Fragments CpMo

Flash photolysis has now been applied to a wide range of metal carbonyl species in solution, including Mn2(CO)10 (37), [CpFe(CO)2]2 (38), and [CpMo(CO)3]2 (39). In almost every case, interesting data have emerged, but, as with Cr(CO)5, the structural information is usually minimal. Thus, the radical Mn(CO)5 has been generated in solution by flash photolysis (37), the rate constant for its bimolecular recombination has been measured, but the experiments did not show whether it had Z>3h or Qv symmetry. Some experiments have been unsuccessful. Although the fragment Fe(CO)4 is well known in matrices (15), it has never been... 

Consistent with our analyses above, CpMn(CO)3 is a known 18-electron compound. The 17-electron CpMo(CO)3 fragment is found as a dimer with an M-M single bond (Figure 4.8). In order to be isolobal with CH3, the latter must use one t2g orbital plus one electron in bonding. Seventeen-electron CpFe(CO)2, with an unused, filled t2g set, is also isolobal with CH3 Further, 15-electron CpMo(CO)2 behaves as if it were isolobal with CH as it is found as a dimer with a short Mo-Mo bond attributed to a Mo=Mo triple bond. Hence, two of the t2g orbitals and three electrons must be used in the bonding. The Fe analog, 15-electron CpFe(CO) also forms a multiply bonded dimer which is isolated at low temperatures by photolysis of [CpFe(CO)2]2 in an inert matrix. It now must utilize one orbital from the t2g set to be viewed as isolobal with CH. 

In solution, the cyclopentadienyl tricarbonyl dimers [CpMo(CO)3]2 and [CpW(CO)3]2 react to form the heterobimetallic complex Cp(CO)3Mo-W(CO)3Cp. However, the reaction does not go to completion a mixture results in which the [CpMo(CO)3]2 and [CpW(CO)3]2 are in equilibrium with the mixed metal compound. The abundance of the three organometallic complexes is governed statistically by the number of CpMo(CO)3 and CpW(CO)3 fragments present. If 0.00100 mmol of [CpMo(CO)3]2 and 0.00200 mmol of [CpW(CO)3]2 are dissolved in toluene until equilibrium is achieved, calculate the amounts of the three organometallic complexes in the equilibrium solution. (Reference T. Madach and H. Vahrenkamp, Z. Naturforsch., 1979, 34b, 573.)... 

The chemical shift value for [ CpMo(CO)2 2(fi-PMes )] at < p=687ppm is in accord with other similar compounds like [ Co(CO)3 2()t-PMes )] at < p=664ppm, and [ CpV(CO)2 2(p-PMes )] at < p=657ppm. The two major contributions to the downfleld shift are the three-membered metallacycle, and the 7r-donor interaction of the phosphorus lone pair with both metal centres. This is not surprising, as the PMes moiety is identical in all three compounds, and the metal fragments have 15 VE (Co and Mo) and 14 VE (V), respectively. They are isoelectronic with respect to the phosphorus moiety, and thus the vanadium centres are allowed to make up their formal electron deficiency by upgrading the metal-metal single bond to a double bond. 

Examples for an isolobal relationship (P is isolobal to CpMo(CO)j) and for the lack of an isolobal relationship (P is not isolobal to Nidppe) are provided in Figs. 7.50 and 7.51. It is quite obvious that the two frontier orbitals of the nickel fragment are not equivalent to the three frontier orbitals of the molybdenum fragment. The consequence of the present case, of course, is that substitution of P with CpMolCO) would result in retention of all of the bonds in the P tetrahedron, whereas substitution with Nidppe would result in a different bonding pattern, as a bond is missing . In fact, [(dppeNi)j( j,-Pj)] does not possess a Ni— Ni bond, whereas [ CpMolCOl l lft-Pj)] does possess a Mo— Mo bond. 

The chemistry of dienes coordinated to the cationic CpMo(CO)j fragment has been exploited many times for complex molecule synthesis. Originally, Faller showed that the cationic molybdenum complex in Equation 11.44 undergoes nucleophilic attack by hydride, deuteride, methyl lithium, and enamines to produce the Ti -allyl complex. As expected, attack of the nucleophiles occurs at a terminal position and exclusively from the face opposite the metal. Trityl cation abstracts a hydride from this allyl product from the face opposite the metal to regenerate a diene complex. Pearson has used this sequence of nucleophilic attack and hydride abstraction to synthesize substituted cyclohexenes with control of stereochemistry as shown in Scheme 11.6. ... 

Scheme 8 illustrates one instance in which attack on the two con-formers 19a and 19b by the enamine of isobutaldehyde gives only Tj -olefin complexes 21 as a mixture of two conformers. To assert the importance of controlling the nucleophilic stereochemistry by the asymmetric CpMo(CO)NO fragment, the two diastereomers 22 and 24 were independently prepared and their alkylation reaction produced 23 and 25, respectively, consistent with the previous model. Notably, the steri-cally demanding phenyl group of 24 does not prevent cis addition to the NO group in the exo isomer. 

Because it is similar to RjSi as an electron-donating group, the CpMo(CO)2 fragment is cap le of stabilizing a P-carbocation center. By analogy with the allyl compounds of silane, borane, and tin, CpMo(CO)2(Tl -cyclohexadienyl) and its 6-substituted derivatives 106 a-c undergo BFj-promoted electrophilic addition to aldehydes and a,P-unsaturated enones by generating cyclohexadiene cationic precipitates 107, as shown in Scheme 22. 

This system gave way to CPM0O2X (X = Cl, CH3) (C), where the cyclopentadienyl Mo fragment is isolobal with (N-N)XMo [14]. This complex can be easily prepared in situ from CpMo(CO)3X (D) and TBHP [30]. A variety of experimental (kinetic) [31] and DFT [32] studies, also including the Cp analogues [33], indicated that an important active... 

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