In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)3] as well as seven-coordinate cationic hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3H]+, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)3] was accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M(PNP)(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)2Cl]+ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)2H]+ in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H2 and the coordinatively unsaturated intermediate [Fe(PNPH-iPr)(CO)2]+ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H2 to give cis-[Fe(PNP-iPr)(CO)2H]+. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D2 and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)2Cl]+. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex trans-[Fe(PNPMe-iPr)(CO)2Cl]+ was prepared, showing no reactions with Zn and H2 under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)2H]+ was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)2]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)2Cl]+ or [Fe(PNP-iPr)Cl2] with an excess of NaHg or a stoichiometric amount of KC8 in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)2] with HBF4 gave the hydride complex cis-[Fe(PNP-iPr)(CO)2H]+. X-ray structures of both cis-[Fe(PNP-iPr)(CO)2H]+ and [Fe(PNP-iPr)(CO)2] are presented.

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Several new octahedral iron(ii) complexes of the type [Fe(PN(R)-Ph)2X2] (X = Cl, Br; R = H, Me) containing bidentate PN(R)-Ph (R = H, Me) (1a,b) ligands based on 2-aminopyridine were prepared. (57)Fe Mössbauer spectroscopy and magnetization studies confirmed in all cases their high spin nature at room temperature with magnetic moments very close to 4.9μB reflecting the expected four unpaired d-electrons in all these compounds. While in the case of the PN(H)-Ph ligand an S = 2 to S = 0 spin crossover was observed at low temperatures, complexes with the N-methylated analog PN(Me)-Ph retain an S = 2 spin state also at low temperatures. Thus, [Fe(PN(H)-Ph)2X2] (2a,3a) and [Fe(PN(Me)-Ph)2X2] (2b,3b) adopt different geometries. In the first case a cis-Cl,P,N-arrangement seems to be most likely, as supported by various experimental data derived from (57)Fe Mössbauer spectroscopy, SQUID magnetometry, UV/Vis, Raman, and ESI-MS as well as DFT and TDDFT calculations, while in the case of the PN(Me)-Ph ligand a trans-Cl,P,N-configuration is adopted. The latter is also confirmed by X-ray crystallography. In contrast to [Fe(PN(Me)-Ph)2X2] (2b,3b), [Fe(PN(H)-Ph)2X2] (2a,3a) is labile and undergoes rearrangement reactions. In CH3OH, the diamagnetic dicationic complex [Fe(PN(H)-Ph)3](2+) (5) is formed via the intermediacy of cis-P,N-[Fe(κ(2)-P,N-PN(H)-Ph)2(κ(1)-P-PN(H)-Ph)(X)](+) (4a,b) where one PN ligand is coordinated in a κ(1)-P-fashion. In CH3CN the diamagnetic dicationic complex cis-N,P,N-[Fe(PN(H)-Ph)2(CH3CN)2](2+) (6) is formed as a major isomer where the two halide ligands are replaced by CH3CN.