The five-coordinate tripodCobalt(II) compound [tripodCo(II)Cl(2)] (2) [tripod = CH3C(CH2PPh2)(3)] undergoes selective substitution of its chlorine groups on activation with KPF6 in the first substitution step and by TlPF6 in the second. Compounds of the type [tripodCo(II)LL'](2+) with two nitrile (3(2+)) or isonitrile (5(2+)) ligands are obtained by this route. Compounds [tripodCo(II)L(2)] containing two equal ligands L are also accessible from Co-aq(II) as the starting material. The syntheses of [tripodCo(II)(CNR)(2)](2+) (5(2+)) and [tripodCo(II)X(2)] (X = CN, NCO, NCS) (6) are reported. Different routes to MpodCo(I) derivatives are described. Depending on the nature of the co-ligands, four-coordinate - [tripodCo(I)NCO] (7) - or five-coordinate - [tripodCo(I)(CNR)(2)](+) (5(+)) - compounds are obtained. Several high-yield syntheses of [tripodCo(I)-alkyne](+) (8(+)) compounds are reported. The compounds are characterised by the usual analytical and spectroscopic techniques including X-ray analysis of selected examples.

Tripod metal entities tripodM are sterically congested systems. The conformations adopted by compounds CH3C(CH2PPh2)3–n[CH2P(oTol)2]nMo(CO)3 (n = 1: 1, n = 2: 2, n = 3: 3) will thus be largely determined by the repulsive forces acting in these molecules. The steric demand of the o-tolyl groups impedes their free rotation and enantiomerization processes referring to the compounds as a whole are sufficiently slow to permit their analysis by NMR techniques. Through a combination of line-shape analysis, EXSY methods, and coalescence experiments, the ΔG‡ values for these conformational enantiomerization processes have been determined as ΔG‡298K = 54.3, 57.9, 65.5 kJ·mol–1 for compounds 1, 2, and 3, respectively. By an exhaustive search on a force field generated hypersurface, activation energies of 53, 57 and 69 kJ·mol–1 have been calculated. Thus, the force field approach correctly reproduces the dependence of the activation energy on the degree of o-tolyl substitution. Moreover, the force field simulation also gives an insight into the individual microsteps of the enantiomerization pathways.

Various efficient strategies for the preparation of tripod ligands with mixed P/S donor sets such as H3CC(CH2PPh2)(CH2SR) (CH2SR′) (6) or three different S donors H3CC(CH2SR)(CH2SR′)(CH2SR″) (5) are described. Compounds 5 are capable of acting as tridentate chelate ligands in complexes such as [H3CC(CH2SEt)(CH2S1Pr)(CH2S1Bu)Mo(CO)3] (9b). Quite alike compounds 6 act as tripod ligands as evidenced by the isolation and structural characterization of three derivatives [H3CC(CH2PPh2)(CH2SR)(CH2SR′)Mo(CO)3] (11a, 11c, 11e). It is shown that under 1 bar CO substitution of one SR group by CO occurs to give [H3CC(CH2PPh2)(CH2SR)(CH2SR′)Mo(CO)4] (12). With 11a, [H3CC(CH2PPh2)(CH2SPh)(CH2SBn)Mo(CO)3], as the starting compound the carbonylation gives exclusively 12a with the CH2SPh group playing the role of the dangling arm. All the new compounds are fully characterized by NMR and mass spectrometric techniques as well as by IR spectra and cyclovoltammetric data and X-ray analysis for the coordination compounds.

Neopentane-derived tripod ligands of the general type HOCH2C(CH2PPh2)(CH2Y)(CH(2)Z) (1; Y, Z = PPh2, SR) are notoriously resistant to ether formation at their hydroxy group. Two routes have been found, which allow the transformation of 1 into ether functionalized tripod ligands ROCH2C(CH2PPh2)(CH2Y)(CH(2)Z) (Y = Z = PPh2: 5, Y = Z = SR: 8). One of these strategies relies upon the eta 3 coordination of 1 in 1 . Mo(CO)(3) (2). By this way the donor groups are efficiently protected and the steric encumbrance of the CH2OH group at the backbone of the ligands is greatly reduced by fixing three arms of the neopentane scaffolding to the metal center. After deprotonation, reaction with electrophiles will produce the corresponding ether derivatives ROCH2C(CH2PPh2) (CH(2)Z)(2) (3). Mesylation of 2 leads to MeSO2OCH2C(CH2PPh2)(3). Mo(CO)(3) (4), which reacts with alkoxides to produce 3 in a sequence of reversed polarity. Ligands 5 [ROCH2C(CH2PPh2)(3)] are liberated from 3 by UV irridation of their solutions in the presence of pyridine N-oxide. Direct etherification of 1 is also possible in some cases after deprotonation of 1 by KOtBu and subsequent reaction with an electrophile RX in the narrow temperature range between -10 and +20 degrees C. By this way, omega-methyl polyglycol ether functions are easily introduced resulting in H3C(OC2H4)(n)OCH2C(CH2PPh2)(3) (5g, h).

