Comment on "The ligand polyhedral model approach to the mechanism of complete carbonyl exchange in [Rh4(CO)12] and [Rh6(CO)16]" by Brian F. G. Johnson, Dalton Transactions, 2015, 44, DOI: 10.1039/C4DT03360D.

Experimental results (recent IR, DFT calculations and modern multinuclear NMR measurements on Rh-containing clusters, together with earlier VT multinuclear NMR measurements) show that the use of the Ligand Polyhedral Model (LPM) to provide a general mechanism for ligand fluxionality in Transition Metal Carbonyl Clusters (TMCCs) in solution cannot be sustained; instead there are numerous examples of only partial CO-migration over either part or sometimes the whole of the Rh-polyhedron as well as rhodium and carbonyl polyhedral rearrangements of Rh9- and Rh10-TMCCs containing an interstitial P when, in the high temperature limiting spectra, all the metals and all the carbonyls become equivalent and show time-averaged values of (1)J(Rh-P) and (2)J(P-CO) respectively.

The synthesis of, and characterization of the dynamic processes occurring in Pd(II) chelate complexes of 2-pyridyldiphenylphosphine.

Pd(II) complexes in which 2-pyridyldiphenylphosphine (Ph(2)Ppy) chelates the Pd(II) centre have been prepared and characterized by multinuclear NMR spectroscopy and by X-ray crystallographic analysis. trans-[Pd(kappa(1)-Ph(2)Ppy)(2)Cl(2)] is transformed into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl]Cl by the addition of a few drops of methanol to dichloromethane solutions, and into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl]X by addition of AgX or TlX, (X = BF(4)(-), CF(3)SO(3)(-) or MeSO(3)(-)). [Pd(kappa(1)-Ph(2)Ppy)(2)(p-benzoquinone)] can be transformed into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(MeSO(3))][MeSO(3)] by the addition of two equivalents of MeSO(3)H. Addition of further MeSO(3)H affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)PpyH)(MeSO(3))][MeSO(3)](2). Addition of two equivalents of CF(3)SO(3)H, MeSO(3)H or CF(3)CO(2)H and two equivalents of Ph(2)Ppy to [Pd(OAc)(2)] in CH(2)Cl(2) or CH(2)Cl(2)-MeOH affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)X]X, (X = CF(3)SO(3)(-), MeSO(3)(-) or CF(3)CO(2)(-)), however addition of two equivalents of HBF(4).Et(2)O affords a different complex, tentatively formulated as [Pd(kappa(2)-Ph(2)Ppy)(2)]X(2). Addition of excess acid results in the clean formation of [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)PpyH)(X)]X(2). In methanol, addition of MeSO(3)H and three equivalents of Ph(2)Ppy to [Pd(OAc)(2)] affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(2)][MeSO(3)](2) as the principal Pd-phosphine complex. The fluxional processes occuring in these complexes and in [Pd (kappa(1)-Ph(2)Ppy)(3)Cl]X, (X = Cl, OTf) and the potential for hemilability of the Ph(2)Ppy ligand has been investigated by variable-temperature NMR. The activation entropy and enthalpy for the regiospecific fluxional processes occuring in [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(2)][MeSO(3)](2) have been determined and are in the range -10 to -30 J mol(-1) K(-1) and ca. 30 kJ mol(-1) respectively, consistent with associative pathways being followed. The observed regioselectivities of the exchanges are attributed to the constraints imposed by microscopic reversibility and the small bite angle of the Ph(2)Ppy ligand. X-Ray crystal structure determinations of trans-[Pd(kappa(1)-Ph(2)Ppy)(2)Cl(2)], [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl][BF(4)], [Pd(kappa(1)-Ph(2)Ppy)(2)(p-benzoquinone)], trans-[Pd(kappa(1)-Ph(2)PpyH)(2)Cl(2)][MeSO(3)](2), and [Pd(kappa(1)-Ph(2)Ppy)(3)Cl](Cl) are reported. In [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl][BF(4)] a donor-acceptor interaction is seen between the pyridyl-N of the monodentate Ph(2)Ppy ligand and the phosphorus of the chelating Ph(2)Ppy resulting in a trigonal bipyramidal geometry at this phosphorus.

The loss of CO from [Rh12(mu12-Sn)(CO)27]4-: synthesis, spectroscopic and structural characterization of the electron-deficient, icosahedral [Rh12(mu12-Sn)(CO)25]4- and [Rh12(mu12-Sn)(CO)26]4- tetra-anions.

