Metal carbonyls matrix isolation

Spectroscopists also saw the potential of reacting ligands with transition metal under matrix isolation conditions. Photolysis of metal carbonyls in organic (383 or inert gas matrices (39) had already been done, but atoms offered the possibility of step-wise addition of ligands. DeKock (40), Turner (41 ), and Moskovits and Ozin (42) made early contributions, but the work of the last two became dominant (32). By 1972, there were the two distinct branches in transition metal atom chemistry, the preparative and matrix spectroscopic studies. 

Matrix isolation studies on transition metal carbonyls and related species. J. K. Burdett, Coord. Chem. Rev., 1978, 27,1-58 (212). 

Nonmetal-atom, matrix-isolation spectroscopy has proved useful in structure and isomer determination of stable, metal carbonyls. Fe(CO)4(NO) was investigated (157) in low-temperature matrices with CO enrichment, and it was demonstrated that the IR spectrum is consistent with C v symmetry (trigonal bipyramid with an equatorial NO), in agreement with X-ray studies (55). The work resolves the dis-... 

Until recently, fast time-resolved IR spectroscopy has been a technique fraught with difficulty. Generally it has been easier to use low temperature techniques, particularly matrix isolation (2,4), to prolong the lifetime of the fragments so that conventional spectrometers can be used. In the last 5 years, however, there have been major advances in fast IR spectroscopy. It is now posssible to detect metal carbonyl intermediates at room temperature in both solution and gas phase reactions. In Section II of this article, we explain the principles of these new IR techniques and describe the apparatus involved in some detail. In Section III we give a self-contained summary of the organometallic chemistry that has already been unravelled by time-resolved IR spectroscopy. 

Matrix isolation has many limitations, even for metal carbonyls (i) it cannot easily be used for charged species2 (ii) very little kinetic information can be obtained because of the restricted temperature range and limited diffusion (iii) the solid matrix cage can effectively block some... 

Despite the limitations, matrix isolation has been used to generate a large number of transition metal fragments containing carbonyl groups. The frequencies of their C—O bands have been measured and these data form a spectral library which has played a central role in the interpretation of time-resolved IR experiments. 

Thus, overall, it is clear that flash photolysis with uv-visible detection is effective in establishing the broad outlines of the photochemistry of a particular metal carbonyl. Intermediates can be identified from their reaction kinetics, and sometimes, with the help of uv-vis data from matrix isolation experiments. Structural information from uv-vis flash photolysis is at best sketchy. Many questions remain unanswered. Time-resolved IR measurements can fill in some of these answers. 

Relatively little work has been done on the flash photolysis of gas phase metal carbonyls, partly because of the low volatility of many of the compounds. Early work by Callear (41,42) provided some evidence for Ni(CO)3 generated from Ni(CO)4 in the gas phase (41) and Fe atoms produced from Fe(CO)5 (42). This latter process has even been used as the basis of an Fe atom laser (43). More recently Breckenridge and Sinai (44) studied the flash photolysis of Cr(CO)6. Their results, interpreted largely on the basis of data from matrix isolation experiments, were in broad agreement with Kelly and Bonneau s solution work (JJ). In particular, they found no evidence for loss of more than one CO group [Eqs. (4) and (5)]. 

The bands due to Fe(CO)4 are shown in Fig. 8. This spectrum (68) was particularly important because it showed that in the gas phase Fe(CO)4 had at least two vq—o vibrations. Although metal carbonyls have broad vC—o absorptions in the gas phase, much more overlapped than in solution or in a matrix, the presence of the two Vc—o bands of Fe(CO)4 was clear. These two bands show that in the gas phase Fe(CO)4 has a distorted non-tetrahedral structure. The frequencies of these bands were close to those of Fe(CO)4 isolated in a Ne matrix at 4 K (86). Previous matrix, isolation experiments (15) (see Section I,A) has shown that Fe(CO)4 in the matrix had a distorted C2v structure (Scheme 1) and a paramagnetic ground state. This conclusion has since been supported by both approximate (17,18) and ab initio (19) molecular orbital calculations for Fe(CO)4 with a 3B2 ground state. The observation of a distorted structure for Fe(CO)4 in the gas phase proved that the distortion of matrix-isolated Fe(CO)4 was not an artifact introduced by the solid state. 

