Phosphinidene complexes, forms

More than 35 years ago, it was reported that the reaction of tri-methylphosphine with the cyclotetraphosphine (CF3P)4 affords the phosphine-phosphinidene complex, Me3P —> PCF3 (43). More recently, it has been demonstrated that stable carbenes are sufficiently nucleophilic to effect the depolymerization of cyclopolyphosphines and cyclopolyarsines to produce carbene-pnictinidene complexes 66-70, the first examples of p-block pnictinidenes (44, 45). Two extreme canonical forms, 71 and 72, can be written for such species. Structure 71... 

The cyclopolyphosphines that were used to prepare the carbene-phosphinidene complexes described earlier were formed by the reduction of higher oxidation state phosphorus compounds, typically di-chlorophosphines, RPC12 (47-49). However, in some cases a separate reduction step is not necessary and it is possible to prepare the car-bene-phosphinidene complex (74) directly by reaction of a stable nucleophilic carbene with RPC12 (44). 

Free 1,2,5-triphospha Dewar benzene derivative 95 is formed in two steps from 3 mol of 44a by oxidative decomplexation of the triphosphacyclohexa-l,4-diyl-2,5-diene ligand of the Hf complex 96 <1997CB1491>. An organometallic [2+1+1] cycloaddition reaction between the phosphinidene complex 97, 44a, and a coordinated CO gives access to the 1,2-diphosphacyclobutenone complex 98 (Scheme 31) <1998CEJ1917>. 

Phosphinidene complexes may considered as synthetic equivalents however, they have the disadvantage that the first formed phosphirane or phosphirene complexes must be transformed in an additional step to the desired uncomplexcd phosphorus heterocycles. The complexed phosphiranes and phosphirenes can be easily decomplexed by several methods Scheme 8 gives an overview <1996CHEC-II(I)297, 1995S941, 1997JA8432>. Initially halogena-tion, for example, with I2, followed by displacement of the metal fragment by A -methylimidazole or 2,2 -biphenyl or decomplexation with l,2-bis(diphenylphosphino)ethane, can be performed. 

As briefly indicated in Scheme 31, 377-azaphosphirene complexes also serve as sources for reactive W(CO)s-phosphinidene complexes under mild conditions, and may be intercepted by phosphaalkynes. Isolable diphosphirene complexes 87c and 87d were formed when the thermolysis of the precursors 86 [R = CsMes, CH(TMS)2] was performed in the presence of P=CN(Pr )TMS (Scheme 36) <1995CC2113, 1997PS545>. 

Trapping electrophilic phosphinidene complexes with alkynes has been used in the synthesis of the new phosphirene systems (341) and (342). In the latter, the phosphirene rings are coplanar and conjugated. The diphosphinines (343) are formed unexpectedly in a head to head dimerisation of l.ff-phosphir-... 

So far, much of the discussion has focussed on the bonding of phosphinidene units to aluminum, gallium, or indium. However, we are also interested in the coordination of phosphinidenes to transition metals - particularly in cases where the phosphinidene unit coordinates in a terminal fashion. As indicated below, terminal phosphinidene complexes, like imido complexes, can exist in angular or linear forms. 

Some terminal phosphinidene complexes may exist in equilibrium with ring forms. 

In 164, no Mn—Mn bond is present and formal double bonding exists between the Mn and Te atoms. The propensity of the 5d transition metals to form M—M bonds may be cited as a reason for these differences, and the situation is similar to the valence isomerism observed in phosphinidene (RP) and sulfenium (SR ) complexes (and their respective heavier congeners) (158-160), in which two types of structures termed open (C) and closed (D) are observed (ML = 16-electron transition metal fragment). 

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