Quadruple transition

Calculations of the electronic structure of the complexes of this type were first carried out for [Tc2C18]3, using the semi-empirical EHT method [146], It was shown that a strong quadruple M-M bond is formed in both the d4-d4 binuclear transition-metal complexes and d4-d5 technetium complexes. The addition of an excess electron does not decrease the Tc-Tc bond strength,... 

Although less numerous than complexes containing heavier transition metals, some cases are known in which there is a quadruple bond between first-row metals. One very interesting complex of this type is [Cr2(C03)4(H20)2] in which the carbonate ions form bridges between the two metal atoms as shown in Figure 21.25. 

As is mentioned in the introductory material, chromium exhibits a propensity to form quadruple bonds that is unmatched by any other first-row transition metaL Although Cr2(02 CQl3)4(H20)2 was first reported over 100 years ago (210), it was not promoted as a quad ruple-bond-containing unit until Cotton and co-workers carried out a redetermination of the x-ray structure and reported a change in the chromium atom separation from the accepted value of 2.64 A initially reported in 1953 (251) to 2.3855(5) A (66, 67). Reduction of chromium trichloride to chromium(II) in aqueous solution with zinc followed by addition of sodium acetate is a convenient route to Cr2(02CCH3)4(H20)2 (204), which can be dehydrated by heating under vacuum to form Cr2(02 CCH3)4. 

The main visible absorption between 600 and 700 nm was deconvoluted into two bands (13,600 cm-1, e = 35 15,700 cm-1, e= 172), with the more intense band assigned as 6 - 7r, since this is the only other allowed transition calculated to lie below 20,000 cm -I. It should be noted that no such bands are present in closed-shell quadruple-bond configurations where no 8 electron is present in the ground state. The weaker band is assigned to the forbidden rr - 6 transition, although the authors are not certain that the solution spectrum requires the existence of this band (70). 

In the Mossbauer spectra of the Fe(R2Dtc)3 complexes, simple quadruple split doublets are observed that are interpreted as the weighted means of the high-spin and low-spin forms (153). It has been pointed out (454) that a rapid transition (>10 7 sec) is not characteristic for complexes with different spin states, and indeed two doublets are observed in the Mossbauer spectra of iron(II) compounds with temperature-dependent magnetic moments. The same... 

Each quadruple point occurs at the intersection of four three-phase lines (Figure 1.2). The lower quadruple point is marked by the transition of Lw to I, so that with decreasing temperature, Qi denotes where hydrate formation ceases from vapor and liquid water, and where hydrate formation occurs from vapor and ice. Early researchers took Q2 (approximately the point of intersection of line Lw-H-V with the vapor pressure of the hydrate guest) to represent an upper temperature limit for hydrate formation from that component. Since the vapor pressure at the critical temperature can be too low to allow such an intersection, some natural gas components such as methane and nitrogen have no upper quadruple point, Q2, and... 

The I-Lw-V line that connects the quadruple point to the water triple point (I-Lw-Vw) (273.16 K, 0.62 kPa), denotes the transition between... 

A comprehension of Figure4.3 has value because a similar phase diagram could be drawn for a natural gas of fixed composition between the quadruple points (Qi and QaJ. The same phase transitions and boundaries would qualitatively occur, with the artificial constraint that all hydrocarbon phases be of the same composition as the original gas. A second useful outcome of binary phase diagrams like Figure 4.3 is the use of the lever rule (Koretsky, 2004, p. 367) at constant temperature to determine relative phase amounts note that the lever rule can be applied for quantitatively correct phase diagrams. 

Figure 5.17 shows a predicted pressure versus excess water composition plot for the ethane+propane+water system at 274 K. At 0.0 mol fraction ethane (propane+ water) sll form at approximately 2 bar, and at 1.0 mol fraction ethane (ethane + water) si form at approximately 5 bar. At the intermediate composition of 0.78 mole fraction ethane, a quadruple point (Aq-sI-sII-V) exists in which both incipient hydrate structures are in equilibrium with vapor and aqueous phase. This point will be referred to as the structural transition composition the composition at which the incipient hydrate formation structure changes from sll to si at a given temperature. 

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