Stereochemistry and bonding in nickel  olefin complexes. The structure of t etramethylethylenenickel 1,2-bis (dicyclohexylphosphino)ethane

Abstract

The structure of tetramethylethylenenickel 1,2-bis(dicyclohexylphosphino) ethane, (CH3)2C2(CH3)2Ni[(C6H11)2PCH2CH2P(C6H11)2], has been determined from 4995 observed, absorption-corrected reflections collected by counter methods. The compound crystallizes in the monoclinic space group C52h  P21/n with cell dimensions a 17.1306(7), b 16.9016(7), c 11.4899(8)» and > 100.082(5)°. The calculated density is 1.147 g/cm3 for Z=4. Least-square refinement yielded a final conventional R value of 0.036. If one coordination site is assigned to the olefin, the nickel atom is three-coordinate. Although no crystallographic symmetry is required, the molecule posses idealized C2 symmetry. The average NiC and NiP bond lenghts are 1.981(2) and 2.156(6)», respectively. The central bond of the olefin has beenlengthened upon coordination to 1.421(3) ». The tetramethylethylene moiety is distorted from planarity with the C(CH3)2 planes bent away from the nickel atom 27.2(6)°. The nickel atom and four atoms bonded to it are not coplanar, the central bond of the tetramethylethylene species forming a 16.5° angle with the Ni,P(1),P(2) plane. A segmented rigid-body analysis of the thermal motion of the molecule indicates that the barrier to rotation of the olefin is 18 kcal/mole.