Photoaquation

Chromium, bis(l,2-ethanediamine)difluoro-photochemistry, 1,393 Chromium, bis(l, 2-ethanediamine)oxalato-coordination isomerism, 1, 183 Chromium, diamminetetrakis(isothiocyanato)-photoaquation chemical actinometer. 1,409 photochemistry reactivity, 1, 398... 

The photoaquation of a transition metal coordination compound (Reinecke s Salt) has recently been developed as an actinometer for the... 

Platinum(ni).—Unlike the photochemistry of [PtXg] " (X = Br or I) in which only photoaquation is observed,flash photolysis of [PtClsV" in its Cl Pt charge-transfer band has been shown to result also in photoreduction. The unusual Pt" complex [PtC ]" is produced via the mechanism... 

A study of the photochemistry of frans-[PtBr2(NH3)4] has shown that photoaquation is the only reaction occurring. However, the observation of transient BrJ in the flash photolysis, together with the formation of HCHO when methanol is solvent, suggests that the [PtBr(HjO)(NH3)4] product is formed not only by simple photoaquation but also via the ion-pair mechanism... 

A detailed study of the photochemistry of Cr(en)33 + has been carried out.40 Both cis- and trans-isomers of Cr(en)2(H20)23 + were isolated as products of photoaquation of this complex.5... 

If we denote cis- and trans-Cr(en)2(H20)2f3 by H2C and H2T, respectively, and the four acid dissociation products by HC, HT, C, and T, then there are four acidity constants (-Kh2c, -Khc, h2t, axidKur) and three isomerization constants ( h2t/h2c, Kht/bp and Kt/c) All of these have been evaluated by Woldby (17), and some rate data on the T — C conversion has been reported by Olson and Gamer (12). Preliminary experiments showed that both photoaquation and photoisomerization did indeed occur, and the detailed investigations described below were therefore carried out. 

The photosensitivity of all six forms was determined qualitatively, and the results are in Table III. The H2T and HT systems were difficult to work with because their high thermal lability made the dark reaction correction large. An additional complication was the secondary photolysis of cis isomer, which made it doubtful if the aquo product observed in the case of irradiated T was caused by primary photoaquation. The results in Table III are, therefore, qualitative and are displayed to provide a general picture of the situation. 

Figure 3. Quantum Yields for Photoaquation and Isomerization of cis-Cr(en)2(011)2+ solutions adjusted to pH 10.5 with ethylenediamine.

Contrariwise, the photochemistry is decidedly one of photoaquation for the cis species, and isomerization for the trans species. 

It might be thought that photoisomerization and photoaquation could be competitive towards a common precursor excited state, but this explanation fails on quantitative testing. Briefly, the quantum yield for photoaquation is too high to accommodate its negative apparent activation energy. Photoaquation must then be competitive with some other process such as deactivation, or return to the original cis species. 

Mechanisms. Since the above conclusions are minimum, it is interesting to see if a more detailed picture exists which can account for all of the observations. Four different sequences (at least) must be provided thermal isomerization, photolysis in the doublet region of wave length, and photoaquation and photoisomerization in the quartet region. 

Bis(2,2 -bipyridyl)ruthenium(II) was anchored onto poly(4-vinyIpyridine) (PVP) (6) 28>29), but the polymer complex is not suitable as photocatalyst, because it is susceptible to photoaquation. A polymer complex containing Ru bpy) + pendant groups was first prepared by reaction of polystyrene (PSt) as shown in Eq. (15)30). 

In hexacyanochromate (III) ion, the lowest doublet and quartet ex- ited states are well separated (Section 8.6). By the use of suitable sensitizers, it is possible to excite the two states separately. By such techniques, it has been established that photoaquation of cyanide ion occurs from the excited quartet state 4T , whereas E state is photochemically inactive and the seat of phosphorescence emission. The intersystem quartet to doublet crossing is not always fast. 

A. Photosubstitution reactions can be aquation, anation or ligand exchange. (i) Photoaquation reactions of the type... 

Hoffman27 recently observed that in the photoreduction reactions of azido-pentaminecobalt(II) cations the predominant first step is photoaquation with loss of ammonia, rather than loss of azide ion. 

The replacement of X by pyridine in the complex PtX(dien)+ to give Pt(dien)py2+ (equation 549) has been studied under controlled conditions with a range of leaving groups X. These data are shown in Table is.1 87-1988 From these data the leaving group order is NOj > H20 > Cl- > Br > I- > N3 > SCN- > NO2 > CN-. Reactions such as these must be carried out under thermal conditions for accurate comparison since photoaquation can occur, albeit with a rather low quantum yield.1989 The volumes of activation of these reactions (equation 550) are all negative. An associative mechanism is proposed for the nucleophilic dependent path, but for the nucleophile independent pathway both associative and dissociative mechanisms need to be considered.1990... 

The photochemical and thermal substitution reactions of CrIn complexes can follow quite different stereochemical courses. The behavior described by equation (32) is typical thermal aquation of trans-[CrCl2(en)2]+ proceeds with retention of configuration, whereas photoaquation is accompanied by a highly stereospecific (>99% cis product) rearrangement.69 With very few exceptions, it has been found that stereochemical mobility is a general feature of Cr111 photosubstitution reactions. 

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