METAL CLUSTER STABILIZED 2-BORNYL CATIONS - A SYNTHETIC, X-RAY CRYSTALLOGRAPHIC, AND EHMO STUDY

Metal cluster derivatives of the 2-bornyl cation, viz. (2-propynylborn yl)(M1-M2), where M1 and M2 can be (C5H5)MO(CO)2 or CO(CO)3, have been synthesized by protonation of suitable precursors. Unlike the uncompl exed 2-alkynylbornyl cations, these metal-stabilized systems do not un dergo the Wagner-Meerwein rearrangement to the corresponding 4-alkynyl camphenyl cations. [(2-propynylbornyl)Mo2(CO)4(C5H5)2][BF4], 22, cryst allizes in the orthorhombic space group P2(1)2(1)2(1) with a = 8.557(2 ) angstrom, b = 12.074(3) angstrom, c = 25.888(4) angstrom, and V = 26 75(1) angstrom3 for Z = 4. [(2-propynylbornyl)Mo(CO)2(C5H5)-Co(CO)3][B F4], 23, crystallizes in the orthorhombic space group P2(1)2(1)2(1) wi th a = 12.647(2) angstrom, b = 13.874(2) angstrom, c = 14.040(2) angst rom, and V = 2463.6(6) angstrom3 for Z = 4. In both cases, the 2-borny l cation leans toward a molybdenum atom such that the Mo-C+ distance i s 2.74 angstrom for 22 and 2.91 angstrom for 23. The preferential stab ilization of the cation by the molybdenum vertex rather than by the tr icarbonylcobalt fragment is rationalized by means of molecular orbital calculations at the extended Huckel level.