Complexes of the type Mo(alkyne)(DIPP)2Cl2, Mo(alkyne)(DIPP)3Cl, and Mo(alkyne)(DIPP)4 (DIPP = O-2,6-C6H3-;-Pr2; alkyne = EtC=CEt or PhC=CPh) have been prepared by adding DIPP salts to Mo(alkyne)Cl4(ether). Mo(C2Ph2)(DIPP)4 crystallizes in the space group P2{2{2 with a = 12.464 (3) Á, b = 19.230 (4) Á, c = 12.268 (2) Á, V = 2940.4 A1 23, Z = 2, mol wt = 983.2, and p(calcd) = 1.110 g cm"3. It is a crowded, distorted-trigonal-bipyramidal molecule in which the alkyne is bound in an equatorial position and almost lies in the equatorial plane. It is assumed that the alkyne is bound in a similar position in the other two molecules. Steric crowding is evidenced by restricted rotation of axial DIPP ligands in Mo(alkyne)(DIPP)3Cl and Mo(alkyne)(DIPP)4, but not Mo(alkyne)(DIPP)2Cl2. Complexes of the type Mo(alkyne)(TIPT)4 (TIPT = "S-2,4,6-C6H2-i-Pr3) have been prepared either by adding LiTIPT to Mo(alkyne)Cl4(ether) (alkyne = MeC=CMe or EtC=CEt) or by adding alkyne to Mo(TIPT)4 (alkyne = MeC=CMe, EtC=CEt, or HC=CH). Mo(C2Et2)(TIPT)4 crystallizes in the space group P2,/n With a = 13.889 (4) A, b = 22.475 (5) A ,c = 22.041 (5) A, ß = 96.97 (2)°, V= 6829.4 A3, Z = 4, mol wt = 1119.9, and p(calcd) = 1.089 g cm"3. It is a crowded trigonal-bipyramidal complex in which the 3-hexyne is bound in an axial position and is constrained to lie along the Mo-S bond of the unique equatorial thiolate ligand. The phenyl rings of the other equatorial thiolate ligands are tilted up around the other end of the 3-hexyne ligand. The three different thiolate ligands do not interconvert rapidly on the NMR time scale at 25 °C, and the rotation of the alkyne about the Mo-alkyne bond axis is restricted. The alkyne ligand in Mo(alkyne)(TIPT)4 can be substituted by another (alkyne') in a reaction that is first order in Mo and zero order in alkyne' when the alkyne is ethyne. In contrast, the alkyne ligand in Mo(alkyne)(DIPP)4 complexes cannot be replaced by another simple alkyne. Electrochemical studies show that both Mo(alkyne)(DIPP)4 and Mo(alkyne)(TIPT)4 complexes can be reduced in dichloromethane, the former more easily than the latter by ~0.2 V. The reduction is an irreversible EC reaction in which only 0.5 electron is consumed; the reduction products could not be identified. Anodic waves can be observed in both the DIPP and TIPT systems, but the oxidations are also irreversible.of Research Resources, for Funds to purchase the X-ray diffraction equipment (NIH Grant S10 RR02243). E.R. thanks NATO for a postdoctoral fellowship. We thank Dr. Peter Bauerle for assistance with the electrochemical studies. Supplementary Material Available: For both structure determinations, tables of final differences and thermal parameters and figures showing fully labeled structures (6 pages); tables of observed and calculated structure factors (64 pages). Ordering information is given on any current masthead page.
