O-Bond complexes

Spectroscopy at variable temperatures enables us to reveal linkage isomerism of adsorption, when certain molecule fonu with the same site two or more complexes with different geometry and chemical properties. The most studied so far is the case of CO in zeolites, when besides the usual C-bonded complexes with the cations or OH-groups, energetically unfavorable O-bonded complexes ai e formed. 

It was suggested that this change in product distribution was due to the existence of an equilibrium between two types of complex, viz., a cr-butenyl-pentacyanocobaltate(III) and a 7r-butenyltetracyanocobaltate(III) 107, 109). However, further study of the kinetics and product distribution suggested the presence of two o-bonded complexes, viz., cr-but-l-en-3-yl and a-but-2-en-l-yl 24a). Direct evidence for the existence of a cyanide-dependent equilibrium between the a- and rr-bonded organocyanide complexes has been obtained from NMR studies of the complex prepared by the reaction of allyl halides with Co—H 109) (see also Section V,C). Both butadiene and crotyl chloride react with Co—H to give the same... 

The same allylic species can be formed from the related 1,3-diene and indeed cw,ci5-l,3-cyclodecadiene yields at least 38% tmns-cyclodecene although the saturation of one double bond which did not involve its neighbor should obviously yield the more stable cis isomer. The result implies that the allylic intermediate, which would be formed initially in the cis,cis conformation, has time to rearrange on the siurface to its more stable cis,trans configuration before it reacts further with hydrogen. The possibility that the ct ,cts-l,3-cyclodecadiene isomerizes first to the less stable cis,trans-diene was discounted. The rearrangement of the allylic intermediate could be accomplished via a o-bonded complex, as indicated on page 142 of Section III,C. 

Overall, the UV-visible, NMR, and electrochemical data for the range of O -bonded complexes of gallium, indium, and thallium porphyrins show that there is a trend from pure a -bonded character of the M—C bond in the alkyl complexes to the much more ionic character observed for the fluorophenyl and acetylide complexes. These more ionic complexes show NMR and UV-visible characteristics more like those of the ionic M(Por)Cl or M(Por)C104 complexes. ... 

In more acidic solutions, monodentate O-bonded complexes are formed.1177,1175 1178 Ring closure is rapid if the pH is raised y, 6 and e amino acid complexes do not undergo ring closure.1179... 

Alternatively, it may occur by 1,2-elimination of HPdCl followed by reverse readdition and reductive elimination. Moiseev and Vargaftik533 prepared the o-bonded complex independently by reaction of PdCl2 with 3-chloromercuri-ethanol and showed that it afforded acetaldehyde on decomposition ... 

The reaction probably follows a course similar to that taken in aqueous solution. In acetic acid, nucleophilic attack of coordinated acetate occurs with simultaneous rearrangement to a o-bonded complex which subsequently decomposes to products, presumably via a 0-hydrogen elimination ... 

Tris(pyrazolyl)borates have been reported also in two chapters on B-H120 and X-H121 activation (X = Si, Ge, Sn) in the book Metal Dihydrogen and o-Bond Complexes, edited by Kubas. 

Coordination and activation of B-H and other X-H and X-Y bonds, in Metal Dihydrogen and o-Bond Complexes, Kubas (ed.), SpringerLink, pp. 417-439, Chap. 13, 2002. 

Transition metal complexes can be classified into mononuclear and dinuclear (or polynuclear) complexes. Depending on the effective atomic number of the metal and the kind of ligands, the complexes can be mononuclear like Fe(CO)s or dinuclear like Co2(CO)g. The characteristic feature of dinuclear metal complexes is that they have a metal-metal bond in their structure. The metal-metal bond is cleaved oxidatively by the reaction with some covalent molecules to form a-bonded complexes. This reaction is regarded as a one-electron oxidation. This is another way of forming o-bonded complexes and is useful for organic synthesis. 

The likelihood that alkyl radicals, like their aryl counterparts, bind to the iron before shifting to the nitrogen is supported by the observation that the type II complexes formed between alkyldiazenes and P450 in the absence of oxygen are converted, in the presence of limited amounts of oxygen, to complexes with an absorption maximum at 480 nm characteristic of iron-carbon o-bonded complexes Furthermore, alkyl diazene-iron tetraphenylporphyrin complexes can be prepared under anaerobic conditions. However, the alkyl-iron complexes are much less stable and less well characterized than the aryl-iron complexes and their involvement in heme iV-alkylation reactions remains to be demonstrated. 

As discussed in Sec. 111-15, the mode of coordination of the pseudohalide ion can be determined by vibrational spectroscopy. Burmeister et al. " found that all NCS and NCSe groups are N-bonded in M(Cp) X2-type compounds, where M is Ti, Zr, Hf, or V, and X is NCS or NCSe. In the case of analogous NCO complexes, Ti, Zr, and Hf form O-bonded complexes, whereas V forms an N-bonded complex. Later, Jensen et al suggested the N-bonded structure for the titanium complex. 

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