Heating (80 degrees C) the electron-precise, Sn-centred, icosahedral cluster [Rh(12)Sn(CO)(27)](4-) under a nitrogen atmosphere affords in sequence the electron-deficient icosahedral [Rh(12)Sn(CO)(26)](4-) and [Rh(12)Sn(CO)(25)](4-) derivatives. The reaction is reversible in solution and the parent compound is quantitatively regenerated upon exposure to carbon monoxide. The reaction course has been unravelled via a combination of Band-target Entropy Minimization (BTEM) IR analysis and X-ray studies. While icosahedral clusters displaying electron counts formally exceeding 13 skeletal electron pairs (SEP) are known, [Rh(12)Sn(CO)(26)](4-) and [Rh(12)Sn(CO)(25)](4-) show for the first time that icosahedral clusters may also be stabilized with a deficiency of SEPs with respect to the requirement based on the cluster-borane analogy. In contrast to the behaviour of the electron-precise cluster [Rh(12)Sn(CO)(27)](4-), the electron-deficient cluster [Rh(12)Sn(CO)(25)](4-) undergoes reversible electrochemical reductions.

The reaction of mixtures of [Rh4(CO)12] and triphenylphosphite with carbon monoxide or syngas as studied by high-resolution, high-pressure NMR spectroscopy.

The fragmentation and redistribution reactions of [Rh4(CO)12-x{P(OPh)3}x] (x = 1-4) with carbon monoxide have been studied using high-resolution, high-pressure NMR spectroscopy. Under the conditions of efficient gas mixing in a high-pressure NMR bubble column, [Rh4(CO)9{P(OPh)3}3] fragments to give mainly [Rh2(CO)6{P(OPh)3}2]; [Rh4(CO)11{P(OPh)3}] is also observed,implying redistribution of the phosphite ligand and/or recombination of the dimers to tetrameric clusters. Fragmentation of[Rh4(CO)10{P(OPh)3}2] is found to be pressure-dependent giving predominantly [Rh2(CO)6{P(OPh)3}2] at low CO pressure (1-40 bar), and increasing amounts of [Rh2(CO)7{P(OPh)3}] at higher (40-80 bar) pressure. Using Syngas (CO : H2 (1 : 1)) instead of CO in the above fragmentations, homolytic addition of H2 to the dimer [Rh2(CO)6{P(OPh)3}2] to give [RhH(CO)3{P(OPh3}] and [RhH(CO)2{P(OPh)3}2] is observed. The distribution of tetrameric species obtained is similar to that obtained under the same partial pressure of CO. On depressurisation/out-gassing of the sample, the original mixture of tetrameric clusters is obtained.

Multinuclear NMR studies of the products resulting from the reaction of pyridine or 2,2'-bipyridine with [Rh4(CO)12].

The progressive addition of anhydrous pyridine, (py), to a solution of [Rh(4)(CO)(12)] in CH(2)Cl(2) under CO, even at low temperature, results in immediate disproportionation to give cis-[Rh(CO)(2)py(2)][Rh(5)(CO)(15)]; further addition of pyridine results in the progressive replacement of CO's by py on the same apical rhodium in [Rh(5)(CO)(15)](-) to give cis-[Rh(CO)(2)py(2)][Rh(5)(CO)(15-x)py(x)] (x = 1, 2). The analogous reactions with 2,2'-bipyridine (bipy) give only [Rh(CO)(2)bipy][Rh(5)(CO)(13)bipy]. IR and low temperature, multinuclear NMR measurements have been used to establish the structures of all the above anions and the structures of [Rh(5)(CO)(13)(bipy)](-) and [Rh(5)(CO)(13)py(2)](-) are subtly different. Under N(2), [Rh(4)(CO)(12)] reacts with py to give [Rh(6)(CO)(16-y)py(y)] (y = 1, 2).

Sn-centred icosahedral Rh carbonyl clusters: synthesis and structural characterization and 13C-{103Rh} HMQC NMR studies.

The reaction of [Rh7(CO)16]3- with SnCl(2).2H2O in a 1 : 1 molar ratio under N2 results in the formation of the new heterometallic cluster, [Rh12Sn(CO)27]4-, in very high yield (ca. 86%). Further controlled additions of SnCl(2).2H2O, or solutions of HCl, or [RhCl(COD)]2, give [Rh12(mu-Cl)2Sn(CO)23]4-. Similarly, addition of HBr to [Rh12Sn(CO)27]4- gives the related cluster [Rh12(mu-Br)2Sn(CO)23]4-. Notably, if the addition of SnCl(2).2H2O to [Rh12Sn(CO)27]4- is carried out under a CO atmosphere, the reaction takes a different course and leads to the formation of the new cluster, [Rh12Sn(mu3-RhCl)(CO)27]4-. All the above clusters have been shown by single-crystal X-ray diffraction studies to have a metal framework based on an icosahedron, which is centred by the unique Sn atom. Their chemical reactivity and 13C-{103Rh} HMQC NMR spectroscopic characterization are also reported.

