Square planar hydrides

We shall exclude from this section all square planar hydride derivatives of the d transition metals, since these are effectively saturated systems and diamagnetic. For other cP systems, compounds such as MXH(CO)(PRj)2 (M = Ru, Os X = 71 donating group such as halide) are diamagnetic and stable, because of the effective saturation ascribed to the 7t donation from X, whereas the corresponding MH2(CO)(PR3)2 complexes are very reactive and non isolable transient species [80]. 

Insertion of olefins into the Pt—H bond of square-planar hydrides /ra 5-[(Pt(H)-(X)(L)a] was originally assumed to involve olefin co-ordination to give a five-coordinate intermediate which underwent migratory rearrangement to the alkyl trans-[Pt(R)(X)(L)a]. Recent MO calculations of the insertion reaction between ethylene and [Pt(H)(Cl)(PH3)a] suggest that collapse of the five-co-ordinate intermediate is... 

Its structure shows the trans-influence of hydride and pronounced distortion from square planar geometry (H—Rh-P 70.7°) owing to steric crowding. (RhH(PPh3)3 is rather less distorted (H-Rh-P 75.8°) [118a], This 16-electron complex shows no tendency to add an extra molecule of phosphine, unlike the less hindered RhH(PEt3)3. It is, however, an active... 

In synthesis (b), the initial product is a 5-coordinate (sp) iridium(III) hydride complex, which is rapidly oxidized in solution to the planar iridium(II) complex. Both of the compounds are paramagnetic with one unpaired electron, as expected for square planar d7 complexes. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

The proposed mechanism starts with a methyl group abstraction on platinum complex 416 with the borane reagent in the presence of diyne 414 (Scheme 105). The square-planar cationic diyne-platinum(n) complex 417 is converted to the octahedral platinum(rv) hydride intermediate 418 through oxidative addition of the hydrosilane. This complex decomposes rapidly with methane release to form another tetracoordinated platinum(n) species 419, followed by platinasilylation of the triple bond. The resulting vinylplatinum 420 undergoes an intramolecular carboplatination to... 

Direct oxidative addition of C-H bonds to square-planar Pt(II) to form octahedral Pt(IV) alkyl hydrides (Scheme 5, a) appears to be extremely uncommon. In fact, there are no unequivocal examples of this reaction pathway. However, a couple of examples have been reported wherein such a direct mechanism may provide the best explanation for the available data. 

Catalysis of hydrosilylation by dimeric or by monomeric rhodium (and iridium) siloxide complexes occurs via preliminary oxidative addition of siUcon hydride followed by elimination of disiloxane (detected by GC/MS) to generate the square planar 16e hydride complex with an already coordinated molecule of alkene (Scheme 7.7). 

The active species in the catalytic cycles are square-planar Pd complexes of the formula [Pd (X)(S)(L-L)]Y where L-L is a chelating ligand with the same or different donor atoms among P, N, O and S X is the growing polyketone chain or hydride S may be a solvent molecule, a co-monomer, or a keto group from the chain. Finally, Y is a counter-anion of weak nucleophilicity in order to avoid competition with the co-monomer for coordination to palladium (Scheme 7.1). 

The hydrido complexes are diamagnetic, and are square planar or five-coordinate. Their stability, in general, increases with the number of coordinated phosphines. In the complex [Ni(BH4)(H)(PCy3)2] (153) the nickel atom is coordinated in the equatorial positions by two hydrogens of the borohydride and by one hydride anion. 

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