Metal-pivot mechanism

A hemispherical socket, which takes the place of the acetabular cup and retains the spherical ball. This hemispherical socket typically is a metal cup affixed into the joint socket by mechanical attachments and lined with a suitable polymeric material so that the ball can rotate within the socket, and so that the stem, via the ball, can pivot and articulate. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Of course the above treatment is not definitive, but it does suggest that the antibonding a 2s MO mechanism can compare favourably with the pivotal resonance mechanism as the primary VB formulation for electron conduction in metallic lithium. 

However, the reason of the appearance of negative impedance is always a chemical/electrochemical process. In most cases the blocking (inactivation) of the electrode (metal) surface is the pivotal (autoinhibition) step in the mechanism behind the emergence of the oscillating behavior. The blocking can be a consequence of adsorption of ions or molecules, chemisorption of molecular fragments, deposition of metals, salts or other compounds, formation of oxide layer etc. In all cases several coupled, consecutive, and simultaneous processes occur. The oscillating behavior appears only at a certain set of parameters (concentrations of the electro-chemically active species, the nature and the concen-... 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

In addition to the aforementioned X-ray analysis to disclose the structure of a few crystalline titanium chelates, and NMR studies have been performed to provide evidence for the chelation structure of a- and /1-oxycarbonyl compounds in solution [33-35]. Approximate solution structures for -alkoxyaldehydes are as shown in Fig. 7 [34]. The mechanism of chelation-controlled reactions of organotitanium reagents has been investigated experimentally [5] and theoretically [36], and the subject has been reviewed [10]. The formation of a chelate structure with titanium metal at the center plays a pivotal role in determining the reactivity and selectivity [37] in many synthetic reactions as shown in the following discussion. 

The motor speed reducer and storage drum are mounted on a metal frame. The frame is attached to the upper support structure by means of a pivoting support shaft. The rotation of the mechanism is resisted by redundant load cells which are used to monitor cable load (i.e., the weight of the control rod plus friction). These devices are used to detect a stuck control rod or a broken control rod cable. 

These are some of the many examples for C-H bond functionalization of (hetero)arenes that are proposed to follow CMD pathway. The pivotal role of base on the metal, mainly the acetato and carboxylate ion which acts as bases for abstraction of the C-H proton was highlighted through these examples. Besides the mechanisms that have been mentioned so far which are known to be operating in C-H activation of (hetero)arenes several others have also been proposed such as Heck type carbometalation mechanism and Se3 type mechanism. However, in this chapter we intend to focus on the more widely applicable ones, which have been described above. 

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