Tripod ligands RCH2C(CH2PPh2)(CH2X)(CH2Y) (1) with X, Y = SAlkyl, SBzl selectively undergo reductive cleavage of the SBzl function with Li/NH3/THF at –40 °C to produce RCH2C(CH2PPh2)(CH2X)(CH2Y) (2) (X, Y = SAlkyl, SH). In these mixed donor set ligands the SH functions are the least coordinative ones such that [η2-{CH3C(CH2PPh2)(CH2SEt)(CH2SH)}(CO)4Mo] (5) is a stable compound with an uncoordinated SH function at the dangling arm of the ligand. If the SH functions of a potential tripod ligand are deprotonated the resulting thiolate functions are found to strongly coordinate in compounds such as [η3-{CH3C(CH2PPh2)2(CH2S)}(CO)3Mo]– which are obtained as their lithium and sodium salts 6. These highly reactive species may be stabilized by encapsulating their alkali counter ions in cryptands and the corresponding salts with [2,1,1]cryptand (7a) and [2,2,2]cryptand (7b) are far easier to handle than the cryptand-free progeners 6. Alkylation of the coordinatively bound thiolate function in 6 leads to the corresponding thioether derivatives [η3-{CH3C(CH2PPh2)2(CH2SR)}(CO)3Mo] (8).

Chiral Tripod Ligands with Phosphorus and Sulfur Donors. Synthesis and Complex Chemistry Neopentane based tripod ligands CH 3 C(CH 2 X)(CH 2 Y)‐(CH 2 Z) with mixed donor groups X,Y,Z (PR 2 , SR, SH) are accessible via two different strategies. The functionalized thietane CH 3 (CH 2 X)ČH 2 SCH 2 allows stepwise introduction of P or S donors by substitution of the mesylate function and subsequent nucleophilic cleavage of the thietane ring. Tripod ligands 5 [CH 3 C(CH 2 X)(CH 2 Y)(CH 2 Z)] with up to three different donor groups (X = SH; Y = PPh 2 ; Z = P(4‐Tol) 2 , SCH 2 Ph, PPh 2 ) are thus available. As an alternative the neopentane derivatives 8 [CH 3 C(CH 2 X)(CH 2 Y)(CH 2 Cl)] which are easily assessible from CH 3 C(CH 2 OSO 2 Me)(CH 2 Br)‐(CH 2 Cl) ( 7 ) by stepwise substitution with phosphides LiPAr 2 (Ar = aryl) may be transformed to 9 [CH 3 C(CH 2 X)(CH 2 Y)(CH 2 SBzl)]. Under controlled conditions the S—benzyl bond of 9 is cleaved reductively without concomitant cleavage of P—Ar bonds. Ligand 5a [CH 3 C(CH 2 PPh 2 ) 2 (CH 2 SH)] reacts with Ni(BF 4 ) 2 · 6 H 2 O to yield the dinuclear species [(CH 3 C(CH 2 PPh 2 ) 2 ‐(CH 2 S)Ni) 2 ] 2+ ( 10 ). The two ligands are bonded in an equivalent way with each of the two nickel centers interacting with one of the two phosphane functions of the ligand; the thiolate functions form μ 2 ‐SR bridges between the two nickel atoms. The SCH 2 group at the four‐membered S 2 Ni 2 ring are in mutual cis position with the four‐membered ring bent along the sulfur–sulfur axis by 70°. The internal strain in 10 is analyzed by comparing its structure with the ones of [(CH 3 C(CH 2 ‐PPh 2 ) 3 NiSPh) 2 ] 2+ ( 11 ) and [(CH 3 C(CH 2 OH)(CH 2 PPh 2 ) 2 ‐NiOH) 2 ] 2+ ( 12 ).