Tetracarbonyl diazaphosphole complexes of group 6B metals: the role of steric effects

Abstract

Thermal substitution reactions of [M(CO)₅L¹_c] with cis diazaphosphole derivatives (L¹= 3,4-dihydro-2,3,4,5-tetraphenyl-2H-1,2,3-diazaphosphole, L¹_c and L¹_t denote cis and trans L¹ respectively) have yielded a mixture of cis and trans(2 : 3 ratio) tetracarbonyl complexes in which the ligands are bonded through the phosphorus atom. In contrast the reactions of [M(CO)₅L¹_t] with the bulkier trans diazaphosphole isomers (L¹_t or L²_t, L²= 5-benzyl-3,4-dihydro-2,3,4-triphenyl-2H-1,2,3-diazaphosphole) have only afforded the trans disubstituted tetracarbonyl [M(CO)₄(L_t)₂]. On the other hand, L¹_t and L²_t have reacted with [M(CO)₅L¹_c] affording the mixed-phosphine derivatives trans-[M(CO)₄L¹_cL_t](L_t= L¹_t or L²_t). The norbornadiene complex [Mo(CO)₄(nbd)] produced only cis-[Mo(CO)₄(L¹_c)₂] when treated with L¹_c in cyclohexane at room temperature, whereas with L¹_t no tetracarbonyl complexes (cis or trans) have been observed. In addition it has been found that cis(or trans)-[Mo(CO)₄(L¹_c)₂] thermally interconverts by a dissociative pathway to the trans(or cis)-[Mo(CO)₄-(L¹_c)₂] to give an equilibrium composition (cis : trans= 2 : 3) whereas trans-[Mo(CO)₄(L_t)₂] did not isomerize. All these different stereochemical results are tentatively explained by invoking fluxional or rigid five-co-ordinate intermediates in which only steric factors play a decisive role in determining the geometry and then the products distribution. The nature of the complexes has been established essentially by i.r. and ¹H n.m.r. spectra. An X-ray study on cis-[Mo(CO)₄(L¹_c)₂] is also reported.