**2. Concerning organometallic compounds occurrence in environment from anthropogenic sources**

The compounds considered in this work are those having environmental implications and are susceptible to threat biota and human health. Thus, we limited this chapter to organic forms of mercury, lead, and tin. These compounds could occur in environment either naturally or deposited as an industrial pollutant.

#### **2.1. Mercury and its organic derivatives**

Mercury (Hg) is a nonessential and extremely toxic trace element that poses global environmental and human health risks [5, 13]. Its biogeochemical cycle was perturbed during the last centuries by anthropogenic inputs. The organic forms of mercury compounds have been used in chlor-alkali plants and coal power station industry, and also in other anthropogenic activities, such as catalysts, fungicides, herbicides, disinfectants, and pigments [5, 12, 13]. Emissions and inputs from those mentioned industrial processes as well as from the combustion of fossil fuels and waste disposal finally resulted in severe environmental contamination [17, 18].

As mentioned in many studies, mercury and its related compounds are considered health hazards, but their toxicity depends strongly on their chemical forms, those organic forms of mercury, such as methylmercury or dialkyl mercury, are considered more toxic than inorganic salts of mercury [19, 20].

(1) Generally, metal speciation marks both the fate as well as the toxicity of metals in environmental compartments, that speciation adverts to the occurrence of the different variety of chemical forms of a specific metal in the environment. Such forms of metals could be free ions, complexes (dissolved in solution or sorbed on solid surfaces) or as forms that have been coprecipitated in major metal solids or which occur in their own solids [3]. Considering scientific reports regarding mercury species toxicological effects, it becomes necessary to speciate mercury [21]. Lindqvist et al. [22] categorized the mercury species compounds into three categories: (i) volatile species (Hg); (ii) reactive species (Hg2+, HgO on aerosol particles, Hg2+ complexes with OH, Cl, Br, and organic acids); and (iii) nonreactive species (CH<sup>3</sup> Hg<sup>+</sup> and other organomercurial moieties, Hg(CN)<sup>2</sup> , HgS and Hg2+ bond to sulfur in fragments of humic matter) [20]. The speciation of mercury and its related compounds affects, besides the degree of toxicity, also its properties (e.g., volatilization, photolysis, sorption, atmospheric deposition, acid/base equilibrium, diffusivity, microbial transformation degree, and pattern) that characterize their fate and pathways in the environment [3]. The organic species of mercury that were found to be important from hazard and toxicological point of view and those are prevalent in environment are as follows: methylmercury (CH<sup>3</sup> Hg<sup>+</sup> ), ethylmercury (C<sup>2</sup> H5 Hg<sup>+</sup> ) [23, 24], phenylmercury (C<sup>6</sup> H5 Hg<sup>+</sup> ) [25], and dimethylmercury ((CH<sup>3</sup> )2 Hg) [26].

#### **2.2. Lead and its organic derivatives**

affect the ecosystem, plants, and other living organisms (micro and macro), and also contami-

As natural components of the Earth's crust and resultant of biogeochemical reactions, organometallic compounds are generally present at low concentrations in natural environmental compartments, as soil or water, but extensive anthropogenic activities over the past 50 years (industrial, mining, agricultural, and urban-extension activities) have greatly increased their inputs in different compartments of our surrounding environment; thus,their presence is

Among all compartments, soil plays an important role in the distribution and fate of organometallic compounds, since often it serves as a major reservoir and sink of these pollutants due to its large absorption capacity [11, 12]. In the terrestrial and aquatic environment, metals occur in both organic and inorganic forms, including elemental forms, salts, and organometallic compounds. In most cases, the mineral form of metals is insoluble thus rendering these

Inorganic species are adsorbed onto soil organic matter and/or metal oxides which can be subjected to biogeochemical processes (e.g., reduction, oxidation, methylation, alkylation, and biomethylation) resulting in highly mobile organic species that further have the ability to form water-soluble complexes in living organism body tissues thus increasing the potential for uptake and accumulation by organisms [13, 14]. Moreover, bond formation between methyl, ethyl, or alkyl groups and metals or metalloids cause changes in their physical properties as solubility or volatility, properties that could significantly affix their fate, pathways, and life cycle both in the original environmental compartment and between interconnected environmental compartments. The rates of all these processes depend greatly on the local conditions, which exist in the relevant ecosystem and the microbial activity, as the pollutant's

species rather unavailable for transport or plant uptake in the short term [5].

leaching rate strongly depends on its specific geochemical properties [15, 16].

