Transition‐Metal Carbonyl Complexes and Electron‐Donating Properties of N‐Heterocyclic‐Carbene–Phosphinidene Adducts

Rhodium(I), tungsten(0), and molybdenum(0) carbonyl complexes of the N‐heterocyclic‐carbene–phosphinidene adducts IPr · PR [ 1a , R = H; 1b , R = Ph; 1c , R = Mes; IPr = 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene; Mes = 2,4,6‐trimethylphenyl] were prepared. The reaction of 1b with [Rh(µ‐Cl)(CO) 2 ] 2 afforded the dicarbonyl rhodium(I) complex cis ‐[(IPr · PPh)RhCl(CO) 2 ] ( 2 ) as the major product together with the tetranuclear complex [{µ‐(IPr · PPh)} 2 Rh 4 (µ‐Cl) 4 (CO) 4 ] ( 3 ). The latter was characterized by X‐ray diffraction analysis. The tungsten pentacarbonyl complexes [(IPr · PR)W(CO) 5 ] ( 4a , R = H; 4b , R = Ph; 4c , R = Mes) were isolated from the reactions of 1a – 1c with [(Me 3 N)W(CO) 5 ], whereas the reactions of 1a and 1b with [(thf)Mo(CO) 5 ] (thf = tetrahydrofuran) gave the corresponding molybdenum complexes [(IPr · PR)Mo(CO) 5 ] ( 5a , R = H; 5b , R = Ph). The molecular structures of the five carbonyl complexes were established by X‐ray diffraction analyses. The IR spectroscopic analysis of the CO stretching frequencies of the carbonyl complexes 2 , 4 , and 5 revealed the strong electron‐donating abilities of the phosphorus(I) ligands.