Chemistry and properties of electron-rich bis(phosphine) dicarbonyl molybdenum compounds

Abstract

The easy displacement of two ligands from Mo(CO) 2(MeCN) 2(PR3) 2 and Mo(CO) 2(MeCN)(PBu 3)(PR 3) permits the synthesis of several new types of complex, whose properties are dominated by the strong back-donation of the Mo(CO) 2(PR 3) 2 moiety. Complexes with aromatic nitriles in place of MeCN are new and unexpected chromophores with only one intense charge transfer absorption band in the visible region. This band and the CN and CO stretching frequencies are solvatochromic. α, β-unsaturated nitriles form similar labile complexes which in several cases give π(C = C)/α(N) doubly bridged dimers. Structural data for the acrylonitrile dimer are given. Four-electron ligands are as easily introduced as in Fe(CO) 3 complexes. The butadiene and the cyclo-octatetraene complex, which is still fluxional at −80 °C, are representative examples for carbon τ systems. The 1-azadiene complexes Mo(CO) 2 -(RN:CHCH: CHR') (PR 3) 2 are protonated by strong acid in different ways, depending on the coordination ability of the acid anion. N-protonation occurs with H 2SO 4 and HClO 4; HX (X = Cl, Br, I) probably prefer C-protonation (and coordination of X − to the metal). Although o-quinones generally oxidise metal carbonyls to diolate complexes, phenanthrenequinone and other 1,2-dicarbonyls can be coordinated to the Mo(CO) 2(PBu 3) 2 moiety. Dithio-oxamides, RR'NC(S)C(S)NRR', show interesting behaviour towards the electron-rich metal. Dithiooxamide ligands (DTO) are poor acceptors in Mo(CO) 4(DTO) or Mo(CO) 3(PR 3)(DTO), but in Mo(CO) 2(PR 3) 2(DTO) dithio-oxamides behave like dithiolene ligands and the acceptor properties of the DTO are similar to those of phenanthrenequinone. The change in ligand properties is discussed on the basis of an electronically induced conformational change.