**2. Concerning organometallic compounds occurrence in environment** 

The compounds considered in this work are those having environmental implications and are susceptible to threat biota and human health. Thus, we limited this chapter to organic forms of mercury, lead, and tin. These compounds could occur in environment either naturally or

Mercury (Hg) is a nonessential and extremely toxic trace element that poses global environmental and human health risks [5, 13]. Its biogeochemical cycle was perturbed during the last centuries by anthropogenic inputs. The organic forms of mercury compounds have been used in chlor-alkali plants and coal power station industry, and also in other anthropogenic activities, such as catalysts, fungicides, herbicides, disinfectants, and pigments [5, 12, 13]. Emissions

nate the food chain (including that of humans) [4–6].

48 Recent Progress in Organometallic Chemistry

becoming a severe problem at worldwide level [7–10].

**from anthropogenic sources**

deposited as an industrial pollutant.

**2.1. Mercury and its organic derivatives**

Inorganic lead is introduced into the natural environment from several sources, but organolead compounds are mainly exhausted into the air from the petroleum industry and automobiles, and then they contaminate soil and water sources [27].

Lead organic forms, such as tetramethyl lead and tetraethyl lead have been widely used as antiknocking agents in fuels. Although in the past decade lead gasoline consumption decreased considerably, there are still countries around the globe that use it [28, 29]. Currently, gasoline used in aviation remained the fuel with the highest alkyl lead content, in those days sources of alkyl lead in surrounding environment are airport fuel terminals, bulk aviation, gasoline plants, bulk leaded racing, and other nonroad vehicles gasoline plants, spills from fuel loading, transfer storage, and fuelling [27].

Alkyl lead compounds, such as tetra-alkyl lead are easily absorbed by living organisms due to their lipophilic character. Their absorption depends on the nature of the compound, exposure time, and nature of organism [27, 30]. Toxicological studies on human beings have been demonstrated that cumulative chronic exposure to organic forms of lead is more toxic than those to inorganic forms of lead [31]. According to Gallert and Winter [30] and Pyrzynska [32], the toxicity of alkyl lead compounds decreases with a decreasing number of ethyl or methyl moieties or with a decreasing number of carbon atoms (ethyl lead → methyl lead) according to the following sequence: R<sup>4</sup> Pb > R3 Pb+ > R2 Pb2+, with R being either –CH<sup>3</sup> or –C2 H5 [27].

#### **2.3. Tin and its organic derivatives**

Organotin compounds are organometallic compounds in which carbons are bonded directly to tin (R*<sup>n</sup>* SnX4-*<sup>n</sup>* , where *n* is between 1 and 4, and R is an alkyl or aryl group) [33]. Organometallic forms of tin have been used as active agents in a wide range of applications in industry, such as stabilizers in the polyvinyl chloride industry, plastic additives production, industrial catalysts, antifouling paints, wood preservatives, and in agriculture as biocide products (insecticides, fungicides, and bactericides) [33–35].

Nowadays, use of organotin compounds as anti-foulant has been banned due to their severe toxic effects on the aquatic organisms [36]. Moreover, use of tributyl tin and triphenyl tin compounds in various industrial applications has raised a great concern in the last decades owing to their serious toxic effects on nontarget organisms when leached into environment even at very low concentrations (ng·L−1) [37]. Besides the fact that they are considered as endocrine disruptors among organometallic compounds, they also possess teratogenic properties and can cause disruption to the reproductive function in mammals, as well as could act as hepatoxins, immunotoxins, neurotoxins, and obesogens [35, 38].

In the following sections of this chapter, we will present quantitative and qualitative data about the presence of these compounds in different environmental compartments and biota samples.