To date, most of the photochemical data available for transition metal complexes comes from condensed phase studies (1). Recently, the primary photochemistry of a few model transition metal carbonyl complexes has been investigated in gas phase (5.). Studies to date indicate that there are many differences between the reactivity of organometallic species in gas phase (5.6) as conq>ared with matrix (7-10) or solution (11-17) environments. In most cases studied, photoexcitation of isolated transition metal... 

The structures of gas phase coordinatively unsaturated metal carbonyls are very similar to those of the matrix isolated species. 

For example, the results in Table 3 suggest that binary carbonyls of copper, silver and gold which have been detected spectrometrically in matrices at very low temperatures27, contain metal-CO bonds which are approximately of the same strength as those in Mn2(CO)i0. Similar considerations apply to carbonyls of palladium and platinum which have also been detected by matrix isolation spectrometry28. All of these binary compounds are unstable with respect to [M(c) + CO(g)J at room temperature. 

The most recent fairly comprehensive review Of the vibrational spectra of transition metal carbonyls is contained in the book by Braterman1. This provides a literature coverage up to the end of 1971 and so the subject of the present article is the literature from 1972 through to the end of 1975. Inevitably, some considerable selectivity has been necessary. For instance, a considerable number of largely preparative papers are not included in the present article. Tables A-E provide a general view of the work reported in the period. Table A covers spectral reports and papers for which topics related purely to vibrational analysis are not the main objective. Papers with the latter more in view are covered in Table C. Evidently, the division between the two is somewhat arbitrary. Other tables are devoted to papers primarily concerned with the spectra of crystalline samples — Table B — to reports of infrared and Raman band intensities — Table D and sundry experimental techniques or observations - Table E. Papers on matrix isolated species, which are covered elsewhere in this volume, are excluded. 

Matrix isolation studies usually permit spectroscopic observation of the species M(CO), M(CO)2,. M(CO) , the coordinatively saturated molecule. In some early studies, species thought to be simple unsaturated carbonyls were in fact carbonyls of metal clusters Mx(CO) a very low concentration of metal in the matrix (e.g., I mol in 104 mol noble gas) has to be used to prevent clustering. All the partially coordinated carbonyls are only matrix species, that is, they only exist when completely isolated from other molecules of their own kind or from CO. The coordinately saturated carbonyls are of more interest in the context of this review. The following new molecules have been reported Au(CO)2 (84a) Ag(CO)3, Cu2(CO)6 (46, 87) Pd(CO)4 (22), Pt(CO)4 (69) Rh2(CO)g, Ir2(CO)g (37) M(CO)6[M = Pr, Nd, Gd, Ho, Yb (100), Ta (24), U (117)]. The Cu, Pd, Pt, Rh, and Ir carbonyls can be obtained by condensing the metal vapors with pure CO at 40 K and then pumping off excess CO to leave a film of the carbonyl. The Cu, Pd, and Pt carbonyls decompose under vacuum temperatures above -100°C, and the Rh and Ir carbonyls dimerize with loss of CO to give M4(CO)12 above -60°C. The gold and silver carbonyls are not stable outside matrix isolation conditions. Unfortunately, the literature is presently unclear about the stability of the Ta and lanthanide hexacarbonyls outside a matrix. 

Carbonyls of some Main Group metals have been detected under matrix isolation conditions, e.g., Alx(CO)2 (42), Gax(CO)2 (91), and Sn(CO) (n = 1-3 ) (87). Bonding in these compounds is presumably of the soft-acid-soft-base class as in borane carbonyl, with only a minimal contribution from 7r-bonding. 

In the last three decades of the twentieth century, following Walter Hieber s retirement, four aspects of the research on mono- and polynuclear metal carbonyl complexes found particular attention. These were the preparation of highly reduced carbonyl metallate anions, the generation of stable metal carbonyl cations, the matrix isolation of uncharged metal carbonyls obeying or not the 18-electron rule and, last but not least, the giant metal carbonyl clusters. 

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