Carbonyl and Nitrosyl 521 Fhen K0h < 1, that is c2 >gh, the equation (31) for k has a real root only >a more limited range of values of 0, the lower limit being 0O = cos^V^o^) i£ead of zero. It is readily seen that the expression for R will be as in (38) vh 0O as the lower limit of the integral.
Garbonyl and Nitrosyl Compounds. 523 generally recognized that in all these the effective atomic number (E.A.N. : ; total number of electrons, shared and unshared, in the molecule ; in these expounds the atomic number increased by 2 for every CO) of the metallic itn is that of the next following inert gas, as is shown above.* This fact, ch explains why monometallic carbonyls are not formed by elements of ,*j. atomic number, is very remarkable. As a rule it is found that the stability oft molecule depends, not on the total number of electrons in the E.A.N., n separately on the number in the core and that in the valency group. In t hte substances, however (which are all derivatives of the transitional elements of?eriodic Groups VI, VII and VIII), the determining factor seems to be the toil number, irrespective of how it is divided between the shared and the uhared electrons ; thus the numbers are (the shared electrons are under- lud):— Cr(CO) : Cr = 2, 8, 14, 12 = 36 Fe(CO) : Fe = 2, 8, 16, 10 = 36 Ni(CO) : Ni = 2, 8, 18, 8 = 36 Mo(CO) : Mo = 2, 8, 18, 14, 12 = 54 W(CO) : W = 2, 8, 18, 32, 14, 12 = 86 t does not, however, seem to have been noticed that there is a further larkable regularity in the composition of the carbonyls, which extends to se that contain more than one atom of metal in the molecule (the poly- 'allic carbonyls). If we calculate the average E.A.N. of the metallic atoms, adding 2 to tfce atomic numbers for every CO, then the difference between result and the atomic number of the next inert gas is always found to be less than the number of metallic atoms in the molecule. If we write the <npound M^CO),, and if m is the atomic number of M, and G that of the itt inert gas, then the equation a _ ?-.» + .% , x - i (1, v 7 X always found to be true. Where x = 1 (monometallic carbonyls) we have n that this holds throughout. The other known pure carbonyls (con ning only metallic atoms and CO groups) are the following ; the molecular This was given by Langmuir (‘ Science,’ vol. 54, p. 65 (1921)) as the cause of the oility of iron and nickel carbonyls, and also accounts for the diamagnetism of these expounds ; Oxley, ‘ Proc. Camb. Phil. Soc.,’ vol. 16, p. 102 (1911).
Carbonyl and Nitrosyl Compounds. 525 t ough a link of this kind. For two such atoms this is secured by joining r un through a single CO, for three, by placing them at the angular points of triangle with a CO in each side, and for four by placing them at those of a : rahedon, with a CO group on each edge.
Carbonyl and Nitrosyl Compounds. 527 1*14 : C — Fe 0*77 + 1*27 = 2*04 : O — Fe 0*70 + 1*27 = 1-97,* the distance between the centres of the terminal oxygen atoms should be 11*5 A. The whole height of the cell according to Brill is 15*8 A., which means that the distance between the centres of two neighbouring oxygens in different molecules is 4*3 A. This is about the usual value ; in graphite two carbon atoms in different sheets (i.e., molecules) are 3*4 A. apart, in benzene hexachloride two such chlorines are 3*74 A. apart.f Thus there is no undue crowding on the threefold axis, and the structure, fig. 1, proposed for Fe2(CO)9 in no way conflicts with the X-ray evidence.
Carbonyl and Nitrosyl Compounds. 529 Some examples of the working out of the E.A.N. and of the size of the valency $pup may be given :— Fe(CO )3(en) : 26 + 2 X 3 + 4 = 36 : 2, 8, 16, H2Fe(CO)4 : 26 + 2 + 2 X 4 = 36 : 2, 8, 14, 12^ Fe(00)4H[Na] : 26+ 2 x 4 + 1 + ! = 36 : 2, 8, 16, 10, # 2 X 26 + 2 x 5 -j— 2 x 4 _ Fe2(CO)5(m)2 . 3X26 + 2 X 9 + 6 Fe3(CO)9Br6 = 34 x = 3.