Monocyclopentadienyl complexes of Mo(II) with 16-and 18-electron configurations of the form (Ring)MoClL x (x ) 2, Ring ) Cp, L ) PMe 2 Ph; x ) 2, Ring ) Cp*, L ) PMe 3 , PMe 2 Ph, L 2 ) dppe; x ) 3, Ring ) Cp, L ) PMe 2 Ph) are described. All of the 16-electron complexes are paramagnetic with an S ) 1 ground state, as shown by magnetic measurements in the solid state and in solution, and by the contact-shifted 1 H NMR spectra. The structure of Cp*MoCl(dppe) was determined by X-ray diffraction methods. The 18-electron complex CpMoCl(PMe 2 Ph) 3 has been synthesized by reduction of {CpMoCl 2 } n with Na in the presence of 3 equiv of phosphine. It has been fully characterized by 1 H and 31 P NMR, chemical analysis, and X-ray structural determination. Thermolysis of a THF or C 6 D 6 solution of this 18-electron species generates the 16-electron paramagnetic Mo(II) complex CpMoCl-(PMe 2 Ph) 2 . The Cp*MoClL 2 (L ) PMe 3 and PMe 2 Ph) systems react with 2-electron-donor ligands, i.e. CO and H 2 , to afford stable 18-electron complexes. The carbonyl derivative Cp*MoCl(CO)(PMe 2 Ph) 2 has also been characterized by X-ray crystallography.

The first examples of stable, mononuclear 17-electron carbonyl complexes of Mo(III) have been synthesized, isolated, and characterized by IR and EPR spectroscopy. Oxidation of Cp*MoCl(CO)(PMe3)2 (1; E 1/2 = −0.48 V vs Fc/Fc+), Cp*MoCl(CO)(dppe) (2; E 1/2 = −0.44 V), and CpMoCl(CO)(dppe) (3; E 1/2 = −0.25 V) with Fc+PF6 - yields [1]PF6, [2]PF6, and [3]PF6, respectively. The IR stretching vibration of the 17-electron oxidation products are 136−153 cm-1 blue-shifted with respect to the corresponding stretching vibrations of the parent Mo(II) compounds. The room temperature EPR spectra show observable coupling to the Mo and P nuclei and indicate a trans geometry for 1 + and a cis geometry for 2 + and 3 +. The four-legged piano stool geometry of 2 + with the phosphines atoms in relative cis positions has been confirmed by a single-crystal X-ray analysis. The X-ray data for [2]PF6·THF are the following:  monoclinic, P21/n, a = 13.7394(14) Å, b = 20.421(2) Å, c = 14.857(2) Å, β = 99.119(8)°, V = 4115.8(8) Å3, Z = 4, D x = 1.469 g·cm-3, λ(Mo Kα) = 0.71073 Å, μ (Mo Kα) = 0.562 mm-1, R(F) = 4.59%, R(wF 2) = 10.77% for 4299 data with F o > 4σ(F o). The thermal stability of the cations decreases with increasing carbonyl stretching frequencies in the order 1 + > 2 + ≫ 3 +. The decarbonylated product of thermolysis of 3 +, [CpMoClF(dppe)(MeCN)]+PF6 -, [4]PF6, which is believed to arise via a 15-electron [CpMoCl(dppe)]+ intermediate, has been isolated and characterized by 1H, 31P, and 19F NMR spectroscopies as well as a single-crystal X-ray analysis. The structure of 4 + shows a distorted pseudooctahedral geometry with the Cp ring and the F atom occupying the two axial coordination sites and the two phosphine atoms of the dppe occupying two cis equatorial sites. The data for [4]PF6·MeCN are as follows:  monoclinic, P21/n, a = 13.160(2) Å, b = 10.942(2) Å, c = 25.010(3) Å, β = 95.225(10)°, V = 3586.4(9) Å3, Z = 4, D x = 1.557 g·cm-3, μ(Mo Kα) = 0.71073 Å, R(F) = 4.11%, R(wF 2) = 11.81% for 5964 data with F o > 4σ(F o). The nucleophilic addition of Cl- to 1 + , 2 + , or 3 + is followed by redox processes to afford mixtures of the parent Mo(II) carbonyl complexes 1−3 and Mo(IV) or Mo(V) products, depending on the nature of the ligands. The mechanism of these reactions has been elucidated through parallel chemical and electrochemical studies.