Photolysis of W

Figure 1. Time resolved absorption of the CO laser P 1,0(10) line following the 351 nm photolysis of W(CO)g. [W(CO)g] = 0.025 toir, [He] = 4.0 toir.

Figure 2. Vibrational energy distribution of the CO product formed via the 351 nm photolysis of W(CO)s. Experimental data are indicated asfl. The lines correspond to results obtained by phase space calculations with an available energy of 40 and 35 Kcal/mole.

In recent work in collaboration with G.R. Dobson, we have examined the flash photolysis of W(CO) (piperidine)L (L = PPh, ... 

One of the more detailed studies of this type (64) has involved the unravelling of the rearrangement modes of the coordinat-ively unsaturated species W(C0)4CS, which was generated by UV photolysis of W(C0)5CS the experiments involved IR/UV spectroscopy, wave-selective photolysis and analysis based on l CO substitution. The results are summarised in Figure 7. These experiments also confirm that W(C0)4CS is initially formed in an excited state, as predicted by the scheme outlined in Figure 3. 

Furthermore, Fischer rendered this chemistry more practical by generating vinylidene complexes of pentacarbonylchromium and tungsten directly in situ from terminal alkynes [9]. For example, treatment ofterminal alkynes with WCO)5(CH2Cl2), generated by photolysis of W(CO)6 in CH2CI2, gave thermo-labile tt-alkyne W(CO)5... 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic generation from a trisilane 280153, thermolytic or photolytic decomposition of cyclic silanes 28114,154,155 and degradation of diazidosilanes 282153,156. The photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies157. The photolysis of w-diazo compounds 283 is the only frequently used reaction path to silenes 284 via a carbene-silene rearrangement8. 

Photolysis of a liquid NH3 solution of (NH4)4[Mo(CN)8] has been reported two photolytic intermediates were identified. Such photolysis was used to prepare Mo(CN)4(NH3)2, which was characterized by spectrophotometric and magnetic studies. Preliminary results concerning the photolysis of [W(CN)8]4 in liquid NH3 have also been reported.329 Improved syntheses of K3[MoO(OH)(CN)4],2H20 and K4[Mo02(CN)4],8H20 have been developed based on the photolysis of K4[Mo(CN)8] in aqueous ammonia.330 A kinetic study of the photosubstitution of K4[Mo(CN)8] in 0.1 M aqueous NaOH has been published,331 as have the results of several other photolyses of the [M(CN)8]" (M = Mo or W n = 3 or 4) ions.332... 

Fig. 9. TRIR spectra obtained 1 fis following 355-nm photolysis of W(CO)e in the gas phase. The solid line is the spectrum obtained with 10 mtorr of W(CO)6 and 80 torr of He. The dashed line is that obtained when the He was replaced by 80 torr of Xe. The negative bands are due to depletion of WlCOle and the positive bands represent the production of naked W(C0)5 (solid line) and W(CO)5Xe (dashed line). The apparent production of two positive hands for naked W(CO)g is a result of the low spectral resolution of the IR CO laser used in these experiments. Reproduced with permission from Fig. 1 in Ref. (52).

Fig. 12. s -FTIR spectrum obtained at Nottingham 1 na after 355-nm photolysis of W(CO)6 dissolved in scXe (1500 psi, 298 K) in the presence of 50 psi of CO. The negative hand indicates depletion of the parent and the two positive bands are assigned to the e and low-frequency ai mode v(C—O) vibrations of WCCOsXe. Note that the high-frequency ai vibration, expected for a molecule with C4 ssrmmetry, was not observed owing to its extremely low intensity. The spectral resolution used in this experiment was 8 cm . ... 

Surprisingly, little new research on the prototypal tung-sten-arene complex see Arene Complexes), r] -Ceih)f, has appeared, possibly because of the low-yielding and elaborate experimental procedures required for its synthesis. Photolysis of W(CO)6 in the presence of ethyne leads to the formation of benzene and rf-CdRf) W(CO)3 (101), and the solid-state molecular structure of this complex was... 

Mo(IV) 142). The second-order rate constant for the formation of the octacyanotungstate(IV) has been determined as 2.9 Af" sec" for the W(IV) system. The product formed (first-order rate constant, 4(1) x 10- sec [Mo] = 0.1 Af) (747) during acid hydrolysis (step III, Scheme 2) is not well characterized but afforded the study of the decomposition kinetics. The overall quantum yield of the photolysis of [W(CN)g] " at pH 1-13 remains constant at ca. 0.98(6) mol/einstein, illustrating also the high-yield conversion of the octacyanotungstate(IV) 141, 142). 

In contrast to the typical behavior of organic compounds discussed above, many photoreactions of transition metal complexes have wavelength-dependent quantum yields (7). Generally, these wavelength effects have been interpreted in terms of more than one reactive excited state of the photolyzed species. The photoreactivity of V(CO) L (L = amine), for example, has been interpreted in this manner with the previously mentioned model of substitutional photoreactivity proposed by Wrighton et al. (42, 49,73). Assuming ligand dissociation to be the only primary photochemical process (Section III-B-1), photolysis of W(C0)5L could produce three primary products ... 

E21.22 Since the intermediate is believed to be [W(CO)5], the properties of the entering group (triethylamine versus triphcnylphosphine) should not aflfect the quantum yield of the reaction, which is a measure of the rate of formation of [W(CO)5] from the excited state of [W(CO)j(py)]. The product of the photolysis of [W(CO)5(py)J in the presence of excess triethylamine will be [W(CO)5( t3)], and the quantum yield will 0.4. This photosubstitution is initiated from a ligand field excited state, not an MLCT excited state. A metal-ligand chai ge transfer increases the oxidation state of the metal, which would strengthen, not weaken, the bond between the metal and a cr-base-like pyridine. 

Flash photolysis of W(CO) in CCl results in the formation of the new complex W(CO)5Cl. The crystal structure of W(CO)5(Bu3P) has been determined and as expected, the W—C bond lengths are slightly shorter than found for W(CO). Photolysis of W(CO)g in n-pentane in the presence of HjS has resulted in the isolation of the first complex containing HjS as a ligand, W(CO)jH2S. The H2S is believed to be S-bonded. 

Figure 23. Plots of In and In [0/(1 — O)] versus P for photolysis of W(CO)5 pyridine ( , upper half), W(CO)5(4-cyanopyridine) ( ), and W(CO)5(4-acetyl-pyridine) ( , lower half) at 436 nm in toluene.

Finally, Katz and subsequently Rooney (386-388) showed that (CO)sW [C(OMe)Ph] catalyzes the polymerization of alkynes via carbene-alkyne and purported metallacyclobutene intermediates (Scheme 34). Later computational work by Hofmann (388b) called into question the formation of the latter species, suggesting that the reaction proceeds directly via the vinyl-carbene. Formation of the alkyne-carbene intermediate by low-temperature photolysis of (CO)5W[C—(OMe)Ph] in the presence of alkynes leads to polymerization upon warming. The use of a pre-formed carbene complex is not required since active polymerization catalysts can be formed by photolysis of W(CO)5 in the presence of terminal alkynes in hydrocarbon solutions. A key step in catalyst generation is rearrangement of a coordinated alkyne to a vinylidene ligand (389). 

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