*1.7. CO Identifier*

The authorized CO identification method consists of the following techniques: Electrochemical assays [59,60], laser infrared absorption [61] and gas chromatography [62]. Moreover, a colorimetric CO sensor facility is an alternative platform, especially for observing the CO behavior in living cellular tissues or/and organs [63,64]. The vibrational spectroscopy technique (such as IR, infrared and Raman) is one of the quickest ways to monitor the CO attached with transition metal through carbonylation at distinctive bond ranges. To date, the plethora method "Myoglobin Assay" famous as "*Gold Standard*" has been in operation for in vitro interrogation of the CO release from CORMs and CORMats. The "Myoglobin assay" is also a worthy and standard method for observing the behavior of the CO release kinetics in a biological environment and monitors the performed activity of CO (Equation (1)).

$$\text{Mg} + \text{CO} \rightarrow \text{Mg-CO} \tag{1}$$

Recently, It has been observed that binuclear Rhodium compounds, i.e., *cis*-[Rh2(C6H4PPh2)2(O2CCH3)2](HAc)2] can detect CO with a substantial selectivity and superior sensitivity [65]. The gas-phase IR spectroscopy is the most reliable and high-resolution technique for analyzing the CO release activity. For directly sensing the CO, various gas chromatography (GC) detectors have been introduced to date including the gas chromatography-mass spectrometer (GC-MS) [66], reduction gas detector (GC-RGD) [67] and thermal conductivity detector (GC-TCD). The fluorescent probe has the ability to recognize the CO-release entity even at a low concentration as compared with the myoglobin assay, but it is unable to operate in a short interval kinetic measurement (for that at least one hour interaction is required) [68,69].

#### *1.8. The Development Phases of CORMs Motifs*

#### 1.8.1. Metal Carbonyl Complexes (MCCs)

To construct the bonding relation between the CO and low valent metal ions for producing carbonyl complexes (M-CO), the M-CO bonds must undergo an inert ambiance along with reducing conditions, which are mostly feasible in organic solvents. Irrespective of a metal physical state, the CO gas can react and develop the volatile metal carbonyl complexes (MCCs) for example, Ni(CO)4 and Fe(CO)5 [70]. MCC acts as a core entity for organometallic transition chemistry. The general representation for MCCs is [Mm(CO)xLy]z±[Q±]z [53], in which, M, L are the basic entity known as

transition metal (B, Cr, Mn, Fe, Co, Mo, Ru, Rh, W, Re, Ir) [71], and ancillary ligand might be the C, O, P, N, S or halide ligand. Furthermore, Q and z represent the counter ion and overall complex charges. If no counter ion is available, then z will be zero. Moreover, m, y, z are calibrated as stoichiometric coe fficients and x and m values should be ≥ 1 [53]. Modern and classic complexes can be distinguished by two determining factors: Low oxidation state (OS) or either very low oxidation state (LOS) and total valance electron occupied in the outer coordination sphere. For 4th, 9th, and 10th groups, the compounds were observed to have 16e- configuration and the rest of the complexes were generally observed having 18e- configuration. Each of these commodities must comply with the chemical, biological and physical characteristics of MCCs. Furthermore, it needs to be precisely selected in the configuration of pharmaceutical CORMs [71].

It is important to note that CORM and CORMats are stable in the aqueous medium, and it is also feasible to store it under ambient environmental conditions like the majority of other pharmaceutical drugs. Their circulation must be ensured, as it needs contact both with the diseased and damaged tissues. Moreover, the potent and non-toxic metabolites may be left behind after the CO removal. This is exactly what is required and regarded as therapeutic features. It sets the basic pattern, in the line of action for the development of such challenge-able MCCs. Mostly, during administration MCCs incorporates with the organic solvent and results in traditional oxygen-free atmosphere. These medical conditions might be di fferent from a variety of other biological surroundings, considering that most reactants and their resulting complexes are uncertain under ambient conditions, i.e., oxygen and humidity [71]. Hypothetically, this biological activity of MCCs remains toxic in nature like Ni(CO)4, and (MeCp)Mn(CO)3 (MMT) as an anti-knock gasoline additive [72]. Hence, the common MCCs chemistry can act as a simple guideline for the development of pharmaceutical CORMs. Currently, this research is focused on novel strategies for establishing MCCs-CORM's activity and specifically for therapeutic purposes.

The above discussion suggested that MCCs when triggered as CORM's, become a competitor for the CO availability during its decomposition. In organometallic complexes the releasing strategy is as follows: A new incoming ligand (L') can push itself to a metal center resulting in a new bond, which establishes and influences on the coordination number. The elevated coordination number then promotes it to elongate the M-CO bonds, and eventually it then breaks. Consequently, the CO is liberated by this method and the new L'-M bond is constructed. This information provides the foundations of the CORMs concept. The chemistry of MCCs provides per se di fferent strategies and has an adverse impact on the CO release (Scheme 1) [53].

**Scheme 1.** CO releases from the ligand-metal CO framework (LnM-CO).

#### 1.8.2. Proposed Strategies for CORMs Development

CORMs based on metal-to-ligand charge transfers (MLCT) morphology. These CORMs are the elementary motifs with organometallic ingredients; corresponding to a series of MCCs occupied at the transition metal core. Other exclusive features are mentioned below:


As abovementioned, the spectroscopic nature of MCCs confirms the identification and recognition of significant trace elements like ruthenium [73,74], manganese [49,74–78], iron [37,79], cobalt [80,81], tungsten [82], osmium [83], molybdenum [82] and rhenium [84]. The developed organometallic carbonyl complexes CORMs are CORM-1, CORM-2, CORM-3, CORM-401, ALF492, CORM-A1, B12-ReCORM-2, Re-CORM-1, CORMA-1-PLA and ALF186.

Along with organometallic complexes, the miscellaneous compounds can be nominated as the CORMs family. In this scenario, numerous nonmetallic compounds [85] have been accomplished by entertaining the CO release such as silica-carboxylates [86], borano-carbonates [87], borano-carbamates [88], xanthene carboxylic acid (XCA) [89], unsaturated cyclic diketones (DKs) [90], methylene chloride (MC) [91,92], meso-carboxy BODIPYs [46], hydroxy-flavones [93] (Scheme 2). Furthermore, 1,2-disubstituted ferrocenes belongs to an aldehyde family, unfavorably elicits the toxicity and its slow release mechanism restricts the researchers from developing another nonmetallic CORMs (NCORMs). The main drawbacks of NCORM are potentially a low CO content releasing, and always producing organic molecules along with the CO moiety. Anyhow, NCORM clinical traits have shown their utility to communicate with biological activity [94].

**Scheme 2.** Various CO-releasing molecules (CORMs) formulation associates with di fferent functional capabilities.

#### *1.9. CORM's Therapeutic Scope*

Strategically, the synthesis route of CORMs development is not the only objective. The main theme of this CORM's innovation is to obtain the therapeutic advantage eventually. The biological significance of CORM is associated with their bacterial performance in cells lines, (i.e., standard myoglobin assay). The important biological roles of CORMs are listed below (Table 1).


