Phosphorus-transition metal triple bonds

In 1995 a breakthrough occurred in this field in February we were able to show the synthesis and the spectroscopic properties of the first complexes of type B [7], and in August the first isolated and structurally characterized complexes containing terminal metal-phosphorus triple bonds (type A) were independently obtained and published in back-to-back articles by the groups of Cummins [8] and Schrock [9]. Since then, a rapid development has occurred in the synthesis and particularly in the study of the reactivity pattern of complexes with phosphorus-transition metal triple bonds. This review chapter will highlight the development in this field by giving an overview from 1995 until the current stage of research. 

By writing about complexes containing triple bonds between phosphorus and transition metals, one has to take into account the triple-bond character of phosphinidene complexes which are in a nearly linear coordination mode (type C) in contrast to the usual bent coordination mode D possessing typical double-bond features. Due to the additional r-donation bonding ability of the PR moiety to the metal atom in type C and the observed bond lengths, this type of complexes has to be included into the classes of metal-phosphorus triple bond compounds. Thus, at the end of this review will appear a chapter highlighting the appropriate compounds of type C. 

The synthesis of stable complexes with transition metal-phosphorus triple bonds is of fundamental importance and opens a novel chapter of a special field of coordination chemistry. The synthesis of analogous complexes with ligands of the heavier homologues like arsenic has partially been carried out [6], while for antimony and bismuth, the elusive M=Sb and M=Bi systems have now moved within reach. Moreover, the experimental and theoretical... 

The preparation of stable complexes with transition metal-phosphorus triple bonds is of fundamental importance and constitutes a novel field of coordination chemistry. The contribution of B.P. Johnson, G. Balazs, and M. Scheer reports the synthesis and isolation of such complexes for transition metals in high oxidation states in contrast, the corresponding complexes with transition metals in low oxidation states were found to exist only as highly reactive intermediates. A synthetic concept to generate directly such intermediates is documented. The present knowledge of the reactivity pattern of all these types of compounds is summarized in chapter 1. 

The extent of metal-P coordination is reflected in the Raman C=C stretch frequency [72]. Uncoordinated dppa has a lower v(C=C) of 2,097 cm than most disubstituted acetylenes (v = 2,190-2,260 cm ) as the conjugated phosphorus lone pair causes 7i-electron depletion from the triple bond (Fig. 2.18). Transition metal complexation suppresses this effect. The magnitude of Av(C=C), i.e., v(complexed dppa) — v(free dppa), then reflects the degree of P M a-bonding. Electron depletion of the triple bond has been inadvertently attributed to the contribution of P(d,7i) orbitals, as well as M P 7r-bonding to the increase in Av(C=C) [72]. 

Abstract In the present chapter we discuss transition-metal-catalyzed phosphorus-hydrogen (P-H) bond addition to the triple bond of alkynes and to the double bond of alkenes, dienes, imines, aldehydes and ketones. Main attention is paid to highlight the factors responsible for development of highly efficient catalytic systems and to carry out the addition reaction with high stereo-, regio- and enantioselectivity. 

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