Ligand bending and tilted coordination in the coordinatively unsaturated NHC complex lateral-bis(N,N′-dineopentyl-benzimidazoline-2-ylidene)molybdenumtricarbonyl – Synthesis and structural investigations

Abstract Reaction of free dineopentyl-benzimidazoline-2-ylidene 1 (ligand: np 2 bimy) with (η 3 -cycloheptatriene)Mo(CO) 3 , molar ratio 3:1, provides the coordinatively unsaturated lateral -(np 2 bimy) 2 Mo(CO) 3 complex 2 . Crystal structure analysis of 2 reveals an almost ideal cis -square pyramidal geometry with tilted coordination of the metal, interplanar angles of the np 2 bimy ligands to the C 4 Mo plane of about 70° and significant bending of the N-heterocyclic carbene rings towards each other. Quantum chemical calculations (B97-D/6-31G(d)[Mo:cc-pVTZ-PP] level of theory) reproduce this peculiar structure and show it to be more stable by 36 kJ/mol than the second energy-minimum structure with trigonal bipyramidal geometry and the carbene ligands in the axial positions. Calculations indicated no significant Mo–C(II) back-bonding or interactions of the empty d-orbital with adjacent p-orbitals of the ligands, implying that the ligand distortions, tilted coordination and the occurrence of a free coordination site are caused by the spatial demand of the N-substituents.