Group 6 Chromium, Molybdenum and Tungsten

Molybdenum is in the middle of the triad elements of group 6. These three metals (from periods 4, 5, and 6) are chromium, molybdenum, and tungsten, which, in their pure states, are relatively hard, but not as hard as iron. They are silvery-white as pure metals, and they have similar oxidation states. Their electronegativity is also similar—Cr = 1.6, Mo = 1.8, and W = 1.7—which is related to their reactivity with nonmetals. 

Group 6 In order of increasing atomic number, these are chromium, molybdenum, and tungsten. Chromium and molybdenum have one eleeiron in their outer shells tungsten has two electrons in its outer shell. Although chromium and molybdenum also have other valences, all of the dements in this group have a 6+ valence in common. 

The uranyl ion has a totally symmetric groundstate28 with Q = 0+. Hence, it is not open to the same kind of controversy as was quite vigorous around 195517,241 about whether uranium belongs to the 5 f group or is a higher homolog of chromium, molybdenum and tungsten, and hence is a 6 d element. It turned out that all non-metallic compounds of the... 

In 1962 and 1963 a pair of papers by Stolz, Dobson and Sheline [23,24] looked at the IR spectra of the then suspected pentacarbonyl intermediates of the group 6 metals chromium, molybdenum and tungsten. These papers provided early support for the idea that one CO ligand is initially lost in the first step of the photoreactions of these carbonyls. Analysis of the CO stretching vibrations managed to rule out other possible species such as the W2(CO)xq anion and the W(CO)5 anion. Analogous results were found for the pentacarbonyls of chromium and molybdenum. The chromium result is particularly relevant for the photochemistry discussed later. 

Chromium, molybdenum, and tungsten are the transition metals in Group 6 of the periodic table. Chromium metal has the outer electronic configuration. W54,yl and forms compounds in oxidation states II- to VI+.1 The extensive organometallic chemistry of Cr is covered in the accompanying Comprehensive Organometallic Chemistry series, and this review is generally restricted to the coordination complexes (oxidation states 0 to VI+). 

Complexes of chromium, molybdenum and tungsten Table 23.5 Oxohalides of Group 6 (mp/°C)... 

No ji-coordination compounds with a coordinate bond from an aryl group to the metal of the type shown in compound 4.19 that are as stable as those in which the ligand atom is nitrogen, phosphorus, arsine, oxygen, or sulfur have yet been reported [19, 20] the synthesis of these 4.19 compounds is difficult, because p-elimination readily occurs in these alkyl compounds and the Group 6 metals chromium, molybdenum, and tungsten as shown in Fig. 4.5. 

Zn, Ni, Cu, and W, yet is the seventh most abundant element overall because Cr is concentrated in the earth s core and mantle (1,2). It has atomic number 24 and belongs to Group 6 (VIB) of the Periodic Table and is positioned between vanadium and manganese. Other Group 6 members are molybdenum and tungsten. On a toimage basis, chromium ranks fourth among the metals and thirteenth of aU mineral commodities in commercial production. 

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