Transition metal complexes with weakly bonded anions

Solvated transition metal cations containing weakly bonded organic ligands are potentially useful as intermediates in preparative coordination chemistry. The coordinated solvent molecules provide solubility of the compounds in various organic solvents, and they can be replaced readily by other ligands with better donor properties.1, 2 The preparation of mononuclear transition metal complexes with weakly bonded anions has been described previously3 and preparative applications of such complexes have been reviewed.4... 

Transition metal complexes with weakly bonded anions, 26 92 Transition metal polyoxo anions, 27 71 Tributylammonium pentatungsto-bis(phenylphosphonate)(4 -), [[(C,H,),NH],[W,O,(QH,PO0 l], 27 127... 

MONONUCLEAR TRANSITION METAL COMPLEXES Part II COMPLEXES WITH WEAKLY BONDED ANIONS... 

Metal carbon insertions of C02 occur in both main group and transition metal complexes.5,19 The reactions result in a strong M—O bond at the expense of a weak M—C bond (together with a C C a bond at the expense of a C O n bond). Insertion into t/weo-W(CO)5 CHDCHDPh gave the threo carboxylate, indicating retention of configuration at the o-carbon, as with CO insertion.29 Cis- RW(CO)4L (R = Me, Et or Ph) showed second-order kinetics toward C02 insertion, first order in anion and in C02. Replacement of CO by a phosphine or phosphite increased the insertion rate.30 Palladium catalysis of C02 insertion into unreactive Sn C bonds in allyl stannanes provided three isomeric tetra-carboxylates from tetraallyl tin. Attempted reactions of tetraalkyl, vinyl or aryl tin complexes did not proceed. Insertion into an intermediate rf -allyl palladium complex was suggested to lead to carboxylate products.31,32... 

Most of the reactions of PH3 with the carbonyls of the transition metals and with a variety of carbonyl derivatives can essentially be classified according to three main types of reactions (i) substitution of CO ligands, of other neutral Ti-acceptor ligands like, e.g. PR3, weak 0-, S-, N-donor ligands, and alkenes, or of anionic ligands, (ii) cleavage of metal-metal bonds, and (iii) oxidative addition. Only in a few cases combinations of two or three of these reaction types or completely different reactions are observed. Most reviews dealing with the chemistry of phosphanes, with complexes of transition metals, or with transition metal carbonyls and derivatives cover only some aspects of the coordination chemistry of PH3 see e.g. [1,4, 14, 16, 22, 24, 27 to 29]. 

These systems were extensively studied with the goal of finding even better, more efficient catalysts. Based on these studies it appears that the key attributes for high activity appear to be three-fold (1) the transition metal center is complexed only by metal olefin Tt-bonds and a metal-carbon rr-bond (which can be part of an allylic system), (2) the metal center is cationic, and (3) the counter-ion is a weakly coordinating anion. 

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