The mechanism of the hydroalkoxycarbonylation of ethene and alkene-CO copolymerization catalyzed by Pd(II)-diphosphine cations.

All the intermediates in the "carboalkoxy" pathway, and their interconversions giving complete catalytic cycles, for palladium-diphosphine-catalyzed hydroalkoxycarbonylation of alkenes, and for alkene-CO copolymerization, have been demonstrated using (31)P{(1)H} and (13)C{(1)H} NMR spectroscopy. The propagation and termination steps of the "hydride" cycles and the crossover between the hydride and carboalkoxy cycles have also been demonstrated, providing the first examples of both cycles, and of chain crossover, being delineated for the same catalyst. Comparison of the propagation and termination steps in the pathways affords new insight into the selectivity-determining steps. Thus, reaction of [Pd(dibpp)(CH(3)CN)(2)](OTf)(2) (dibpp = 1,3-(iBu(2)P)(2)C(3)H(6)) with Et(3)N and CH(3)OH affords [Pd(dibpp)(OCH(3))(CH(3)CN)]OTf, which, on exposure to CO, gives [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf immediately. Labeling studies show the reaction to be readily reversible. However, the back reaction is strongly inhibited by PPh(3), indicating an insertion/deinsertion pathway. Ethene reacts with [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf at 243 K to give [Pd(dibpp){CH(2)CH(2)C(O)OCH(3)}]OTf, that is, there is no intrinsic barrier to alkene insertion into the Pd--C(O)OMe bond, as had been proposed. Instead, termination is proposed to be selectivity determining. Methanolysis of the acyl intermediate [Pd(dibpp){C(O)CH(3)}L]X (L = CO, CH(3)OH; X = CF(3)SO(3) (-) (OTf(-)), CH(3)C(6)H(4)SO(3) (-) (OTs(-))) is required in the hydride cycle to give an ester and occurs at 243 K on the timescale of minutes, whereas methanolysis of the beta chelate, required to give an ester from the carbomethoxy cycle, is slow on a timescale of days, at 298 K. These results suggest that slow methanolysis of the beta chelate, rather than slow insertion of an alkene into the Pd--carboalkoxy bond, as had previously been proposed, is responsible for the dominance of the hydride mechanism in hydroalkoxycarbonylation.

Synthesis and structural characterization of two novel heterometallic clusters: [Rh4Pt2(CO)11(dppm)2] and [Ru2Rh2Pt2(CO)12(dppm)2].

Two novel heterometallic octahedral clusters [Rh(4)Pt(2)(CO)(11)(dppm)(2)](1) and [Ru(2)Rh(2)Pt(2)(CO)(12)(dppm)(2)](2) were synthesized by the reaction of [Rh(2)Pt(2)(CO)(6)(dppm)(2)] with [Rh(6)(CO)(14)(NCMe)(2)] and Ru(3)(CO)(12), respectively. Solid state structures of 1 and 2 have been established by a single crystal X-ray diffraction study. Two dppm ligands in 1 are bonded to one platinum and three rhodium atoms, which form an equatorial plane of the Rh(4)Pt(2) octahedron. Two rhodium and two platinum atoms bound to the diphosphine ligands in 2 are nonplanar to give an octahedral C2 symmetric Ru(2)Rh(2)Pt(2)(dppm)2 framework. The (31)P NMR investigation of and (1D, (31)P COSY, (31)P-[(103)Rh] HMQC) and simulation of 1D spectral patterns showed that in both clusters the structures of the M(6)(PP)(2) fragments found in the solid state are maintained in solution.

The effect of mechanistic pathway on activity in the Pd and Pt catalysed methoxycarbonylation of ethene.

All the intermediates involved in the platinum catalysed methoxycarbonylation of ethene have been characterised by in situ NMR; the low activity of platinum catalysts in this reaction is shown to be due to trapping of the active intermediates by carbon monoxide at every step in the catalytic cycle and to the ready reversibility of the product forming reactions.

Polymeric anionic networks using dibromine as a crosslinker; the preparation and crystal structure of [(C4H9)4N]2[Pt2Br10].(Br2)7 and [(C4H9)4N]2[PtBr4Cl2].(Br2)6.

The reaction of M[PtX3(CO)] (M+ = [(C4H9)4N]+, X = Br, Cl) with an excess of Br2 gives the new platinum(IV) salts, [(C4H9)4N]2[Pt2Br10].(Br2)7, 1, and [(C4H9)4N]2[PtBr4Cl2].(Br2)6, 2, which, in the solid state, contain strong Br Br interactions resulting in the formation of polymeric networks; they could provide useful solid storage reservoirs for elemental bromine.