Ligand exchange/substitution associative

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

A main objective of this work is to develop the relationship between the many reaction pathways leading to ligand substitution at square-planar molecules. Concentrating on more recent results to illustrate the processes under discussion, we examine in detail the evidence for operation of the less common and sometimes controversial routes such as dissociative ligand exchange (6). It cannot be stressed too much, however, that the field is still dominated by associative reactions, so to maintain a balance, as well as to provide the now necessary comparative evidence, we also cover the essential features of nucleophilic ligand replacements. 

The same concept applies to substitution reactions (e.g., ligand exchange) in metal complexes, for example, trans [PtCl2(py)2] + 2NH3 trans [PtCl2(NH3)2] -F 2 py. The same two substitution mechanisms are operative (see Section 6.9). Coordination chemists speak of associative (or adjunctive) mechanisms and of dissociative (or disjunctive) mechanisms. 

Carbonyl replacement dominates the observed photoreactions of Co complexes, and with CO-like ligands, total substitution is possible. Table 1 lists examples. The complex CofCOjNO is unusual in that the primary photoprocess is a linear to bent NO transformation, followed by an associative displacement of CO from this shortlived intermediate. Photolysis in CHClj leads to incorporation of chloride in the complex if PPhj is present, then Co(PPh3)2Cl2 is formed " if NO is present then [Co(NO)2C1 j is generated Trialkylsilane exchange in RjSiCo(CO) (Table 1) proceeds via oxidative addition of RjSiH to the intermediate formed by CO loss, followed by reductive elimination of R SiH. This is true for surface-confined analogues as well d. 

Square-planar Exchange in square-planar complexes is associative, and is influenced by the trans effect, complexes whereby some ligands facilitate substitution of trans ligands. 

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

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