The infrared spectra of compounds of the type RM(CO)3C,H, (M = Mo or W; R = (CH,),Sn, o-alkyl groups, halides, etc.) exhibit the expected three v(C0) frequencies (2A' + A"). The relatlve positions of the A" and the lower A' frequencies interchange upon variation of the electronegativity of the R group. Values for the Cotton-Kraihanzel stretching force constants k l and k2 have been calculated from these v(C0) frequencies assuming thc ratio k , / k , = 2 for the interaction force constants. From these force constants the o and n parameters of Graham were calculated. The o and n parameters were found to be very similar for analogous RM(CO)3C,HS and RMn(CO), compounds.
ICanadian Journal of Chemistry, 47, 2959Chemistry, 47, (1969 Recently we have been studying the relationship between the z-cyclopentadienyl proton nuclear magnetic resonance (n.m.r.) chemical shifts and the carbon-oxygen stretching force constants in cyclopentadienylmetal carbonyls in an attempt to investigate the T-cyclopentadienyl n.m.r. chemical shift as a possible indicator of the negative charge on the metal atom (1). Evaluation of the carbon-oxygen stretching force constants in monocarbonyl and dicarbonyl derivatives from thev(C0) infrared (i.r.) frequencies is a very simple problem. This paper discusses the much more difficult problem of evaluating the carbon-oxygen force constants from the v(C0) frequencies in compounds of the type C,H,M(CO),X (1: M = Mo or W; R = oneelectron donor (2)).These C,H,M(CO),X compounds, 1, have C, symmetry. A group theoretical analysis predicts three v(C0) frequencies (2A' + A") which are related to the four force constants 'Fellow of the Alfred P. Sloan Foundation, 1967-1969. ZPostdoctoral Fellow, 1967 (k,, k,, kd, and k,) by the following secular equationsSince there are four force constants but only three v(C0) frequencies, a solution of the secular equations is only possible if one additional relationship between two of the force constants is used. This additional relationship requires an additional assumption and/or approximation. Since the stretching force constants k, and k , are both larger and of greater interest than the interaction force constants kd and k,, the additional relationship used was an assumption of a constant value for the ratio kd/k,.The constant value of 2 for kd/ks was selected for the following reasons: (1) The metal orbitals required for bonding in C,H5MX, were first considered. The five metal orbitals s, p,, p,, p,, and d,2-y2 are used for the forward o-bonding of the metal atom to the z-C,H, ligand and the four X groups. The two metal orbitals d,, and d,, are used for the two orthogonal forward z-bonds of the metal atom to the z-C,H, ring. This leaves the two metal orbitals dZz and d, available for retrodative bonding. Linear combinations of these two metal orbitals can form the following two hybrid orbitals JI-= 2-*[4(d,.> -4(dxy)lThe lobes of one of these hybrid orbitals will be of suitable symmetry to engage in retrodative Can. J. Chem. Downloaded from www.nrcresearchpress.com ...
