Second and Third Series Transition Metals

The dominant valency state of molybdenum in aqueous solution is +6, although at reduced both the +3 and +4 states can form. Hexavalent molybdenum is anionic, whereas no hydrolysis data have been reported for molybde-num(lV). Molybdenum(III) is cationic and should behave in a similar fashion to chromium(III). The ionic radius for sbc-coordinate Mo has been reported by Shannon (1976) to be 0.69 A. Stability constants for the monomeric hydrolysis species of molybdenum(III) have been reported. 

All of the isotopes of technetium are radioactive. The longest-lived isotope is Tc with a half-life of 2.6 million years. Technetium has six oxidation states from divalent to heptavalent, with the first three forming cations. However, hydrolysis data are only available for tetravalent technetium that exists in solution as the oxo-cation TcO.  

Rhodium can occur as a cation in the monovalent, trivalent and tetravalent states. Although the latter of these is likely to be unstable in water, two studies by Kalinina etal. (1977) and Kalinina and Lyakushina (1977) have reported stability constants for the oxo-ion, RhO . There is no supporting evidence for either of the species proposed or the form in which rhodium(lV) exists, and, as such, the data are not accepted. A solubility constant for rhodium(l) oxide is available (van Muylder and Pourbaix, 1974), although it is not clear from where this datum was originally sourced. The reported solubility is log /Csio = 3.31, but no confirmatory evidence has been presented for this solubility. Rhodium(III) is the dominant aqueous form of the element, and some stability constant data are available for the first monomeric hydrolysis species RhOH . Data are not available for any other species. Shaimon (1976) has indicated that the ionic radius of Rh is 0.665 A. 

In aqueous solution, palladium exists in the +2 valency state. The solubility of PdfOHljfam) has been reported as well as stability constant data for the first three monomeric hydrolysis species. No data are available for polymeric species or for the solubility of crystalline palladium hydroxide. Palladium(II) has the most stable hydrolysis species (by a number of orders of magnitude) of any divalent cation, which suggests that the coordination number of palladium might be as low as two. If so, this would indicate that the formation of Pd(OH)3 is unlikely or that 

Hydralyds of Metal Ions, First Edition. Paul L. Brown and Christian Ekberg. 

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Mass spectral data suggest that metal-metal bonds between first row transition metals are weaker than corresponding metal-metal bonds between second and third row transition metals. Thus in the series... 

It is clear from the table that the maximum oxidation state of the group increases from left to right, peaks at Group 7 for the first row and at Group 8 for the second- and third-row transition metals, and then steadily decreases toward the end of the transition metal series. Within a group, the heavier elements can achieve higher oxidation states than first-row members (see, for example. Group 8). 

Thus for M(II) ions of the first transition series A usually varies from about 90 to 145 kJ mof (7500-12000 cm" ) while for M(III) ions A varies from 170-300 kJ mof (14000-25000 cm ). Such generalisations are less applicable to second- and third-row transition metals, but normally A increases by 30-40% on moving from the first to the second row, and by 30-40% on moving from the second to the third row. The magnitude of A is also dependent on the symmetry and geometry of the ligand field. For example, A is less for tetrahedral than for octahedral complexes with the same donor atoms (A , 4/9 A eJ, Figures 1.3 and 1.4. 

As mentioned above, some chemistry of a few heavier elements is also of concern in the development of the geosphere and of living systems as we shall see later. A striking case is the chemistry of molybdenum (Mo) and tungsten (W), which we take here with vanadium (V). The first two elements are in the second and third series of transition metals and all three are found in higher combining ratios and with a greater preference for S rather than O, W less so than Mo (the... 

Nitridotetracyano Complexes of Second and Third Series Early Transition Metals Johann G. Leipoldt,... 

Interelectron interactions depend on the size, namely the greater the ion size, the more distant the electrons from each other, the less repulsion between them. Hence B and C decrease with a decreasing of oxidation state, from the first transition series to the second and third series and from the first to the last ions within each of the series. For an ion in a crystal the overlapping of transition metal and ligand orbitals leads to a decrease of B and C, namely... 

The metals which form this type of complex by reaction with dioxygen are mainly the later second and third row transition series metals, notably Ru°, Os°, Rh1, Ir1, Pt° and Pd°. It is not surprising to see metals noted for their ability to undergo oxidative addition reactions amongst those which form this type of mononuclear dioxygen complex as a formal two-electron reduction of dioxygen is required for complex formation. The other metals known to form mononuclear peroxo-type complexes with dioxygen are Ni°, Co1 and one example of Co . 

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