Carbonyl complexes covalent

Fig. 7 Os(II) bipyridyl carbonyl complex covalently linked to both free base and Zn(II) tetraphenylporphyrin [45]...

IR spectroscopy of adsorbed carbon monoxide has been used extensively to characterize the diluted, reduced Cr/silica system [48-54,60,76,77]. CO is an excellent probe molecule for Cr(ll) sites because its interaction is normally rather strong. The interaction of CO with a transition metal ion can be separated into electrostatic, covalent a-dative, and 7r-back donation contributions. The first two cause a blue shift of the vco (with respect to that of the molecule in the gas phase, 2143 cm ), while the last causes a red shift [83-89]. From a measurement of the vco of a given Cr(II) carbonyl complex, information is thus obtained on the nature of the Cr(II)- CO bond. 

Some writers feel it important to distinguish carefully between covalent and metallic radii. Others suggest that a self-consistent set of atomic radii - some covalent, others metallic - can be devised. Such a collection is presented in Table 4.1. In cases where both a covalent and a metallic radius can be obtained, the agreement is variable. For example, the metallic radii of atoms of the Group 1 elements are 20-30 pm greater than the corresponding covalent radii, taken from the M-M distances in M2(g). The Mn-Mn distance in (CO)5Mn—Mn(CO)5 is 293 pm, which compares with 274 pm calculated from the metallic radius of Mn. The electronic environment of the Mn atom in the carbonyl complex is, of course, very different from that in the elemental substance. 

Hyslop and coworkers reported an Os(II) bipyridyl carbonyl complex (Fig. 7) covalently linked to both free base and Zn(II) tetraphenylpor-phyrin [45]. The Os(II) chromophore, having a CO ligand, has a luminescence maximum of 589 nm in CH2CI2 and excitation of the complex results in energy transfer to the free base porphyrin. In the mixed Os(II)/Zn(II) complex,... 

Based on the foregoing experimental results, the versatility of metal carbonyls and their derivatives in their reactions with liquid NH3 may be summarized as follows (/) substitution of CO or other ligands by NH3 without change in the oxidation number of the transition metal in question (2) conversion of covalent carbonyl complexes into ionic compounds by addition of NH3 molecules (3) base reactions" in which the transition metal is reduced to a carbonyl metalate with complementary oxidation of a CO ligand to CO(NH2)2 (4) valence disproportionations with... 

Similarly, several other covalent molecules, such as halogens, organometallic compounds, and carbon tetrachloride, take part in such reactions. Susuki and Tsuji reported that addition of carbon tetrachloride to olefins and carbonyla-tion are catalyzed by dinuclear metal carbonyl complexes like [7r-CsHsFe(CO)2]2 and [7r-CsH5Mo(CO)3 ]2 S8-S9>. 

The project by Sun et al. has been extended to photochemical production of H2 by a Ru(II) photosensitizer covalently linked to a proton reduction catalyst of dinuclear Fe carbonyl complex [128,129] (Fig. 25, left). The researchers synthesized (but did not isolate) a Ru(II) terpy complex connected to an [Fe2( x - SCH2N(Ph)CH2S)(CO)6] complex by an acetylene spacer [129] (Fig. 25, right). However, the photoproduction of H2 has not yet been reported. 

This limited range of oxidation states can be explained by the lack of covalent bonding, which plays a major role in the stabilization of transition metal compounds in both low and high oxidation states. For instance, in metal carbonyl complexes, snch as Fe(CO)s, the high electron density at the metal center is reduced by overlap between the filled metal d orbitals and the empty CO Jt orbitals (Figure la). Similarly,... 

Compounds in the zero oxidation state are represented mainly by a number of unstable carbonyl complexes that decompose well below room temperature and some better characterized zero-valent complexes with bulky aromatic hydrocarbons. Their stability depends in part on the low 4f 5d promotion energies, which allow the formation of covalent bonds. Their synthesis requires special conditions and they are not further discussed here. 

The reaction of Os3(CO)j2 with the alumina surface is different from that observed with the silica surface. The first step is the oxidative addition of anA1—OH group into the Os—Os bond which gives the species (H)Os3(CO)io(OA1c ). However it is much less stable than on silica it decomposes above 150 ""C to give mononuclear Os(II) species with liberation of 3 moles of H2. In contrast to what is observed on silica, these surface species are linked to silica by two covalent bonds Os(OAlC)2. Those osmium(II) carbonyl complexes are not easily reduced by H2 to metallic osmium. In contrast to (H)Os(CO)3(OSi ), the Os(CO)3(OAl C) species is not a catalyst for olefin hydroformylation. In order to achieve a reduction to Os metal, it is necessary to reduce Os(II) with H2 at 400 for 20 hours. The metallic particles which are thus obtained are much smaller than on silica since their size is situated around 8 A. 

Metal-metal bonds and covalent atomic radii of transition metals in their n-complexes and polynuclear carbonyls. B. P. Biryukov and Y. T. Struchkov, Russ. Chem. Rev. (Engl. Transl.), 1970, 39,... 

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