Molybdenum valence orbitals

The approximate correctness of the relative values of fav given in the table is supported by the observed values of the metallic radii. Values of R for ligancy 12 are shown in Figure 1, with a straight line with slope that represents the change in radius with atomic number for constant valence and type of orbital, as given in Figure 3 of an earlier paper.17 The points lie above the line by amounts that indicate a monotonic decrease in valence to both sides of molybdenum. 

CT transitions occur in Mo(IV) oxo-molybdenum-bis(dithiolene) complexes since the dxy acceptor orbital is completely filled. Therefore, it has been proposed that a thermally induced intramolecular electron/charge transfer (valence tautomeric) process is operable in [MovO(qdt)2J1 and responsible for the observed thermochromism. Such a S —> Mo dXi CT process formally results in the conversion of [Mo 0(qdt)2] to the Mo(IV) species LMoIV0(qdt Kqdt) 1 (Fig. 46), which is consistent with the attenuation of both the S —> Mo dxy CT and the dianionic qdt n —> n bands. 

The instability of the electronic structure caused by the unfilled inner orbit of a transition metal leads to a very variable valency. Molybdenum exhibits valencies of 2,3,4,5 or 6 in different compounds, and it is considered that it has zero valency in its hexacarbonyl Mo(CO)g. Because of this variability in its valency, many of its reaction products are mixtures of compounds in which it has different valencies. In solid form such products may be quite homogeneous in composition, and best represented by a nonstoichiometric molecular formula. The hypothetical simple compounds present in such products cannot be separated readily from one another because valency shifts occur in processing. 

The six unpaired electrons of the atoms of chromium, molybdenum and tungsten may form three double bonds with three carbonyl groups. Three unfilled p orbitals remain, which may be filled by the donation of an electron pair from each of three carbonyl groups. The structure of these compounds will therefore be represented by the valence-bond structure XXII ... 

---