Dynamics and fluxionality in metal carbonyl clusters: some old and new problems

The Clusters [Ma(ECp*)b] (M=Pd, Pt; E=Al, Ga, In): Structures, Fluxionality, and Ligand Exchange Reactions

“…The fact that ME 4 clusters are kinetically inert in contrast to M 2 E 5 could be reasoned by taking into account that splitting only one M À E connectivity in the case of the M 2 E 5 unit is of course likely to cost less activation energy per M 2 E 5 molecule than complete dissociation of ME 4 in ME 3 and free E. This situation is comparable to the well-studied classical metal-carbonyl cluster complexes, with the terminally bound carbonyls generally being less reactive than the bridging ones. [26,27] The relative inertness of Pt 2 Al 3 Ga 2 (1 d) in the presence of phosphines is in agreement with this mechanism, as the rupture of the M À E bond of the bridging AlCp* ligand would require more energy in comparison to the bridging GaCp* unit in the related Pt 2 Ga 5 (1 a). However, reaction of Pt 2 Al 3 Ga 2 (1 d) with CO does lead to [Pt 2 (CO) 2 (m 2 -AlCp*) 3 ] over a period of two days at room temperature.…”

Slow Migration of a Phosphorus Ligand between Two Metal Centers

“…[1][2][3][4] One of the most frequently mentioned textbook examples is the fluxional behavior of the carbonyl ligands in the bis(cyclopentadienyl)diiron complex [(h 5 -C 5 H 5 ) 2 Fe 2 (m-CO) 2 (CO) 2 ] (1), which proceeds via interchange of the bridging and terminal CO ligands according to the Adams-Cotton mechanism. [2][3][4][5][6] While many ligands, such as CNR and NO, exhibit similar fluxional behavior, it has long been asserted that phosphorus ligands PR 3 are unable to participate in such fluxional processes. [5] This belief is most clearly illustrated by the dynamic NMR-spectroscopic study of the ring-bridged system [(h 5 ,h 5 -C 5 H 4 Me 2 CCMe 2 C 5 H 4 ) 2 Fe 2 -(m-CO) 2 (CO)P(OPh) 3 ] (2), in which even the migration of CO is blocked by P(OPh) 3 .…”

Internal carbon monoxide exchange and CO dissociation in cobalt carbonyl carbene complexes. A density functional study

“…However in metal carbonyl clusters the so-called tripodal rotation of the M(CO) 3 moiety is a commonly observed exchange process as well [11]. It is known that the interconversion processes take place with rather small activation energies, namely 3 and 8.4 kcal/mol at B3LYP/ LANL2DZ level [12], which are smaller than the 11.7 ± 0.6 value measured by solid-state 13 C NMR spectroscopy [13].…”

“…The free energy of activation is 7.4 kcal/mol (8.5 kcal/ mol for the methylenecarbene complex) indicating the process as a very fast one. It is worth comparing the geometry of the product of the internal CO exchange (11) with the starting complex (1). In 11 the difference between the Co-CO cis and Co-CO trans bond lengths is smaller than in 1, but it is very close to them in the methylenecarbene analogue 3, however the Co-CO axial distance is by ca.…”

“…Transition states for the CO exchange between terminal and bridging positions (9,10) and the isomer of 1 formed after the CO exchange(11).…”