Coordination complexes stepwise

Cyanide anation in DMSO has proved useful in the preparation of cyano complexes (Scheme 29). 25 Cyanide displaces coordinated DMSO cleanly but deprotonates rather than displaces coordinated water. The preparation of trans- and ds-[Cr(CN)2(NH3)4]C104 starts from [Cr(DMSO)2(NH3)4]3+ or [CrCl(DMSO)(NH3)4]2+. The insolubility of trans-[Cr(CN)2(NH3)4]C104 permitted its isolation from solution, but it was necessary to separate the cis isomer by ion exchange. Rapid trans to cis isomerization of bww-[Cr(DMSO)2(NH3)4]3+ occurs on the addition of CNT before anation produces the dicyano complexes stepwise, both apparently via c -[CrCN(DMSO)(NH3)4]2+, which in an uncommon rearrangement gives a mixture of trans- and cis-dicyano complexes. Substitution of DMSO by CN" in water is stereoretentive. 

Cr(CO)6 react with phthalonitrile in hot 1-chloronaphthalene to precipitate the ft form of CrnPc which is readily oxidized to CrI11(Pc)OH in air.197,198 The hydroxide coordinates other ligands to form six-coordinate complexes, Crni(Pc)(OH)X (X = ROH, H20, CN, SCN, SeCN). The hydroxy group is substituted by CN in cone. KCN solution and Cri PcXChOj - is formed. Acetic anhydride acetylates Cr(Pc)(OH)(OH2) in a stepwise fashion to Cr(Pc)(OAc)(OH2) and Cr(Pc)(OAc)(HOAc). 

FIGURE 4.14 Stepwise addition of O2 to four- and five-coordinate complexes of... 

The role of protonic acids in the stepwise oxidative addition of five-coordinate complexes is well illustrated by reactions of acids with the osmium(O) complex (XXXV). Hexafluorophosphonic and tetrafluoroboric acids form stable adducts (XXXVla) and (XXXVIb) which have been characterized as 1 1 electrolytes (39, 103). Similar cationic adducts (XXXVIc) have been formed by reaction of Os(CO)3(PPh3)2 with dry HCl and HBr gas [103). The osmium(II) hydrido cation (XXXVI) possesses trans CO groups (marked with an asterisk) which are mutually labilized through the trans effect. In the presence of a nucleophilic conjugate anion such as bromide one of these CO groups is replaced to yield a neutral adduct... 

For this weak ligand a stepwise mechanism consisting in the MeCN dissociation followed by the coupling from the resulting three-coordinated complex was predicted. This value corresponds to the overall energy barrier for the reductive elimination reaction through this mechanism... 

Because transition metals even in a finely-divided state do not readily combine with CO, various metal salts have been used to synthesize metal carbonyls. Metal salts almost always contain the metal in a higher oxidation state than the resulting carbonyl complex. Therefore, most metal carbonyls result from the reduction of the metal in the starting material. Such a process has been referred to as reductive carbonylation. Although detailed mechanistic studies ate lacking, the process probably proceeds through stepwise reduction of the metal with simultaneous coordination of CO (90). 

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