COMMUNICATIONS TO THE EDITOR
3404
THE ELECTRONIC STRUCTURES OF METAL NITROSYLS AND CARBONYLS'
Sir :
VOl. 84 M.O. Levels
Metal orbitals
This note presents a molecular orbital (m.0.) energy level scheme for the large class of metal nitrosyls and carbonyls which contain one particularly strong M-NO or ill-CO bond; this interaction therefore dominates the over-all ligand field. For a distorted octahedral structure of the type (GV)shown in Fig. 1, the bonding is described as
Ligand orbitals
I
f "N
Po
LI
/
/
L, /
xj'
IGg. 2.-iMolecular
I
orbital energy level scherne for M(L)b-
(CO) or M(L)&(NO)complexes.
trons (six from Fe, one from NO) to place in the m.0. level scheme shown in Fig. 2 . This gives a 2B1I L, { [e(rb)]4[b~]z[bl(u*)]1} ground state. Thus the I'ig. 1.-Ilistortrd ovt;ti;rtiral structure for MtI,)d(SO) or complex formally contains F e + and hTO+; this hT;L)j(CO) coniplexes. formulation is consistent with the observation of an infrared band a t 1735 cm.-I which is usually asfdlows: a strong u bond (al) between the N(or C) signed4 to coardinated KO +. 2s orbital and the ndz2 metal orbital; two strong The solution electronic spectrum of F e ( N 0 ) n-bonds (e) involving the nietal ndxr and ndYa [S2CN(C113)2]2 below 30,000 cm.-' consists of three orbitals and the NO(C0) T (2pH,2py) antibonding charge transfer band systems, with single maxima orbitals; a u bond (b1) between the nietal ndX%-,,2 a t 17,100 tin.-' ( E = 1200 in benzene) and 20,900 orbital and the (ul - uz + u 8 - u 4 ) ligand orbital crn.-' ( E = 1900 in benzene) and a double peaked combination; the ndxv nietal orbital is non-bond- band with ma.xinia a t 27,500 cni.-l ( E = 5750 in itig if metal-L n-bonding is ignored; finally, the acetone) and 23,900 cn1.l' ( E = 5750 in acetone). metal (12 1)s and ( n 1)p orbitals form four Thcse charge transfer bands are assigned to the iiiore u bonds (al, al, e) with the ligand three one electron transitions bl(u*) -+ e(n*), The essential featurcs of an m.0. energy level bz -+ e(.*) and e(nb) 4e(rr*) in order of increasing scheme for the bonding situation described above energy. Note that all the transitions are from are given in Fig, 2 . The ordering of the one m.o.'s mainly located on the metal to e(n*), which c!ectron n i . 0 . ' ~is fixed for the model chosen, since is essentially localized on NO.6 Thus the energy c ( n b ) is more stable than b?, and al(u*) is more differences of the charge transfer band maxima, unstable than bl(u*). This is true whether or not after suitably correcting for differences in interthe other axial ligand (L5) is present. electronic repulsion energies of the excited s t a t e ~ , ~ The spectral and magnetic properties of Fe- yield the positions of the one electron xn.0. levels (NO) [S:CN(CH3)z]2and hfn(C0)sX compounds bl(u*), b2 and e(Tb). This type of calculation gives will now be discussed in order to illustrate the use- A E [ b l ( u * ) - b,] = 9,150 crn.-l and AE [bl(u*) fulness of the derived m.0. scheme. The complex e(+)] = 13,650 cm.-l Thus a band is predicted Fe(N0) [S2CN(CH&I2probably has a rectangular(1) J . Lewis, Scicnce Progress, 506 (I9XJ). based pyramidal structure, analogous to the struc( 5 ) (a) A simple calculation shows t h a t the expected energy Of ture known3 for Co(K0) [S2CN(CH&I2. In addi- the metal d t o e(**) charge transfer is consistent with t h e assignments tion to the ligand u electrons, which will always given here. T h e ionization potential of N O is 74,400 cm-1. T h e valence ionization potential for a d orbital electron in F e + is about occupy the bonding u levels, there are seven elec- 82,000 cm.-l.' Thus a reasonable value of 22,000 cm.-' for the r-
+
+
( I ) Acknowledgment is made t o the donors of T h e Petroleum Resrarch Fund. administered by the American Chemical Society, for support of this research. ( 2 ) (a) I n this model the most stable 0 and r metal orbitals are made available t o the K O ( C 0 ) ligand. (b) For C4, symmetry, see C . J. Ballhausen and IT. B. Gray, Znorg. Chem., 1 , 111 (19W). (3) P. R. H.Alderman, P. C,. OKston and J. M. Rowe, J. Chenz. S O C . , 008 flSG.2).
interaction (the *-interaction in VOz is about 25,000 cm. correctly places the charge transfer bands, (b) T h e intense e ( r b )[ndxz,yzl e ( r * ) charge transfer has been identified in a number of other metal nitrosyl complexes: the "brown ring" complex, Fe(HnO)rNO* +, a t 32,200 cm.-1,12 Co(NO)[S?CN(CHs)2]?,a t 31,200 em.-',' and Cr(CS)hNO$-, a t 43,560 cm,-l,'* (I;) I
