C-13 NMR-SPECTRA OF DITHIOCARBAMATES - CHEMICAL-SHIFTS, CARBON-NITROGEN STRETCHING VIBRATION FREQUENCIES, AND PI-BONDING IN THE NCS2 FRAGMENT

From the 13C NMR spectra of 71 dithiocarbamates, the chemical shifts of the carbon atom of the NCS2 moiety are correlated to the π bonding in the NCS2 fragment. With the aid of pattern recognition techniques, the dithiocarbamates are divided into five classes: (i) the free ligands; (ii) “normal” oxidation state transition metal dtc's; (iii) “normal” coordination number main group dtc's and organic dtc's; (iv) “high” oxidation state transition metal dtc's; (v) “low” coordination number main group dtc's. Normal, high, and low refer to the fractional oxidation number (FON), a useful quantity, defined as the ratio between oxidation number and coordination number. “Normal” is FON = 1/2- A high FON refers to a high oxidation state or a low coordination number and correlates with a high v(C-N) and a low δ(N13CS2). As such, the FON is a characteristic property distinguishing classes ii and iii from classes iv and v. It is not the difference between transition metals and main group element but rather the difference between symmetric and asymmetric dithiocarbamate bonding that distinguishes classes ii and iv from classes iii and v. In a semiempirical way δ(N13CS2) could be expressed as a linear function of the sum of the CN, CS1, and CS2 π-bond orders, and v(C-N) of the CN π-bond order. The sum of the π-bond orders is derived to be maximal for equal π bonds and to decrease with increasing inequality of the three π bonds. Therefore, the free ligands (class i), with nearly equal π bonds, are located at the upper limit of b values; the compounds with high v(C-N) (classes iv and v) have low CS and low total π-bond orders, thus low δ values; and the compounds with asymmetrically bonded dtc (classes iii and iv) have low total π-bond orders and low δ values. For the various classes estimates of mean CN and CS π-bond orders, calculated from δ(N13CS2) and v(C-N) values, are given. Other effects, which influence 5(N13CS2), are the substituent on nitrogen and the period to which the central metal belongs. © 1979, American Chemical Society. All rights reserved.