overall, these systems are more stable than the corresponding silica-supported Re complex, and display reachvities better than those of the well-known corresponding bis-aUcoxide homogeneous derivahves. The better performances of these systems compared to their homogeneous analogues are probably due to the optimized coordination of the metal center in combinahon with a site isolation of the... 

Figure 9 Qualitative MO scheme of the isoelectronic complexes [Fe(MoS4)2]3 and [Co(MoS4)2]2, and analogous bis(thio-metalato) complexes of Fe and Co with half-occupied and completely occupied (/-levels , respectively. The braces link together MOs of approximately the same energy (see also ref. 39).

In order to estimate and compare the magnitude of the M-B interactions in these isoelectronic complexes, a whole set of structural and spectroscopic parameters determined experimentally and/or computed theoretically were considered. This includes the M - B distance the ratio r between the M -B distance and the sum of covalent radii (to take into account the different sizes of the metals involved), the pyramidaliza-tion of the boron environment XB, the rlB NMR chemical shift <5 11B, the difference AqB between the charge at boron in the metal boratrane and the free ligand TPB, the difference A M between the charge at the metal in the metal boratrane and that in the related borane-free complex [M(i-Pr2PPh)3], and the NBO delocalization energy A NBo associated with the main donor-acceptor M-B interaction found at the second order in the NBO analysis (Table 2). Only the conclusions of this detailed analysis will be recalled here ... 

Among the earliest examples of M(/u-H)2 M systems structurally investigated are the isoelectronic complexes H2Re2(CO)8 179> and [H2W2(CO)8]2- 18°). The structure... 

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., d8 (Ni2 +, Co+), d( (Fe2+), d4 (Cr2 + ). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d10 electronic configuration of Cu+ is consistent with the fact that isoelectronic complexes of Ni° and Pd° are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-... 

The protonation leads to a strong increase of the v(CO) frequencies. The exactly analogous v(CO) frequency increase was observed when the thiolate donors were stepwise alkylated with Me30BF4 or Et30BF4. While the resulting thiol derivatives proved too labile to be isolated, the isoelectronic alkyl derivatives could be crystallized and characterized by X-ray structure analysis. Series of isoelectronic complexes were obtained, which comprise up to four species as in the case of Fe(CO)2(S2C6H4)2 2, [Fe(CO)2(S4)], [Fe(CO)2(S4-Et)]+, and [Fe(CO)2(S4-Et2)]2+. 

The isoelectronic complexes Re(NO)(PPh3)Cp 2(p-C=CC=Q (96/ Ph),249 Fe(dppe)Cp 2(p-C=CC=C) (107)258 270 and Ru(PPh3)(L)Cp 2 ( i-C=CC=C) (109)262 all feature fully occupied, closed-shell singlet structures for their HOMOs. The experimental structures are in accord with this description, and display bond lengths consistent with a poly-yne-like structure. These complexes, and closely related phosphine derivatives, display two (Re), three (Fe) or four (Ru) 1-e oxidation processes. 

The nitrosonium cation [NO]+ is isoelectronic with CO and accordingly many mixed nitrosyl-carbonyl complexes are known. For electron counting purposes, the neutral molecule is considered to act as a 3 (or occasionally 1) VE donor. Thus various series of isoelectronic complexes can be envisaged (Table 3.5). The majority of synthetic routes to nitrosyl-carbonyl complexes involve (i) photochemical CO substitution or metal-metal bond cleavage by NO (ii) electrophilic attack by nitrosonium salts, e.g. [NO]BF4 or nitrosyl halides (e.g. C1NO) upon electron-... 

Series of compounds such as V(CO)6, Cr(CO)6, and Mn(CO)6+ are said to be Isoelectronic complexes because they have the same number of electrons distributed in very similar structures. Isoelectronic ligands are CO and NO+ or CO and CN, for example. Strictly speaking, CO and CS are not isoelectronic, but as the difference between O and S lies in the number of core levels (the valence shell being the same), the term isoelectronic is often extended to cover such pairs. A comparison of isoelectronic complexes or ligands can be useful in making analogies and understanding trends. ... 

Before it is possible to interpret on a rigorous basis the behavior of the carbonyl stretching frequencies of a series of isostructural and isoelectronic complexes complete vibrational analyses are necessary. However, it is only within the last few years that far-infrared 137) and laser Raman 84) spectrometers have become available generally. Hence, in the general absence of the data they have provided, earlier complete analyses were limited to the spectra of simple metal carbonyls (for which such information was available). Even for these complexes, the number of force constants exceeds the number of observable frequencies, and model force fields had to be used. Since Urey-Bradley type force fields proved to be unsuitable for carbonyl complexes 86,105, 106), Jones 80-82) developed a resonance interaction valence force field which reduced the number of force constants by interrelating several on the basis of orbital overlap. This approach is not readily adaptable to less symmetrical substituted carbonyl complexes. Alternative models had, therefore, to be investigated. 

The isolobal species Ir(CO)3, Co(CO)3, CR, and P may also be combined in several different ways. As mentioned previously, Ir(CO)3, a 15-electron fragment, forms [Ir(CO)3]4, which has symmetry. The isoelectronic complex Co4(CO)j2 has a nearly tetrahedral array of cobalt atoms, but has three bridging carbonyls and hence C3V symmetry. Compounds are also known that have a central tetrahedral structure, with one or more Co(CO)3 fragments [which are isolobal and isoelectronic with Ir(CO)3] replaced by the isolobal CR fragment, as shown in Figure 15-3. This is similar to the replacement of phosphorus atoms in the P4 tetrahedron by Co(CO)3 fragments P may also be described as isolobal with CR. 

---