Alkyne ligands as three-electron donors: crystal structures of [Mo(CO)(MeC₂Me)₂(η-C₅H₅)][BF₄] and [Mo(NCMe)(MeC₂Me)₂(η-C₅H₅)][BF₄]

The cations [Mo(CO)(MeC2Me)2(η-C5H5)]+ and [Mo(NCMe)(MeC2Me)2(η-C5H5)]+ crystallise with ‘three-legged piano stool’ geometry, the three legs comprising the two bonds between Mo and the centre of the alkyne together with the bond to either CO or MeCN. For both species, and in accordance with theoretical considerations (which are discussed), the alkyne bonds lie parallel to the Mo–CO or Mo–NCMe directions, and the alkyne ligands function formally as three-electron donors. For the carbonyl species the bonding of the alkyne to the metal is slightly asymmetric [Mo–C 2.072(4) and 2.130(4)A, as compared with 2.060(4) and 2.068(4) in the acetonitrile complex] and these bond distances also reflect the expected tighter binding in the acetonitrile complex. Crystals of [Mo(CO)(MeC2Me)2(η-C5H5)][BF4](1) are monoclinic, space group P21/c(no. 14) with Z= 4 in a unit cell of dimensions a= 8.994(4), b= 14.059(8), c= 13.255(5)A, β= 116.23(3)°. The structure has been refined to R 0.048 (R′ 0.052) for 2 144 reflections at 220 K. Crystals of the complex [Mo(NCMe)(MeC2Me)2(η-C5H5)][BF4](2) are orthorhombic, space group P212121(no. 19), with Z= 4 in a unit cell of dimensions a= 8.780(4), b= 11.656(4), c= 16.225(6)A, R 0.032 (R′ 0.032) for 2 466 reflections at 200 K.